Friday, December 17, 2010

What a Geologist Sees - Part 24 [Original Post Date 10/31/08]

I am sure that many folks have seen rounded boulders before. Have you given much thought to how they got rounded?

One possibility is that the boulders were rounded in a river channel subject to "big ass" floods, necessary to move and abraid the boulders against each other.

But more often, the boulders are rounded in situ (in place) by a process called spheroidal weathering. On the Georgia Piedmont, most often we see spheroidal weathering in granites and granitic gneisses.

A couple of other places where I have seen spheroidal weathering of granites is Texas Canyon, AZ and Sequoia National Park. [While traveling with my college roommate Dave in 1974, we were in our campsite in Sequoia, stretched out on the hood of the car, leaning against the windshield watching the stars. In the dim light of nearby lanterns, we could see that we were surrounded by rounded granite boulders. After a while, I became aware that there seemed to be an "extra boulder". Turning on the flashlight, that "extra boulder" stood up (it was a bear). At that point, we decided the car was a better place to sleep that night, rather than the tent.]

An additional place where I have seen spheroidal weathering, of volcanic ash-flow tuffs, is in the City of Rocks State Park in New Mexico. [I may place a photo in a later post of this particular place.] With the softer ash-flow tuffs, wind erosion may have also played a role in rounding the boulders, in that particular locale.

What is destined to become rounded boulders starts off as fractured bedrock, broken by what we call brittle deformation. At depth, with the greater confining pressure and heat, rocks become plastic (softened) and undergo folding, stretching, and other forms of ductile deformation. At shallower depths, without the confining pressure and the flexibility provided by the higher temperatures, the rocks become fractured and jointed during earthquakes and other heavings of the earth, leaving angular corners.

As weathering and erosion proceed over the countless millenia, the downward percolating water and natural acids attack the minerals on the outer margins of the rock, by way of the joints and fractures - in the shallow subsurface. The chemical breakdown of minerals by chemical reactions is called chemical weathering.
There are several chemical weathering processes, the one that attacks the feldspars in the granites is primarily hydrolysis.

In areas prone to occasional freezing, the expansion of ice in the fractures (and microfractures) provide even more surface area. This is one process in what we call physical weathering.

Over time, the combined processes of physical and chemical weathering attack the corners of the rock, rounding them off. As erosion carries away overlying soil and degraded rock, the boulders become exposed, wherein the processes of chemical and physical weathering continue, yielding the rounded boulders as seen in the photo.

[The scars seen on the photos are likely from the bulldozers used to "gather" the boulders during construction at the nearby Nature Center parking lot.]

[Visit this post for links to previous posts in this series.]

What a Geologist Sees - Part 23 [Original Post Date 10/10/08]

Providence Canyons, locally called "the Little Grand Canyon", in Stewart County, Georgia, provides a stark look at what can happen when erosion takes hold in an area underlain by a soft sand.

The canyons are generally on the order of 125 feet deep and are only perhaps 150 years old.

The supposed genesis of the canyons was due to rain running off the corner of a church and beginning a small gully. The surface of this area is locally covered by the residuum of the Paleocene-aged Clayton Limestone, which has been reduced to a iron-rich, reddish clay. One this clay veneer is breached by a gully, the underlying soft, deltaic sands of the Cretaceous Providence Sand. There is essentially nothing to stop the downward erosion or the widening of side canyons. At some point in the future, the area will probably reach some sort of equilibrium as a series of low sandy hills, separated by sand-clogged, braided streams.

As an aside, exposures of sand such as this serve as "recharge zones" for sand aquifers and elsewhere, the subsurface Providence Sand does serve as an aquifer.

[Older posts in this series, before Part 22, are linked here.]

Wednesday, December 15, 2010

What a Geologist Sees - Part 22 [Original Post Date 10/10/08]

Most folks know a river or creek meander when they see one (or more).

Usually we see them in Coastal Plain settings or other places where the stream gradient (feet/mile drop in elevation) is low, especially if the materials underlying the stream are soft Coastal Plain sediments and/or floodplain deposits.

When a stream's gradient is steep, as in a mountain stream, there is a tendency for gravity to control erosion, i.e., the erosion is vertical - down-cutting as we call it. This vertical down-dutting (being slightly redundant) results in sharp "V"-shaped valleys, with no flood plains.

So when you see deeply-incised meanders, such as in the above photo, that suggests that the meanders were established under low-gradient conditions at a higher Base Level (see this post for an explanation of Base Level). Then a rapid drop in Base Level and/or a rapid uplift of the land "preserved" the meanders as the river cut downward through the Colorado Plateau sedimentary rock layers.
The Grand Canyon also illustrates this same rapid uplift/drop in Base Level, as does the area containing the Black Canyon of the Gunnison River, in Colorado, though without the meanders.

[Previous posts in this series are linked here.]

What a Geologist Sees - Part 21 [Original Post Date 9/05/08]

Understanding the concept of Base Level is an important part of understanding the behavior of streams, erosion, and subaerial (terrestrial) stream deposition.

"Base Level" (mentioned briefly here) as a concept was defined by John Wesley Powell and is the lowest point to which a stream can erode at any one point. More info is presented here.

Ultimate Base Level is Mean Sea Level, i.e., rivers and streams cannot erode any deeper than sea level.

Over geologic time, Ultimate Base Level changes with changes in sea level. The primary causes of Mean Sea Level are variations in polar icecap coverage (more ice = lower sea level) and variations in plate-tectonics oceanic rift zone activity. When rift zones are more active, the sea floor is bulged upwards by the rising mantle plumes, which displaces sea water and raises sea level. When rift zone activity slows, sea level drops. Add to this local changes in the elevation of continental margins. When the edge of a continent rises, that mimics a sea level drop (aka a Regression) and vice versa when the edge of a continent sinks, that mimics a sea level rise (aka a Transgression).

When the Ultimate Base Level rises, the "Ultimate Stream Gradient" (from the ultimate stream/river source) is decreased, resulting in more deposition. There will be deposition also in any "drowned" portions of previously exposed river systems. When Ultimate Base Level drops, the Ultimate Stream Gradient increases (steepens), resulting in more erosion.

Within every river and creek system, there are inumerable "Local Base Levels", points below which the upstream river (or creek) cannot erode. Local Base Level controls the upstream-gradient on that particular river/creek until the next higher-elevation Local Base Level.

In the second photo, there is a small example of a Local Base Level, established by this small outrcop in Chickamauga Battlefield Park. Upstream from this locally resistant outcrop, the stream is prevented from down-cutting any further. The next ledge or other outcrop upstream then establishes the next upstream Local Base Level.

As changes in sea level affect the Ultimate Base Level and the Ultimate Gradient, changes in a Local Base Level affects Local Stream Gradients. When a dam is constructed (or when a landslide naturally forms a pond/lake), that raises the Local Base Level, resulting in more deposition. When Lake Lanier was constructed a little more than 50 years ago, the rising "pool elevation" raised the Local Base Level of all streams entering the lake. In the upper photo, you see the exposed lake-bottom sediments, contributed by local hillside erosion and by deposition of sediments contributioned by this particular small stream, over the course of 50 years.

When the extended drought and dam releases caused a lowering of the pool elevation, that lowered the Local Base Level and resulted in the few inches of erosion (down-cutting) that you see in the upper photo. When the lake returns to its Full-Pool elevation, the upstream Local Base Levels will again rise and down-cutting will cease and sedimentation will resume.

There are characteristic stream behaviors that are affected by the steepness of the stream gradient (Ultimate and Local). When you have a steep gradient (low Ultimate Base Level or Local Base Level), you have more down-cutting in the stream valleys. When the Base Level suddenly rises, the upstream valleys are "back-filled". Remember, a subsidence of a continental margin can mimic a Base Level rise.

On Coastal Plains and River Deltas, where the gradient is very low, lateral erosion causes migration of the stream channels and the resulting meanders. And usually in this situation, you have wide floodplains and/or local low-relief topography. When you have a low-gradient feature, such as river meanders, in an area with great topographic relief, that can tell you of a rapid drop in Ultimate Base Level or a rapid uplift of the landmass, such as with the Colorado Plateau (discussed in the next post of this series) or with some other regional feature, such as the land surrounding the Black Canyon of the Gunnison River, in Colorado.

Changes in Local Base Level (including some caused by human activities) can have smaller scale effects on local streams (as decsribed above regarding Lake Lanier).

[Previous posts are linked here.]

Minerals That I Would Like to Collect [Original Post Date 8/22/08]

[Yeah, this would only appeal to the occasional geologist visitor.]

Having been involved with Geology for more than 35 years, there are a few minerals that I would like the experience of having "dug" myself or at least to have found them on a mine dump. Some of these I do own specimens of, having bought them or traded for them. But I would like the "wow" experience of having found at least a recognizable specimen, by my own hand.

