Friday, March 6, 2015

Geo-Tutorial I - A Brief Minerals & Rocks Primer


Student geologists generally study (and learn to identify) individual minerals first, before we learn to recognize them as components of rocks.

After we gain a working knowledge of minerals, we usually study igneous rocks second, as those are the original sources of most minerals.

Minerals are naturally occurring, inorganic compounds that are solid at normal atmospheric temperatures. They have orderly internal structures, defined characteristics, and a defined composition (or range of compositions due to ionic substitution), with a few exceptions.

Some minerals are elemental, i.e., the consist of a single element, e.g., diamond - but most are compounds consisting of one or more cations (positive ions) and one or more anions (negative ions).

We classify minerals by their anions, e.g., minerals related to pyrite (FeS2) are called sulfides, usually the anion is S or S2. These can include iron copper sulfides (chalcopyrite), silver sulfides, copper sulfides (cuprite), zinc sulfides (sphalerite), lead sulfide (galena). When sulfur is present in the formative stages of these minerals, whether in the original molten form, or in high-temperature, high-pressure mineralized water - called hydrothermal solutions, and if there are a variety of metals present, it is not unusual to find several different sulfide minerals together in the same ore body. This is the nature of the "massive sulfides" that were mined in the Ducktown, TN area and elsewhere.

The nature of the chemical bond between the cation(s) and the anion affect its chemical and physical and characteristics.

The four minerals in the rock above are all included in the Silicates class, wherein the anion consists of a silicon ion and three or four oxygen ions, which act as a single ion when bonded with a cation. Quartz is chemically an oxide (SiO2), but structurally, it is related to the other silicates. The vast majority of important "rock forming minerals" are silicates. Silicate-dominated igneous rocks range from the dense, iron-rich basalts found in Hawaii to the lighter, quartz/feldspar-rich granites that one sees at Yosemite National Park or Stone Mt., GA.

The term "rock" is a little less precise, as a rock is an aggregate of one or more minerals. We use texture (crystal sizes, mineral ratios, and overall composition) to classify igneous rocks. The rock pictured here is a piece of granitic igneous rock with a pegmatitic (large-crystals) texture and it contains four identifiable minerals. This particular specimen was collected along the Appalachian Trail on the north side of Springer Mt., GA. The biotite mica measures about 1-inch across, horizontally.

The presence of potassium feldspar and quartz identify the rock as granitic, while the relatively large crystal sizes identify the rock as being "pegmatitic". The large grain (or crystal) sizes of this rock are the key to this identification. Pegmatites are irregularly-shaped igneous bodies that fill fracture zones and because of the significant presence of pressurized water in the magma, larger crystals form (in a pegmatite in South Dakota, the lithium mineral Spodumene occurs in crystals 40 feet long). In a former pegmatite mine in Georgia, "books" of muscovite mica 5-feet across were mined.

In igneous rocks, the key to crystal size is the rate of cooling and the quantity of pressurized water. Molten lavas at the surface cool relatively quickly because of their exposure to the atmosphere, thus their crystals are generally small, if they are visible at all. If a lava contains some large crystals, these crystals were already solidified before the lava was erupted. Obsidian forms when a lava flow enters a lake, river, or the ocean and the lava is chilled. The ions are present, but there wasn't time for the mineral bonds to form, resulting in the formation of volcanic glass.

Molten magmas, below the surface solidify more slowly, resulting in larger crystals, especially when more water is present. The crystals of the Elberton Granite, northeast of Athens, GA generally measure 1 - 2 mm and the estimated cooling time for the granite to solidify was 1 million years (laboratory experiments with high-pressure furnaces provide some of this information). In a molten magma, there is a defined progression in which minerals crystallize, as the magma cools, different minerals solidify. The Bowen Reaction Series shows the temperature range in which certain major silicate minerals solidify. Quartz is the last major mineral to solidify and the first to melt. In the above sample, the order is biotite, potassium feldspar, muscovite, and quartz.

Generally, minerals that form as well-defined crystals do so because they have "room to grow", i.e., they are the earlier minerals to solidify in a magma (or lava) so they can grow within the remaining molten material or they have a cavity (a fracture, a gas-bubble, or other void) into which to grow. The later minerals solidify, the less room there is to allow crystal growth.

Thursday, March 5, 2015

A Few of My Favorite (Geology) Things - Part 1


My academic work and employment have covered a number of different branches of Geology.  This particular vignette falls within the science of Paleontology - the study of fossils. Studying fossils such as the Class Echinoidea (at left) can also extend into the study of layered rocks and the study of ancient ecosystems.

Echinoids compose a particular class within the Phylum Echinodermata.  Echinoderms, in broad sense include Asteroids (starfish), Crinoids (sea lillies, sea feathers), Echinoids (sand dollars, sea urchins), etc., and some extinct, weird critters such as Blastoids and Cystoids.