In no specific order, they are:

1) Topaz - it has been reported from Graves Mt., GA and I have a mystery crystal from that locality that I would like for someone to verify, yes or no.
2) Garnets - with good crystal faces, at least 3/8 inch in diameter.
3) Biotite mica - some thin "books", at least 2 inches by 2 inches square.
4) Millerite - a fibrous nickel sulfide, found in small geodes near Halls Gap, KY.
5) Zircon - a decent crystal, at least 3/8 inch on any particular side.
6) Brookite - a titanium dioxide mineral from Magnet Cove, AR.
7) Wulfenite - have a number of purchased crystal specimens, would like to find one.
8) Apatite - a decent, recognizable crystal. I think I have some small apatite crystals in marble from Tate, GA and in some pegmatite material from Maine, but they are all in matrix.
9) Lazulite - some decent crystals from Graves Mt., GA. I have some weathered and broken specimens, but no decent ones.
10) Spodumene - a lithium silicate, I have been to one locality in NC, but didn't find any.
11) Selenite (gypsum) - from Jet, OK, for instance. I have found gypsum in several localities, but not any decent crystals.
12) Barite - crystals from Cartersville, GA. I have barite crystals collected from Graves Mt., and the Bishop Cap Hills in southern New Mexico. But I have missed field trip opportunities for the Cartersville area.
13) Vivianite - a nickel phosphate. Some small masses of vivianite have been found inside of fossil oyster shells on the Georgia Coastal Plain, near Providence Canyons. The vivianite was deposited inside the shells by groundwater action.
14) Talc - just some decent, light colored masses of the mineral, not the metamorphic rock (which I already have).
15) Epidote - just a decent sized, 1/2 inch length or longer crystal. I have smaller crystals, but not big enough for non-collectors to appreciate.

What a Geologist Sees - The Series - Phase 2 [Original Post Date 8/18/08]

Phase 1 is indexed here.

Part 11 - Appalachian/Cumberland Plateau, limestone, Silurian fossils
Part 12 - Georgia Coastal Plain, water well construction
Part 13 - Southern New Mexico Quaternary Volcanics, Kilbourne's Hole, base-surge deposits
Part 14 - Southern New Mexico Quaternary Volcanics, Kilbourne's Hole, volcanic bombs, xenoliths
Part 15 - Xenoliths & other rock inclusions
Part 16 - Industrial uses of mica
Part 17 - Florida Coastline, external sedimentary structures, ripple marks, raindrop impressions
Part 18 - Southern New Mexico Quaternary Volcanics, Aden Basalt features
Part 19 - Glacier National Park, structural geology, thrust faults
Part 20 - Colorado Plateau, Monument Valley, sedimentary layers

What a Geologist Sees - Part 20 [Original Post Date 8/18/08]

[As referenced here, Stratigraphy is the study of layered rocks.]

Monument Valley, which straddles the AZ/UT border, is one of those places on my Top-10 Want-to-Visit List (which will be the subject of a later post).

In the course of my classes, whether they be Environmental Science or Geology, I remind my students that in the future, if they do a little "homework" before traveling, they will enjoy the trip more. Especially if they have kids to entertain.

To a non-scientist, it might seem that thinking about the geology might distract from the enjoyment of the natural and stark beauty, but to me it doesn't. To me, visualizing about "what it used to look like" at various points in geologic history adds to the wonder. To address the subject of the first photo (actually all three slides were taken by my Dad in 1980), the mesas, buttes, and most of the spires of Monument Valley are remnants of a formerly-continuous sheet of Permian and Triassic sedimentary rocks. The millions of years of weathering and erosion have brought forth the wonderous landforms we see, not only in Monument Valley, but elsewhere in the Colorado Plateau.

The four formations shown in the second photo are but a small portion of the "Colorado Plateau Stratigraphic Section", i.e., all of the layered rocks that occur within the defined area of the Colorado Plateau. [This USGS webpage lists all of the geologic units that occur within the Colorado Plateau, some of which only occur at the margins and extend into adjacent regions. This USGS webpage lists all of the National Parks and such that are present within the Colorado Plateau (and the Colorado River Basin). Monument Valley itself is not administered by the National Park Service.] A simple geographic definition of the Colorado Plateau, which covers 50,000 square miles, is presented here by Encarta. The Moenkopi and Organ Rock Formations are present to the north at Canyonlands National Park, while the de Chelly Sandstone is present to the south at Canyon de Chelly National Park. The Permian de Chelly Sandstone is derived from eolian sand-dune deposits. At Canyonlands, the White Rim Sandstone lies between the Moenkopi and the Organ Rock, thus making it the "Stratigraphic Equivalent" of the de Chelly, though the environment of deposition was different, suggested to be nearshore sand dunes for the White Rim.

In arid climates, topographic slopes are often defined by their underlying rock types in ways that are different from humid climates. The contrast between arid-climate "slope formers" and "cliff formers" is illustrated by the mesa (I think it is Sentinel Mesa) in the second photo. In arid climates, shales are "slope formers", while sandstones and limestones are "cliff formers". Alternating shales and sandstones/limestones produces a "stair-step effect", with the slope angle related to the percentage of shale vs. the other two rock types.

[I can give a more detailed description of why limestones behave differently in the contrasting climates, but that would take up more space.]

The third photo is of Agathla Peak, which is different from the other landforms in Monument Valley. It is an eroded "volcano neck", i.e., the central spire represents solidified magma within and/or below the original volcano. It is more resistant to erosion than the slope material that made up the flanks of the volcano. The volcano neck can also be referred to as a "feeder pipe". Shiprock is another good example of this type of landform. Agathla Peak is some 1,500 feet higher than the surrounding plateau.

[Additional info may be added later.]

What a Geologist Sees - Part 19 [Original Post Date 6/08/08]

Glacier National Park is one of the places that I hope to visit, someday.

[For the moment, a photo taken by a friend will have to suffice until I can get there to take my own.]

The feature pictured at right is Chief Mountain, which consists of Proterozoic limestones sitting atop Cretaceous shale and sandstones. This is totally "out-of-whack" with "what should be". Under normal conditions, the Concept of Superposition applies, wherein in a series of layered rocks (usually sedimentary, but could apply to some volcanics), the oldest layers are at the bottom and the youngest are at the top. The Proterozoic is much older than the Cretaceous.

The presence of a "thrust fault" is the reason for the existing condition. During periods of tectonic deformation, where there is lateral compression, sometimes sheets of rock will break loose and get "shoved on top of" younger layers. The lateral breakage is usually along a shale layer. In this case, the thrust fault is referred to as the Lewis Thrust Fault or the Lewis Overthrust. Erosion after the period of faulting isolated Chief Mt. from the remainder of the thrust sheet, thus we call Chief Mt. a "klippe", which is a type of "erosional outlier", i.e., it is laterally separated from similar rocks.

The mesas and buttes of Monument Valley are also erosional outliers, but they have no underlying thrust fault, so they are not klippes.

There are plenty of thrust faults in the Appalachians, including the Georgia Piedmont, though they are obscured by soil and vegetation cover. There is a thrust fault (name forgotten) exposed in an outcrop on the west side of I-285, north of the intersection with I-20 and near the Chattahoochee River, but the traffic makes it difficult to pull over and get a good photo.

[I will fill in some more details later...]

Sunday, December 12, 2010

Learning Something New...Every Day (Well Almost)

From the well-known geoblog "Highly Allochthonous", comes a post (from a few months ago) about a recent 5.0 earthquake in eastern Canada.

Scrolling through the post, I learned that the St. Lawrence River follows a small graben system, related to a failed rift-system related to the rifting of the supercontinent Rodinia and the opening of the Iapetus Ocean, about 1 billion years ago.

Similar to the New Madrid Fault System along the Mississippi River, crustal weaknesses due to the old breaks make the area susceptible to earthquakes.

[When time permits, I plan to write a bit more about this newly discovered (from my perspective) geologic feature.]

What a Geologist Sees - Part 18 [Original Post Date 5/06/08]

Is this the world's smallest volcano? Maybe. [See the rock hammer for scale.]

This is a small example of a volcanic spatter vent. Usually spatter vents are late-stage events in the life cycle of an eruption, as the volcano is "losing its punch."

This spatter vent and the spatter mound shown below are just a couple of small, yet interesting features of the Aden Basalt Flows, which are part of the Potrillo Volcanic Field in southern Doña Ana County, New Mexico. The cinder cones of the northern parts of the Potrillo Volcanic Field are visible to the south of I-10, west of Las Cruces, New Mexico. Kilbourne's Hole, discussed in What a Geologist Sees - Part 13, is also part of the Potrillo Volcanic Field.

Most of the Aden Basalt flows are the result of fissure eruptions (lava flows erupted from open fractures in the ground surface), except in the northwestern part of the Aden Basalts, where Aden Crater (a small shield volcano) is present. Aden Crater is thought to have formed after the bulk of the fissure eruptions. Aden Crater (the subject of a future post) lies at the probable intersection of the Robledo Fault and the Aden Volcanic Rift (a buried fracture zone).

Spatter vents erupt by spitting small to moderate clots of partially solidified basalt (of the consistency of taffy).

The lower photo is of a mound of spatter material within Aden Crater itself. The stadia rod (used in mapping), leaning against the right side of the spatter mound, is about 6 feet long (for scale).

These are just a few of the many interesting volcanic features to be found in the Aden Basalts. Many typical features of basaltic vulcanism are preserved in the Potrillo Volcanic Field, along with some oddities that have not been seen elsewhere (for another post).

In another future post, I will show some different types of volcanic "ejecta" (aka volcanic bombs), which are found associated with some of the volcanoes of the Potrillo Volcanic Field. Just a suggestion, if you ever drive out into this area, carry plenty of water, have a good map, and let someone know where you are going.

Geology in the News - Fossil Thievery in Peru

Fossil "piracy" and smuggling, in the high deserts of Southern Peru, are the subject of this New York Times article.

A few snippets from the article:

"Nestled between the Andes and the Pacific, the sparse desert surrounding this outpost in southern Peru looks like one of the world’s most desolate areas. Barren mountains rise from windswept valleys. Dust devils dance from one dune to the next.

But to the bone hunters who stalk the Ocucaje Desert each day, the punishing winds here have exposed a medley of life and evolution: a prehistoric graveyard where sea monsters came to rest 40 million years ago. These parched lands, once washed over by the sea, guard one of the most coveted troves of marine fossils known to paleontology."

[I am posting these, as news links don't seem to last forever.]

"Discoveries here include gigantic fossilized teeth from the legendary 50-foot shark called the megalodon, the bones of a huge penguin with surprisingly colorful feathers and the fossils of the Leviathan Melvillei, a whale with teeth longer than those of the Tyrannosaurus rex, making it a contender for the largest predator ever to prowl the oceans.

“This is perhaps the best area in the world for marine mammals,” said Christian de Muizon, 58, a paleontologist at the Natural History Museum in Paris who led an expedition here in November. He ranks the Ocucaje (pronounced oh-coo-CAH-heh) and adjacent sections of desert with top fossil areas like Liaoning Province in China, where ashfall famously preserved plumed dinosaurs."

As this may be a relatively-newly discovered area, it may be that it is largely unexplored by professional paleontologists. Another consideration, perhaps similar to Argentina, Peru may not have the financial resources to properly collect and study the fossils in this remote area.

"Peru is astonishingly rich in archaeological and paleontological sites, so much so that the issue is part of a delicate political debate here. The loss of national treasures to collectors from abroad has set off concerns about sovereignty, perhaps best exemplified by the feud between Peru and Yale University over Inca artifacts taken by Hiram Bingham, the American explorer typically credited with revealing the lost city of Machu Picchu to the outside world a century ago.

For now, the Ocucaje remains open to just about anyone who wants to search for fossils here. Peruvian law, while vague, classifies fossils as national patrimony and requires fossils found in the country to remain in Peru, unless special permission is granted."

There are concerns about mining company trucks passing through the area and the damage that might be done by them, also.

"But enforcement and preservation here seems like a distant dream. The government controls the desert but leases parts to mining companies, which could damage or destroy fossils. Looters have already ravaged archaeological burial sites on the desert’s fringes. The police rarely even enter the area."

Perhaps, the government/universities could "cut a deal" with the mining companies to encourage them and their employees to help preserve the fossils and protect the area from looters.

"On the streets of Ica and nearby towns, visitors can already see such fossils — and buy them. Merchants sell fossilized shark teeth, about the size of a man’s hand, at prices from $60 to $100 apiece. They say other fossils are available, at higher prices. “Ocucaje yields many bones,” said one merchant, Marcos Conde, 35.

...Meanwhile, seizures of illegally obtained fossils are increasing, surpassing 2,200 this year, compared with about 800 last year, largely at Lima’s international airport, said José Apolín of the Ministry of Culture’s office of recovery. Sometimes officials stumble upon large fossils by chance; in 2008 the police found a jawbone thought to be that of a mastodon in the cargo hold of a bus.

Recent discoveries elsewhere in Peru are raising interest in the country’s fossils and the potential for more trafficking. Almost 14,000 feet high in the Andes, for instance, a mining company controlled by Australian and Swiss investors announced a startling discovery last year: more than 100 dinosaur footprints embedded in walls of stone.

Rodolfo Salas, paleontology curator at Lima’s Natural History Museum, said evidence that his institution obtained, including photos of fossils for sale by private dealers, showed that the Ocucaje was especially vulnerable. He said the trade was supported by huaqueros, or looters of archaeological sites, who turned to fossil hunting."

Promoting a cooperative atmosphere between the mining companies would require some "give and take" between both entities. As for smaller fossils, e.g., shark's teeth, etc., it is harder to police, especially considering the impoverished conditions of the area. $60 to $100 for a large shark's tooth is going to make things difficult as to getting the peasants "on board" in preservation efforts.

Preserving sharks' teeth are not the most important issue here, as long as scientists can collect a wide array of teeth to determine the number of species present and if there are any new ones. The primary concern is of the preservation of vertebrate skeletons. Usually after the death of a vertebrate organism, scavengers and predators tend to disarticulate (scatter) the bones, so complete (or even partial) skeletons are rare. Another consideration, looters don't have time to excavate complete skeletons, so they often steal the skull. That has happened here in the United States, also. Even among well-meaning collectors from past decades, not being able to collect the skeleton, they made off with the skulls, leaving future vertebrate paleontologists to wonder "Where is the skull?", at the time of discovery of the skeletons. The skull is vitally important in the identification process and once the skull have been removed from a site without proper documentation of the location, much of the scientific value has been lost.

[At another time, I will offer opinions on the "professional versus amateur" collector issues as it relates to fossils and archeology.]

What a Geologist Sees - Part 17 [Original Post Date 3/25/08]

I am generally a "mountains and desert" person, but when I do go to the beach, I always like to get some photos of ripple marks and other "sedimentary structures".

Ripple marks and the raindrop impressions are actually termed "external sedimentary structures", as this is what we see when we see the top of a sedimentary layer.

I have seen numerous examples of ripple marks preserved in the "rock record" in sandstones, but I have not seem them combined with draindrop impressions. Usually, when we see symmetrical ripples in a sandstone, it is indicative of a beach environment, where the ripples and raindrops can be obscured by the next storm tides or the next heavy rains, while the sand was still soft and unconsolidated.

I have seen ripples with rain-drop impressions in red siltstones (finer-grained than the sandstones), which were deposited in a tidal flat setting. Sometimes in those cases, after the exposed layer had dried and hardened somewhat, it was covered over and preserved by the next layer.

[So while I may look at the pretty girls, I am taking photos of clouds, waves, ripple marks, and sand dunes, so as not to be labeled a "dirty old man".]

What a Geologist Sees - Part 16 [Original Post Date 2/28/08]

Before you conclude that I have completely lost my mind, it is an old toaster, circa 1930s.

I included this photo in this series to show the sheets of muscovite mica behind the center electrodes. This illustrates one of the uses of muscovite, as an electrical insulator. You can also find mica used in the newer, vertical-type toasters.

[While showing this to one of the teachers at my junior college campus, he remarked that there is an organization devoted to collecting old toasters. I asked him whether the toaster collectors got "toasted" during their social events at their shows.]

In general, if you have a rock with small, aligned mica flakes, it is probably a metamorphic rock, such as a schist. If the mica flakes are larger than 3/4 inch across, it is probably an igneous rock.

Because of the "flaky" nature of mica, large pieces are known as "books" and they usually occur in irregular igneous intrusions called "pegmatites". Mica books 5 feet in diameter have been mined in Georgia pegmatites in the past. If memory serves me correctly, mica has been mined in the Ball Ground, Thomaston, and La Grange areas of Georgia, as well as several places in western North Carolina. Pegmatites also include many other interesting minerals.

Synthetic mica has been in production for more than 50 years, making mining mica less of a necessity.

To Any Visitors...a Reminder

If you have happened across this blog, I am in the process of copying my science (mostly Geology) posts from my original, eclectic blog. This is being done for those who wish not to delve into politics or for those that disagree with my politics. Whatever.

I considered reposting using the original dates in the post options, but that would give the impression of this blog being older than it is (this is just an idea I had a couple of days ago).