They are characterized by a pentameral symmetry, shown in the five "petals" of the fossil sand dollar at left. The echinoids are divided into Regular Echinoids (sea urchins, pencil urchins) and Irregular Echinoids (sand dollars, sea biscuits, sea cookies, and heart urchins). And they are one of my geo-hobbies".  I have been lucky enough to have had a couple of good collecting sites in my field mapping areas in SW Georgia.

The Regular Echinoids appeared in the fossil record during the Ordovician Period.  Generally the Paleozoic urchins are discovered as crushed specimens with some of their spines and plates present. Carefully cleaned and restored examples appear on eBay from time to time. Many of these types come from the Pennsylvanian shales in the Brownwood, TX area.  The Irregular Echinoids (heart urchins) began appearing in the fossil record during the Mesozoic Era, in the Jurassic Period.  The sand dollars began appearing in the fossil record during the latest part of the Cretaceous Period or early Tertiary Period.

The specimen above Periarchus pileussinensis is one that I collected from the Late Eocene Tivola  Limestone (approx. 40 million years old) in the old Medusa Cement Company quarry near Clinchfield, Houston County, Georgia, on the inner Coastal Plain. It was during one of my undergraduate field trips, probably in 1973 or 1974.  The Eocene Epoch (56 to 34 million years ago) of the Tertiary Period was a time of global warmth (palm trees in Alaska, crocodiles in the Dakotas), high sea levels, and great echinoid biodiversity.

Late Eocene sedimentary rock units on the Georgia Coastal Plain include the Clinchfield Sand, the Tivola Limestone, the Twiggs Clay, the Ocmulgee Formation, the Sandersville Limestone, and others, extending from south of Augusta southwestward to the Bainbridge, GA area. Much of the Florida Peninsula is underlain by Eocene limestones, such as the Ocala Limestone and in many surface exposures of the Ocala Limestone, echinoid collectors find their idea of heaven.

The Periarchus genus is found in North Carolina, Georgia, Alabama, Florida, and Mississippi and was present from the earliest Late Eocene until the end of the Eocene, when the genus, along with others, became extinct, perhaps due to the impact of the "Chesapeake Bolide".

In Georgia, the preceding species to P. pileussinensis was the Periarchus lyelli in the Clinchfield Sand (and elsewhere).  Also in Georgia, the next succeeding species in the lineage was Periarchus quinquefarius in the Sandersville Limestone.  Most of the Periarchus specimens found in Georgia measure about 3.5 to 4 inches in diameter, the size of the modern day Atlantic Coast sand dollars.   The smaller Protoscutella in the older Middle Eocene of the Carolinas, Alabama, and Mississippi may represent an ancestor to Periarchus, though most Protoscutella specimens are generally 1/2 to 1/3 the diameter of Periarchus.  To my knowledge, Protoscutella has not yet been found in Georgia (though I have tried).  And to my knowledge Protoscutella and Periarchus have not been found in the same layers.  When there are multiple fossil-bearing layers present in an area, Periarchus is always found in units younger than Protoscutella.

[Another time, I will post more photos of some of my Favorite Things.]

What a Geologist Sees - Part 1

No, I don't need glasses... nor a therapist, just 'cause I see stuff like this when I look at Grand Canyon photographs.  We  geologists are afflicted in this manner.  So just politely say "that's nice" to humor us.  The photo is from the south rim of the Grand Canyon (maybe from Hopi Point or somewhere near there).

Normal people are not expected to remember all of these terms, so I will explain a few of them so next time you visit the Grand Canyon (or the first time), even if you can't remember the specifics, you can revel in what geologists call "deep time". [Seriously, if you spend a little time reading articles on the internet before visiting these National Parks, you will enjoy them more.] 

Above the Inner Gorge, virtually all of the rocks are sedimentary rocks, the earliest of which were deposited on a continental shelf and when the bulk of the continent was flooded by a shallow inland (epeiric) sea, the rest of them were deposited, though there is a 100 million-year-long gap in the layers (more on that later).

In the geologic science of Stratigraphy (the study of layered rocks), when there is an interruption of the geologic record, represented by an erosion surface, we call these "Unconformities, of which there are four types; 1) Nonconformities; 2) Angular Unconformities; 3) Disconformities; and 4) Paraconformities. Types 1, 2, & 3 are visible in this Grand Canyon photo.  On the left side of the photo, an arrow points to a "Nonconformity", where the Cambrian-age Tapeats Sandstone overlies the Proterozoic Brahma Schist (metamorphic rocks). To the right of the "Normal fault", the Brahma Schist is overlain by the Proterozoic Bass Limestone (also a Nonconformity here), the reddish-colored Hakatai Shale, and the cliff-forming Shinumo Sandstone, all of which are very old sedimentary rocks.  The Brahma Schist, the Vishnu Schist and the Zoroaster Granite represent the first of two mountain ranges that were formed, then eroded away before the Cambrian Tapeats Sandstone was deposited approximately 540 million years ago.