I don't have time to blog everyday, as in addition to my part-time Junior College teaching position, I have a full-time online job with a consulting firm that serves the oil and gas drilling industry. The purpose of quickly posting all of the existing Geololgy posts is to allow me to write new ones when time and the spirit move me. On my original blog, the "What a Geologist Sees" series is up to Part 33, with 34 in progess.

Saturday, December 11, 2010

What a Geologist Sees - Part 15 [Original Post Date 2/11/08]

Inclusions and xenoliths. [OK, ya'll. Next time you play Scrabble, if you wind up with an "x", if you can cobble together "xenolith", that should score a lot of points.]

When there are individual components of an igneous or sedimentary rock that markedly-contrast with the matrix, the inquiring mind wants to know - where did that (or those) come from? When the contrasting piece is within an igneous rock, it is a xenolith, as highlighted in previous What a Geologist Sees posts 3 and 14. When the contrasting piece occurs within a sedimentary rock, it is an inclusion.

With the upper slide, I neglected to mark the locality, but I think it is the Eagle Mts. in West Texas. The matrix is on the right, while the xenolith (composed of inclusions) is on the left. The lens cap shows the scale of the inclusions within the breccia boulder that was incorporated into pyroclastic flow. The breccia boulder is older than the pyroclastic matrix and the breccia clasts (pieces) are older than the breccia boulder itself. The pyroclastic ash flows produce the welded ash flow tuffs when the ash flow movement ends. [That is for a separate post.]

The lower photo shows conglomeratic sediments. Each pebble came from a rock older than the conglomerate to which it now belongs. In this case, all of the quartzite pebbles in this conglomerate seem to be from a similar source, but sometimes some conglomerates (with different source rocks) can be quite colorful.

[Before going further, a breccia is composed of angular gravel-sized pieces of rock. A conglomerate is composed of rounded gravel-sized pieces of rock.]

Most conglomerates are associated with old river-channel deposits, the conglomerate in the lower photo represents old Chattahoochee River deposits, out of the current river channel.

Because of their angular corners and edges, breccias are generally found near where they are formed. The rock that produced the angular pieces can be broken by a number of different processes including landslides, faulting, meteor impact, and volcanic explosions (the probable source of the breccia in the upper photo, as this photo was taken inside of an old caldera-type volcano, where explosions would be the rule, rather than the exception).

As stated before, both examples illustrate the Concept of Inclusions that originated with James Hutton and Charles Lyell as one method of determining a relative timeline, without knowing the absolute age of any of the events. With the upper photo, there are at least four events recorded; 1) The brecciation (breakage) of the rock fragments within the boulder; 2) The cementation of the angular breccia fragments; 3) The separation of the boulder from its original locality; and 4) Inclusion of the boulder in the pyroclastic ash flow.

As part of another timeline, the stretched, brown pumice fragments existed before the pyroclastic flow, so they are also xenoliths, of a sort.

What a Scientist Sees - Part 1 [Original Post Date 2/27/08]

Part of being a scientist is about being observant of one's surroundings, so from time-to-time I may include things out of the realm of geology.

Photography of mushrooms (and things related) has been a hobby of mine for a while. When I walked into our backyard a few days ago, I noticed this "yellow stuff" on some lichen-encrusted tree limbs on the ground.

Checking one of my reference books, I found a photo of "Witches Butter", another name for which is yellow brain fungus. Supposedly it is edible and I believe I saw some reference to Witches Butter Soup. But of course, don't use my identifications as an OK to go ahead and eat anything that looks like this.

The name sounds like something out of a Harry Potter movie, perhaps a potion ingredient.

A Ptiny Pterodactyl! [Original Post Date 2/12/08]

One never knows what one will find at Free Republic.

While web/blog surfing today, I found a post on the discovery of a tiny Jurassic pterosaur, in northeastern China in the western part of the Liaoning Province. The wingspan of the young critter, Nemicolopterus crypticus, was about 10 inches.

The original Live Science article suggests that the ptiny pterosaur (which may have been a juvenile), was of the a same "family tree" as Quetzalcoatlus, which boasted a wingspan of more than 30 feet. BTW, Quetzacoatlus was first discovered in Texas, as one might expect.

What a Geologist Sees - Part 14 [Original Post Date 2/08/08]

As a follow-up to Part 13, the upper photo is of a broken "volcanic bomb" from Kilbourne's Hole maar volcano, showing the mantle xenolith within. The average mineral grain-size within the xenolith is approximately 2 mm in diameter. The rock peridotite is generally composed of olivine and a pyroxene, in the case of Kilbourne's Hole, I think the pyroxene is enstatite. The composition of the peridotite xenolith defines it as being "ultramafic". The basalt that encloses the xenolith is defined as "mafic". The ultramafic minerals are the first to solidify in a cooling magma, followed by the mafic minerals, which include olivine and pyroxene, as well as some amphiboles and calcium plagioclase feldspars. The slow rate of cooling is responsible for the visible grain sizes in these xenoliths contrasted with the more rapid rate of cooling of the enclosing basalt lava.

In the few places in the world where these types of xenoliths have been erupted, the xenoliths provide a little insight as to the mineral composition of the upper mantle. In both cases shown here, the matrix or host rock is a basalt.

One other mantle xenolith locality in the western United States is San Carlos, AZ, on the Apache reservation east of Phoenix. The photo below is of a sample of the San Carlos xenoliths.

In "What a Geologist Sees - Part 3", I described the concept of "Inclusions", which originated with James Hutton and Charles Lyell. Inclusions, such as these xenoliths, are older than the rock itself, i.e., the solidification of the xenoliths had already taken place when they were included in the rising magma body.

What a Geologist Sees - Part 13 [Original Post Date 2/02/08]

One of the many interesting geologic features of the greater El Paso area is Kilbourne's Hole, located approximately 30 miles WNW of El Paso, on the La Mesa Surface (a plateau), in southern Doña Ana County, NM. This area lies within the Rio Grande Rift and is included in the Potrillo Volcanic Field, which covers approximately 500 square miles. Some of the northernmost Potrillo cinder cones are visible on the southside of I-10, west of Las Cruces, NM.

Kilbourne's Hole is a low-relief, asymmetrical volcanic crater referred to as a "maar" volcano. It measures approximately 1.7 miles by 1 mile (see the first link for an aerial photo). A short distance south lies a smaller maar, Hunt's Hole. The two maar volcanoes lie within the southern end of a small, triangular-shaped graben basin, bounded on the west by the Robledo Fault and on the east by the Fitzgerald Fault (the two faults converge near the Mexican border). To the north-northeast of Kilbourne's Hole lie the Gardner Cones and the Afton Basalts. The distal ends of some of the Afton flows are visible in the background of the upper photo and covered at least a portion of what is now Kilbourne's Hole, before Kilbourne's Hole was formed. Due north of Kilbourne's Hole lie the Aden Basalts (where I did my Master's Thesis) and Aden Crater, a small shield volcano at the NW "corner" of the Aden Basalts. [Most of the Aden Basalts were erupted from "fissure eruptions" and are termed "flood basalts.]

For years, the origins of Kilbourne's Hole and Hunt's Hole were the subject of much speculation, including their being meteor-impact craters. The formation of a maar in the Philippines in the 1960s provided the needed answers.

Evidence suggests that Kilbourne's Hole formed as the result of repeated, large steam explosions, probably over the course of a few weeks to a few months.

The Aden and Afton Basalts were erupted onto the surface of the Camp Rice Formation, which includes river sediments from the prior meandering of the Rio Grande River during the Pliocene and Pleistocene Epochs of the late Tertiary Period/early Quaternary Period. When buried, river gravels and sand may serve as aquifers and it is believed that as a rising magma body contacted a shallow sand aquifer and the resulting steam explosions produced the crater. [These explosions are termed "phreato-magmatic explosions".]

Normally, basaltic eruptions like those in Hawaii and Iceland, do not produce much ash, unless there are phreatomagmatic explosions. When the explosions take place, the steam explosions pulverize the erupting lava before it can reach the vent, producing ash that is composed of minute particles of volcanic glass, rock fragments, crystals, and dust (each ash eruption will have different percentages of these components).

With some maar eruptions, a pyroclastic "tuff ring" of ash forms around the margins of the maar. The deposition of the tuff ring is probably at least partially controlled by prevailing winds during the explosive events. The ash itself is deposited as a pyroclastic "base surge" from; 1) The direct lateral components of explosive events; and 2) As lateral deposits around the maar after the collapse of the vertical ash column, i.e., after the explosions, some of the ash remains in the troposphere as "pyrocumulus" clouds and the rest collapses and spreads as it reachs the ground.

The undulating "cross-bedding" of the ash, seen in the photos, is produced by the formation and migration of ash dunes and ripples during and shortly after the eruptive events. The tuff ring is deposited on top of the Afton Basalt flows and exposures of the Camp Rice Formation.

Also produced by the explosions are ejecta, aka "volcanic bombs". In the case of Kilbourne's Hole, the volcanic bombs are sometimes cored by "mantle xenoliths", i.e., pieces of the Earth's mantle (carried in the magma/lava) that were already solidified before they reached the surface. When broken open, these mantle xenoliths reveal their composition of olivine and enstatite. When the olivine grains are big enough and of the proper color and clarity, they are termed "peridot" and some of the Kilbourne's Hole xenoliths have produced good-quality peridot.

[I will include a photo of one of these xenoliths in an upcoming "What a Geologist Sees" post. I wish I had collected more of these volcanic bombs when I lived in the El Paso area and I wish I had taken more slides of Kilbourne's Hole while I was working to the north in the Aden Basalts. I did collect some different types of volcanic bombs from within Aden Crater and from another maar to the west, Riley Maar.]

[I am not sure if there has been a Master's Thesis done on an analysis of the tuff rings and the distribution of the volcanic bombs at Kilbourne's Hole.]

All of the features within the Aden-Afton graben are geologically young, less than 2 million years in age, erupted during the Pleistocene Epoch of the Quaternary Period. These volcanic features and the others of the Potrillo Volcanic Field are related to the crustal thinning of the Rio Grande Rift, a crustal feature which is thought to extend from (approximately) the Big Bend area of Texas/Mexico northward into south-central Colorado (some interpretations may vary).

What a Geologist Sees - Part 12 [Original Post Date 1/29/08]

Just a brief lesson on water well construction and aquifers and other stuff like that.

The upper photo is of a poorly constructed/ poorly maintained residential drinking water well, probably on the order of 120-150 feet deep. It was located on the inner Coastal Plain, a few miles south of Augusta, GA, in Burke County.

Though we (the Ga. Geologic Survey Tritium Project) never subjected this particular well to a detailed examination by geophysical logging, whereby several devices are lowered into the well to do a variety of tests, it probably "bottomed out" and was drawing water from the Late Eocene Utley Limestone Member of the Clinchfield Formation or perhaps the overlying Griffins Landing Member of the Dry Branch Formation (also a limestone or a limey clay in places) or the Irwinton Sand Member of the Dry Branch Formation. Overlying the Dry Branch Formation was the Late Eocene Tobacco Road Sand and that is what makes up the sandy soils seen surrounding the wellhead.

There are two areas of concern regarding the construction of this well. The first involves the apparent lack of a "grout seal". When a drill rig drills a borehole, because of the sloughing of sediments, the borehole is rarely ever a perfect cylinder. To prevent the sloughing of sediments that would fill the well, usually well casing is installed consisting of Schedule 40 or Schedule 80 (for deeper wells) PVC pipe that is from 4 to 8 inches in diameter. With this well, the rust on the exposed casing suggests a steel or cast iron casing (it has been almost 10 years since I worked this area).

With the installation of the casing, the area between the borehole and the casing is called the "annular space" and it must be properly sealed to prevent surface pollutants from reaching the aquifer. Around the "screen zone", where the water enters the well casing, coarse sand (sand pack or gravel pack) is usually introduced to keep the water flowing from the aquifer into screen zone. Above the "gravel pack" is where the first seal should be applied. Below the water table, pellets of bentonite clay are introduced by way of a small pipe from the surface. Once wet, the bentonite pellets will swell and seal the annular space, if properly applied. Above the water table and all the way to the surface, a thin slurry of concrete seals the rest of the annular space. At there surface, the well owner is supposed to pour a concrete surface pad to act as the first line of defense against surface pollutants reaching the aquifer.

That is how it should be. A closer look at the surface area (amid the debris) surrounding the wellhead (in the upper photo) suggests that the area slopes inward towards the well casing. This is not good. This suggests that rainwater has been washing the sandy soil into the ungrouted (or poorly grouted) annular space. Along with the sand, whatever else might be on the ground surface (chicken poop, dog poop, etc.) is susceptible to being washed down the annular space, possibly reaching the aquifer if there is no grouting at all.

If you are ever in the position of buying a piece of property and the wellhead looks like this, at least get the water tested by the county health department or walk away from the deal. Due Diligence applies to things more than land titles and termites.

In the photo below, these three GGS monitoring wells (one mile or so from the above well) exhibit properly installed surface pads to top off the annular space grouting. [There are three wells drilled into three separate aquifers at different depths to test for tritium among other things. Above background (but below EPA MCL) levels of tritium were found in the shallow aquifer well (the same one serving the well in the upper photo). One goal of our study was to make sure that similar levels of tritum had not reached the deeper aquifers (which it had not).

If the residential well (drilled to serve a couple of trailers) had at least a surface pad, that would have offered some protection from surface pollutants.

What a Geologist Sees - Part 11 [Original Post Date 1/24/08]

[Click on image to enlarge.]

As a teacher, I would call this a "Bonanza slab", as there are several types of fossils on a single limestone slab. It illustrates the concept of biodiversity (or species diversity) in a Sub-Tropical shallow continental shelf setting or perhaps a shallow carbonate platform setting.

This limestone slab is from the Silurian Rockwood Formation exposed west of Chattanooga, TN, off I-24.

Among the larger fossils are brachiopods, rugose corals, tabulate corals, bryozoa, and trilobite fragments. There are fragments of smaller fossils, such as stalked echinoderms.

According to a professor at Univ. of Tennessee - Chattanooga, the Rockwood Fm. is an offshore equivalent of the Red Mountain Formation, which consists of red shales, siltstones, and sandstones deposited in tidal flat and delta environments. The Red Mountain Formation was deposited due to the erosion of the Taconic Highlands, which lay to the present day east and southeast. The Taconic Highlands, uplifted during the Late Ordovician Period represented the first uplift of what would become (much later) the Appalachian Mountains.

What a Geologist Sees - The Series Thusfar [Original Post Date 11/29/07]

[This post was designed to allow readers to peruse a list of subjects covered in the series, to that point.]

Thusfar, subjects covered include:

Part 1 - Grand Canyon, sedimentary layers, unconformities.
Part 2 - Colorado Plateau, sedimentary layers, unconformities.
Part 3 - Georgia Piedmont, igneous geology, granite, xenoliths.
Part 4 - Georgia Piedmont, river gravels, topography changes.
Part 5 - Georgia Piedmont, alluvial fans, sediments.
Part 6 - Southeast New Mexico, oil wells, drill rigs.
Part 7 - Georgia Piedmont, diabase igneous dikes.
Part 8 - Diagnostic Mineral Characteristics - cleavage.
Part 9 - Georgia Piedmont, saprolite vs. fresh metamorphic & igneous rocks.
Part 10 - Georgia Piedmont, river gravels, topography changes.
Part 10B - Georgia Piedmont, river gravels, topography changes.

I plan for more. As the Georgia Piedmont is my "backyard", I will not ignore it, but I plan to present some other locales and concepts, too.

What a Geologist Sees - Part 10B [Original Post Date 11/29/07]

More river gravels.

I used to think river gravels were boring stuff (as compared to collecting fossils or neat mineral specimens) until I learned more about old river terraces (floodplain remnants) and exposed gravels on hilltops and hillsides. In these settings, the gravels tell us about how much a long-existing river can migrate laterally over time.

These gravels are on the opposite side of the peninsula from the old raceway grandstands. The peninsula lies within an incised meander of the Chattahoochee River and the old river channel cuts across the hill. Meanders are usually present in areas of low-gradient, usually on coastal plains, closer to the ocean. That it is an incised meander suggests that the meander system was locally established (as just described), then with a rapid drop in base-level, down-cutting preserved the meanders.

Where there are sedimentary rocks in the drainage basin, sometimes you can find petrified (permineralized) wood in river gravels. I used to find small rounded pieces of petrified wood in some of the old Rio Grande gravels west of El Paso.

I hope to return to the park to check out the large chunk of rock pictured just below the tree roots. From the surrounding river pebbles, you can see that this chunk is far larger than the rest, suggesting that "there is more to this story". It would take a great deal more energy to move this size rock, as opposed to the adjacent pebbles. In the exposed gravels along the edges of this peninsula, I don't recall having seen other clasts this large.

There are several possibilities.

[1] When an ocean-going iceberg melts, it drops any cobbles & boulders held within to the seafloor. We call these "erratics" or "glacial erratics", when there is an out-of-place large chunk of rock within smaller-grained sediments. During a prior ice age, might there have been a chunk of ice in the ancestral Chattahoochee that dropped this larger chunk of rock?

[2] Another possibility is that this chunk fell into the river from a nearby (now eroded) bluff or rolled down a slope into the river.

[3] Another more interesting possiblity is that this is a chunk of permineralized wood, i.e., a chunk of wood that was buried in the river gravels and mud and was permineralized by silica in the groundwater after burial. I considered that when I shot the photo, but sundown was approaching and the park was about to close, so I didn't remove it from its position to more closely observe the characteristics. A chunk of wood that large would be light enough to be moved along with the gravels, prior to its permineralization.

If it was permineralized wood, "woodn't" that be neat! [Sorry.] As that sort of thing is not that common on the Piedmont, I would take some more photos before disloding it from the vertical face. Something like that might be worthy of a short scientific paper, perhaps if any internal cell walls are preserved, a paleo-botanist might be able to determine the genus of the wood, based upon the internal structures.

[4] One more possibility is that on the nearby exposed shoreline, amid the weathered, saprolitized metamorphic rocks, there are areas of hard quartzite. This chunk of rock could simply be a chunk of quartzite that was "ripped" from the river floor (you can see the scoured bottom below the gravels) by a flood and "rolled" to its present location. It is easier for fast-moving water to roll an object than it is for it to carry it.

[There, without your realizing it, I have "walked you through" the concept of "Multiple Working Hypotheses".]

I have proposed four possible ways that this large chunk of rock (maximum dimension is perhaps 1.5 feet) came to be placed within these smaller pebbles in the old river bottom. There could be other possibilities that I have not yet considered. Speaking from my bias as a field scientist, this is why we learn to "brainstorm" with other scientists and when properly trained, we engage our own imaginations. If the chunk of rock is not permineralized wood, then that Hypothesis is discarded and the others subjected to greater scrutiny. That is the way science works. And because of the ravages of time and erosion, there may not be a single "best" answer.

Unlike lawyers and politicians, we scientists understand that our initial opinion may not be the correct one. That is why power-grabbing politicians want to restrict scientific discussions and healthy skepticism, as it gets in the way of their desires for a nice, neat, controlled situation.

What a Geologist Sees - Part 10 [Original Post Date 11/24/07]

[A follow-up to the last post.] Up the hill from the old raceway grandstands - in a small cliff that normally marks the edge of the peninsula at Laurel Park - is this outcrop of old river gravels overlying saprolite, similar to what I wrote about in "What a Geologist Sees - Part 4". The previous examples of old river gravels over saprolite are on the west side of Duluth, also associated with the Chattahoochee River.

To refresh your memory, saprolite is the heavily-weathered "rotten rock" that has had its structural integrity destroyed by mineral dissolution (actually conversion to clays by hydrolysis). The term is usually used to describe former igneous and metamorphic rocks and traces of the orginal textures and structures must be discernible, otherwise we call it "residuum" - in this area, that would be Georgia red clay.

This hillside exposure of the old Chattahoochee River gravels overlying saprolite is an example of a "topographic inversion", i.e., this used to be the bottom of the river, in the lowest part of the valley. Now it is on a low hilltop, perhaps 1/4 to 3/8 of a mile from the river channel (prior to the filling of Lake Lanier in the late 1950s). A rough guess might be that these gravels are probably some 30-40 feet above the now-covered river bed.

On the opposite side of the peninsula, these river gravels have been eroded and the exposed shoreline is covered with the rounded pebbles and cobbles. I am considering doing a little gold panning, from sand trapped in dips in the exposed bedrock along the shoreline, as the Chattahoochee drainage basin includes Dukes Creek (near Helen, GA), which is a former gold-producing area. The idea is just to satisfy my curiosity. There are areas in which old river gravels have been mined for gold.

Again, I bring this up as a way to pique someone's curiosity. Most people don't give a thought to the river ever being anywhere but besides its current floodplain. What did the land look like before that 30-40 (maybe more) feet of vertical down-cutting took place, along with the lateral migration of the river? Questions such as these might arouse a kid's interest in science. In walking the drought-exposed shoreline, I am sure that numerous people every day see the gravels overlying the weathered bedrock, but I doubt that it really "sinks in" as to what it really means.

[I am so easily entertained.]

What a Geologist Sees - Part 9 [Original Post Date 9/26/07]

The photo at top is of an embankment of saprolite, which is colloquially called "rotten rock". This embankment was exposed during a short period of local construction. Saprolite is the term used to describe exposures which display some original rock structures, but because of intense chemical weathering, the internal structures of most of the minerals have been broken down to the point where the "rock" can be crumbled in your hand - thus the term "rotten rock". The processes that yield saprolite are what produce the inorganic components of soils.

If the exposure has been reduced to a structureless clay, e.g., the famous Georgia red clay, we call this "residuum".

The lower photo is of the original "country rock", a fresh, hard biotite gneiss, a metamorphic rock that is similar to granite. This particular gneiss (silent "g", rhymes with "rice"), probably part of the widespread Lithonia Gneiss, is composed of potassium (orthoclase) feldspar, plagioclase feldspar, quartz, biotite mica and probably muscovite mica, along with some accessory and trace minerals.

The progression from gneiss to saprolite takes place through chemical reactions involving water and natural acids over long periods of time. In the process of hydrolysis, hydrogen cations replace the metallic ions (K, Na, Ca) that occur in the mineral structures of feldspars, micas, and other silicate minerals. The removed metallic ions then become part of the dissolved minerals in ground water (and surface waters) and are flushed out of the system. Any iron bearing minerals, including silicates are altered by oxidation, in addition to hydrolysis. There are other chemical weathering processes, but these two are probably most prevalent.

At or near the surface, quartz is generally fairly stable, so probably most of the original quartz remains, while the feldspars and micas have broken down into various clays.

In addition to the chemical weathering, whereby the actual chemistry of the minerals change, there is also physical weathering, where the rock is broken into smaller pieces by natural processes, yielding more surface area upon which the chemical processes can occur, i.e., both types can occur simultaneously and tend to enhance one another.

It is a little unusual, in this area, to see the fresh parent rock and the resulting saprolite in the small area of a stand-alone drug store construction site.

On a side issue, if there is enough quartz present (as opposed to clays) and there is enough porosity and permeability (hydraulic conductivity), saprolites can sustain small, residential drinking water wells, in this area.

What a Geologist Sees - Part 8 [Original Post Date 9/19/07]

Despite all of the fancy lab equipment these days, it is still necessary for a geologist to often make field identifications of minerals and rocks.

As mineral color may be affected by impurities (trace elements), one of the diagnostics that we use for mineral identification is "mineral cleavage".

Cleavage is the characteristic by which when a mineral breaks, it leaves behind a flat surface. The number of cleavage directions and the quality of the cleavage (on any and all of these cleavage directions) is affected by the internal structure of the mineral. Perfect cleavage in a mineral is because of inherent planes of weakness within the crystal structure (lattice). Micas are examples of minerals with one direction of perfect cleavage and cleavage surfaces are parallel to each other, on opposite sides of a mineral specimen.

Various minerals can have 1, 2, 3, 4, even 6 directions of cleavage (as in Sphalerite - the zinc, iron sulfide mined on the Gore family property in Tennessee). In minerals with more than one direction of cleavage, the angles between the cleavage planes are important.

In the examples above, we have three common minerals, all of which may be transparent to translucent and colorless in the absence of impurities. One way of distinguishing between them is to recognize the differences in cleavage (as explained in the photo). Halite (table salt) breaks into almost perfect cubes with 90 degree angles between all three directions of cleavage - which we call "cubic cleavage". Galena, a heavy, silvery lead sulfide also has cubic cleavage.

Calcite has three directions of cleavage, but none of them are at 90 degree angles to the others. This we call "rhombic cleavage".

Selenite (gypsum) occurs in several forms, in this clear crystalline form, it's one strong direction of cleavage causes it to break into thin, flat sheets, while a secondary, weaker direction of cleavage affects the shape of the specimen margins. [Hint: Selenite is one of only two common minerals that are softer than your fingernail, i.e., you can scratch it.] Differing degrees of Hardness (resistance to scratching) can be used also as a diagnostic tool. [That may be covered in a future post.]

If a broken surface is not smooth enough to classify as a cleavage surface, then it is termed as "fracture". Quartz is an important mineral that does not have cleavage (some suggest it has a very weak cleavage, but for the sake of simplicity, we will stick with the concept of no cleavage for quartz). Pyrite, olivine (peridot) and garnets are examples of other minerals with no cleavage. Some minerals, especially in the "quartz family" exhibit a distinctive type of fracture - "concoidinal fracture", the curving type of fracture that you see in glass. The fracture link explains some of the other types of mineral fracture.

Geology students commonly study individual minerals first, in hand-size samples (if possible) to become familiar with the cleavage and other characteristics of the minerals, before we start studying minerals as smaller components of rocks. Usually we study igneous rocks after minerals, as igneous rocks are the original source of most minerals.

Cleavage can have a larger impact than some imagine. When flat, platy minerals such as micas are present in certain rocks, especially metamorphic rocks, if they are aligned with each other, they can form zones of weakness, which can affect the way rocks break. Knowing the cleavage characteristics in a diamond is important as it affects how a larger diamond can be split into to numerous smaller diamonds for faceting purposes.

What a Geologist Thinks About...[Original Post Date 9/15/07]

often while reading or hearing a news report on a "man vs. nature" issue, is - "Why is this happening?". From The Seattle Times, by way of WND - there is a story on serious coastal erosion problems on the northern side of Willapa Bay along the southwest Washington State Pacific Coastline (see inset map associated with the article). Though there are some explanations within the article, we realize that there may be more than one reason why this is happening.

From the Seattle Times article:

..."This two miles of shoreline at the northern confluence of the Pacific Ocean and Willapa Bay, 12 miles south of Westport, is believed to be the fastest-eroding beach on the Pacific Coast. It has lost about 65 feet a year to the sea since the late 1800s. More than 100 homes, including the entire town of North Cove, have already disappeared, many of them in the past 20 years."...

Shorelines are unquestionably active - deposition takes place in some areas, while erosion takes place in other areas. The first question would be - "Is there something humans have done to make this natural process worse?" Can we estimate how long this rate of erosion has been taking place?

Sometimes along shorelines, erosion control efforts, e.g., jetties, groins, seawalls,...may slow erosion in their intended area, while making it worse somewhere else. And it may take decades to see the results. We have to look carefully before spending additional millions of taxpayer dollars, when any "fix" may be only temporary.

One of my cousins rents a beach house every summer at St. Simons Island, GA. It didn't used to be a beach house. Up until the late 1950s/early 1960s, there used to be another row of cottages along the shoreline. The loss of this row of cottages was not the result of ongoing regional shoreline migration, but the effects of a couple of hurricanes. [Yes, before 1970, there was another "active hurricane cycle", which may have started in the 1940s.]

As for the Pacific Coast in SW Washington State:

..."George Kaminsky, a coastal engineer for the state Department of Ecology who has studied the erosion since 1993, thinks it has become "self-feeding."

Sand from the Columbia River built up a sandbar at the mouth of Willapa Bay, channeling the water flowing out of the bay straight into Cape Shoalwater. And as the cape eroded, the sand built up the sandbar even more.

Pinpointing the original trigger is difficult, Kaminsky said. But man-made jetties likely have halted a natural sand migration that could reverse the erosion."...

This local interruption of sand migration is a form of "sand starvation", i.e., a normal supply of sand might have protected the shoreline.

And though this Seattle Times article doesn't address this possibility, there could be a larger "sand starvation" issue along the coastline. This Wikipedia map shows more than 2 dozen dams in the Columbia River drainage basin (watershed). Prior to the construction of these dams, the down-river perpetual movement of sand, as "bedload" and as "suspended load" (during storms), kept an ample supply of loose sand available for distribution at the "whims" of longshore currents.

Each one of the dams on this map disrupts the normal river transport of sand, ultimately resulting in there being less sand reaching the Pacific Ocean than there was prior to the construction of the first dam. The cumulative effects of the existence of these dams (for decades) is that there is "sand starvation" along portions of the coastline.

This is a problem that takes decades to reveal itself and other than the "deep ecologist" dream of dynamiting everyone one of these dams, there is probably not an easy solution. The dams are a source of relatively clean energy and the environmental disruptions associated with building the dams has already taken place. To remove the dams (or open them permanently) would again represent a series of environmental disruptions. Besides, it would take decades for the sand supply to "resume" it pre-dam transport pattern.

Bad Water in Bangladesh... [Original Post Date 9/06/07]

is the subject of this Moonbattery post, which references this UNESCO report, and this American Thinker article. It is about naturally-occurring arsenic contamination of shallow groundwater in Bangladesh. [At least seven years ago, Dr. Seth Rose, Georgia State University Geology Dept (and a damned nice guy) gave a talk before the Atlanta Geological Society about this very subject.]

For a variety of reasons, Bangladesh is desperately poor. From the memory of Dr. Rose's talk, though the UN "takes it on the chin" in the Moonbattery post, he suggested that the British Geologic Survey may have had some culpability in telling the Bangladesh government that "it was OK" for people to install the bamboo "tubewells", which are physically pushed into the shallow ground, to reach the ground water. Below a clay layer, there is a deeper aquifer, with cleaner water, but it is too deep for the bamboo tubewells. Because of the widespread poverty, it is difficult to access drill rigs to drill the deeper wells.

It is my understanding that arsenic is relatively easy to test for and according one of the articles, it is not that hard to remove. But for some reason, the UN has sat on this information for some 30 years, while the "little brown people" of Bangladesh were progressively poisoned. If you are not familiar with the basics of arsenic toxicity, it is possible to build up a tolerance for arsenic, but in large enough amounts, the results include chronic illnesses and perhaps some forms of cancer. This could be a reason for Bangladesh's inability to rise above its improverished conditions (because most of its people are sick, most of the time.)

Can you imagine the world reaction if Union Carbide (or Exxon/Mobil) had been responsible? The presence of arsenic is not due to industrial pollution, but to the natural conditions of weathering of arsenic-rich rocks and erosion and deposition.

So what is it? Just "governmental" dithering by the UN or is it a form of population control?

[Update: The UNESCO article is quite informative and it explains the geological reasons for the arsenic in the water. As for who can help Bangladesh, as Bangladesh is mostly Muslim, have any of their brethren nations offered help in drilling new water wells? I guess we could ask Senator Patty Murry - "Now that Osama bin Ladin is no longer building roads and schools in Afghanistan, is he going to help Bangladesh?".]

Friday, December 10, 2010

Beer as a Geological Side-Interest

As beer is a favored beverage of Geologists, this is the initiation of a post that will be completed time permits.

BTW, in our local chapter of the BCCA (Brewery Collectibles Club of America), we have 3 Geologists and one Meteorologist.

To be brief, we will divide the "Beer Era" in the United States into the 1) Colonial origins to mid-1970s (Phase 1); and 2) The Craft-Brewing Era (Phase 2).

Of a side interest in my collecting of beer cans and related items of "breweriana" are those items that; 1) Have Geological or Geographical-related names, e.g., Oxbow Beer, Rocky Mountain Beer, Obsidian Stout; or 2) Have Geological or Geographical-related images on their labels; or both.

A Geologist/beer can collector could also highlight cans found during Geological Field Work. For me, that has been rather sparse.

Photos are planned for inclusion in this post, later.

What a Geologist Sees - Part 7 [Original Post Date 9/12/07]

To many geologists, rock quarries are playgrounds, especially if there is more than one type of rock present. Unfortunately, because of today's litigious society, it is difficult to gain access to quarries and to do it without the constraints of a tour group is even more difficult.

My Dad and I initially visited this local "road gravel" quarry about 1975, when you could just drive in there on a Sunday and as long as you stayed away from the machinery and the vertical quarry walls, you were OK.

By the time I went back to do some field work for my "undergrad thesis", during the Summer of 1976, I had to beg for a release form and then I was only allowed about 50 minutes, covering two different visits. Nowadays, the quarry is a very popular place for tours (every Thursday) and because of the popularity with school groups, you have to make reservations about 6 months in advance and for some reason, they don't allow photographs. Hmm. [This photo was taken in 1976.]

The light-colored rock is a metamorphosed granite, called a "gneiss" and it consists of quartz, two different feldspars, and two different micas, along with a host of minor accessory and trace minerals. When rocks are metamorphosed over a large area, this is called "regional metamorphism".

The black colored rock is a diabase dike. Diabase is a dark-colored, fine-grained igneous rock, similar to basalts that one sees in lava flows in Hawaii, Iceland, New Mexico, Idaho, and elsewhere. It is largely composed of calcium plagioclase feldspar, pyroxene, and olivine.

An igneous dike is a tabular (flat) body of rock that intrudes and cuts-across pre-existing rock and local structures. A tabular body of igneous rock that is parallel to local structures is a sill.

The relationship between the two rock bodies is termed a "cross-cutting relationship", wherein the dike cuts-across the pre-existing rock, thus the dike is the younger rock, even if we do not know the absolute (radiometric) ages of either rock body. In the 1700's, James Hutton recognized this concept.

When this dike (about 3 feet wide) was intruded into the gneiss "country rock" (presumably related to the fracturing of the crust during the rifting of Pangea), the heat of the intrusion triggered some minor changes to the gneiss through "contact metamorphism". This was a "dry intrusion", i.e., it didn't contain much pressurized water, so the "zone of contact metamorphism" in the gneiss - adjacent to the dike - is only about 5 - 6 inches wide. Pressurized water helps ions move around, triggering more mineral changes.

So, with the small width of the intrusion and the paucity of pressurized water, the changes to the gneiss were rather minor. These diabase dikes are common in the Piedmont of Georgia, South Carolina and North Carolina. They can range in width from 3 inches wide in this quarry to more than 1,000 feet in South Carolina. Virtually all of them are oriented NW-SE, cutting across the rest of the regional geologic structures. They are all presumed to have been intruded during the Triassic and Jurassic Periods of the Mesozoic Era (the age of the dinosaurs).

What a Geologist Sees - Part 6 [Original Post Date 9/06/07]

At right is a scanned slide that I took during a UTEP Geology field trip 25 years ago. We were somewhere in Southeastern New Mexico.

An environmentalist would probably "have a cow" over the sight of a large oil drilling rig, complain about the environmental damage, and begin assessing the carbon footprint of the oil from this well, if it had discovered oil.

This type of rig, for its time, was the largest of the land-based drill rigs, capable of reaching depths of some 30,000 feet, the depths of the deepest U.S. gas wells in the Anadarko Basin (western Oklahoma and adjacent states). Though deeper depths might be possible, generally the geothermal gradient is too high (it gets too hot), making the metals in the drill bits too soft and causing the drilling mud to start to boil. With that type of heat, generally any petroleum products have been "cooked away", leaving mostly carbon dioxide and/or carbon residue.

So, when a geologist sees a drilling rig, we see jobs for geologists, engineers, roughnecks, roustabouts, truck drivers, surveyors, machinists,... We see tax revenues. We think of the drill crews that go 8 hour shifts - 24/7 until the well is done. Even when it is 11 degrees and blowing snow on Christmas night in Oklahoma. We see risk-taking investors. We hope they find something, while understanding that they might not.

In this particular case, this was a dry hole, a "duster". By the time all was said and done, there was probably a couple of million dollars (at least) dropped on this dry hole. Someone's gamble didn't pay off. Any well generates some geologic data and you find out "where the oil ain't". At this time (1982), we were taught that wildcat wells (those at least a quarter mile from a producing reservoir) had a success rate of 10%! Nine of ten failed to produce.

I believe with modern technology - this figure is now about 50%. This is what we have to do in order to foster a vibrant economy which will produce the energy-saving technologies of the future. In other words, we have to use more energy now in order to save energy later. Conservation is important, but starving our economy of energy will not bear good fruit.

What a Geologist Sees - Part 5 [Original Post Date 8/27/07]

Sometimes we can learn about large geomorphic (land-shape) features by observing small analogs. In this case, by observing small-scale erosion and deposition at a construction site, we can see how alluvial fans are formed (and then later eroded).

When rain water moves in a uniform flow across an inclined surface, we call that sheetwash. Sometimes small surface irregularities concentrate the flow into small rivulets which may erode small rills and when rivulets combine, they may produce gulleys, as has happened at this construction site.

Differences in soil compaction on this slope may have facilitated the gulley erosion seen in these three gulley examples. Though these gulleys are short, they still illustrate how sediments are carried in flood conditions. While the storm water is in the channel (in this case the gulley), it is confined by the walls of the channel (or in a larger example, the walls of a canyon), which keeps the water velocity higher. When the flood waters leave the confinement of the gulley (or canyon), the loss of lateral confinement results in the rapid slowing of the water and the deposition of most sediments carried by the waters. Over time, when unrestricted by the presence of vegetation, this builds a fan-shaped deposit at the mouth of the gulley (or canyon).

Of the three gulleys in this photo, the one of the left has the best preserved alluvial fan. The middle gulley probably had a similar-sized alluvial fan at one time, but it was "dissected" (eroded) by subsequent waters from the largest gulley (on the right).

Generally, alluvial fan growth in the Appalachians is hindered by the presence of trees and other vegetation during the ongoing erosion of the mountains. If alluvial fans are present, their visibility is further hindered by said trees and vegetation. Also, ongoing, routine rain events tend to erode the alluvial fans, which are generally formed by short-term, high-intensity storm events.

The more arid Western United States is a much better place to view alluvial fans, large and small. Subsequent photos (in later posts) will show more examples of alluvial fans. The largest alluvial fans are generally at the mouths of mountain canyons, where the canyons empty into broad valleys, particularly in the Basin and Range Province. In some western cities, e.g., El Paso and Albuquerque, large alluvial fans may be favored sites for building fancy homes - with a good view of the broad valley below - without regard as to how the alluvial fans formed. It is not generally an issue until the occurrence of a 100-year flood, a 500-year flood, or especially a 1000-year flood.

Thus if you can afford a "million dollar view" of a valley and behind you lies the mouth of a mountain canyon, just give it a little thought before you sign the papers.

What a Geologist Sees - Part 4 [Original Post Date 8/19/07]

Have you ever noticed rounded river pebbles, on a hill-top or on some sort of plateau? Not just a few scattered pebbles, related to landscaping, but widespread occurrences and hillside exposures of seemingly intermixed soil and pebbles? On top of a hill?

How far did your sense of curiosity take you? Have you considered that "rivers move" over time? I am sure that many non-scientists have learned a little about how rivers meander and migrate, vis-à-vis the Mississippi and other rivers with broad valleys. But how often do we stop and think about the hilly terrain around us and how it changes over time?

This particular Topozone map shows the area in which both of these exposures were found. Notice the present location of the Chattahoochee River, approximately 1/2 mile north and west of this portion of Peachtree Industrial Boulevard in NW Gwinnett County, GA.

The upper photo area (Site 1) is now covered by a housing development. The site was approximately 200 yards due North of the intersection of Peachtree Industrial Blvd. and Abbotts Bridge Road, which crosses the Chattachoochee River. The present day Chattahoochee River channel is approximately 40 - 50 feet below the elevation shown at the highway intersection. From the lower left quadrant of the map, where Peachtree Industrial Blvd. enters the map, I have traced gravels for about 2/3 of its extent on this map, on both sides of the highway, to about the midway point between "Industrial" and "Blvd" notations on the map.

Exposures can be on eroded hillsides, in roadside ditches, creek valleys, and in construction sites. Because of present-day development of the area, it is difficult to trace the gravels for a greater distance to the southwest along P'tree Industrial Blvd, off of this particular linked map.

The lower photo area (Site 2) can be found in a small creek valley adjacent to Peachtree Industrial Blvd, probably 1/4 mile from Site 1. If you look at the linked map (if it works), in the lower left quadrant of the map, you will notice a series of long, parallel buildings (a storage area). Site 2 is across the small creek and up a side-valley. There are other exposures of river gravels overlying saprolite along this creek, directly across from the storage area site.

[While on the subject, saprolite can be described as "rotten rock", i.e., rock that has been so totally chemically-weathered, it has lost all of its structural integrity and can be crumbled by hand, though you can still see structures and textures in the outcrop.]

In both cases, the upper surface of the saprolite was once the eroded bottom of the river, which was then covered by the river sands and gravels. The contact is easily seen in the Site 1 photo, while at Site 2, the contact is shown by the dashed line. At one point, this essentially was "the lowest point in the valley". The present-day course of the river was, at that time, upland areas that had not yet been eroded by the lateral migrations of the river, as it also cut into the Piedmont soils and saprolite.

[In the Site 1 photo - "Poorly-sorted" refers to the wide variety of grain sizes, ranging from clay size to coarse-gravel sized particles. This is normal for mature rivers in this particular setting. The "Wentworth Scale" is one way of classifying grain sizes. If you have been to the ocean (or have seen sand dunes), where the sand particles are essentially all the same size, that is classified as "well-sorted".]

At the margin of the lower left quadrant of the map, where PIB "leaves the map" is one of our favorite local Mexican restaurants. If I am sitting in the restaurant and point out to those at the table that river gravels underlie the restaurant and all that is within sight, they might find that interesting, for the moment, but they will never see the fascination with trying to understand the nuances of "where the river used to be" versus where it is now. And other than the momentary "Wow, I didn't know that.", more thought will not be given to the subject.

That is why geologists are notorious for "talking shop" when we get together at parties and such. Cause almost no one else finds this stuff fascinating. Maybe that is why we become more eccentric as we get older (or maybe as our brains petrify). Maybe this is why we talk to ourselves (aside from teachers "practicing" their lectures).

There can be instances where tracing old river channels (and related sediments and sedimentary rocks) - in the subsurface - can be useful. Sometimes there can be mineralized zones associated with old river channels (which once-covered) served as conduits for the mineralized fluids. Sometimes the porosity and permeability of the sediments/sedimentary rocks may make them suitable for aquifers or oil reservoirs. If such river sediments lay beneath a proposed landfill, the sediments might serve as a conduit for leachate (landfill leakage) to reach an aquifer, thus rendering the site unsuitable for landfill use.

[Oops, I did it again. Yammered on for too long.]