After these mountains were weathered and eroded, the rising seas covered the area and deposited a thick sequence of sedimentary and volcanic rocks, called the "Grand Canyon Supergroup".  These rock layers were then "block faulted" (tilted, but not folded), yielding the second mountain range, which was then eroded prior to the Cambrian Period sea-level rise. When horizontal sedimentary rocks overlie older, tilted sedimentary rocks, the erosion surface between them is called an "Angular Unconformity". Where that particular arrow is pointed, the Hakatai Shale (and the cliffs of the Shinumo Sandstone) represented an island surrounded by the Tapeats Sandstone. When sea level continued to rise and the shoreline moved toward the present-day southeast, the Bright Angel Shale was deposited over the exposed Shinumo Sandstone. If you enlarge the image, you may be able to see the layers of the Hakatai Shale tilting to the right (in relation to the horizontal Bright Angel Shale above). 

The Grand Canyon is one of the sites referred to as "The Great Unconformity" due to the gap between the Proterozoic Grand Canyon Supergroup and the Lower/Middle Cambrian Tonto Group (Tapeats Sandstone and Bright Angel Shale). The Franklin Mountains, within and north of El Paso, TX represent another exposure of this 500+ million year gap, between the underlying Proterozoic igneous, metamorphic, and sedimentary rocks and the overlying Middle Cambrian Bliss Sandstone and Silurian Fusselman Dolomite.

As sea level continued to rise (for millions of years) eventually flooding most of the continent, the Muav Limestone was deposited on top of the Bright Angel Shale. These three oldest sedimentary rocks units compose the "Tonto Group" and represent a continual rise of sea level that can be traced to 100 miles SE of El Paso, TX.  After the Muav Limestone was deposited, there was an interruption in the sequence, sea level dropped (and/or the land rose). The next time the sea covered the area and left layers behind was 100 million years later, with the Redwall Limestone, deposited during the Mississippian Period.  The eroded surface between the Muav Limestone and the Redwall Limestone is a "Disconformity" and rocks representing the entire Ordovician, Silurian, and Devonian Periods are missing. [There are some Devonian rocks elsewhere in the Grand Canyon.]

The coverage of the area by the sea that deposited the Redwall Limestone happened very rapidly. Limestones are generally deposited in shallow, tropical waters at a distance from any large landmasses. Normally, when sea level rises gradually, the sequence is sandstone overlain by shale overlain by limestone - telling you of the gradually deepening waters.

[The remaining layers of the Grand Canyon section of the Colorado Plateau (above the Redwall Limestone) can be described another time.]

I hope this little geology lesson hasn't put you to sleep. The Grand Canyon is a fascinating place as it is unusual to see so much geologic time represented in such a small area. The oldest metamorphic rocks in the Inner Gorge are about 1.7 billion years old. The oldest horizontal layer is about 540 million years old and the youngest horizontal layer (in this photo) is the Permian Kaibab Limestone, deposited about 240 million years ago, so the horizontal layers in the Grand Canyon represent about 300 million years (estimated) of geologic time, with 100 million years missing (remember the Disconformity).  The canyon itself is thought to be only about 5 to 10 million years old. Why the entire Colorado Plateau underwent this rapid uplift, but remained essentially horizontal and largely un-deformed and largely un-faulted is something of a mystery to geologists and the source of many lively discussions. [Yeah, I left out some details, otherwise this post would be ten paragraphs longer.]

[This "What a Geologist Sees" series is being presented in a chronologically-reverse order, the opposite of how it was originally posted in the retired blog.  There might be a gap or two due to an editing error.]

What a Geolgist Sees - Part 2




This scanned-slide was taken during my UTEP Geology Summer Field Camp field trip 30+ years ago. I don't remember exactly where it was, but it is probably in the vicinity of Canyonlands National Park, near Moab, UT.

This is to further illustrate the definition of an "Angular Unconformity" and while we are "at it", the concept of  "Superposition", and "Slope-forming" vs. "Cliff-forming" sedimentary rock units.

To address "Superposition" first, that is the concept that when observing a sequence of layered rocks (usually sedimentary), if the sequence has not be severly deformed, then the oldest layers are at the bottom and the youngest are at the top. Perfectly logical when you think about it, but it was a big deal when conceived by Nicolas Steno in the 1600s.

Another of Steno's principles was "Original Horizontality", i.e., when originally deposited virtually all sedimentary rocks are horizontal.  So when we look at the above photo, we can see that the reddish-colored layers are slightly inclined to the right and are overlain by the horizontal grayish-colored layers. [It may be a little more complex than this, but as I don't remember the exact location, I will keep it simple.] 

In reconstructing past events, the reddish-colored shales (which form slopes in dry climates) were deposited, then the area underwent some minor geologic deformation, which resulted in the gently tilting shown here. Then after some weathering and erosion of the reddish layers, the gray layers were deposited above the eroded surface (the Angular Unconformity).

Above the gray shales are the cliff-forming layers, probably siltstones and sandstones in this case. In dry climates, limestones can form cliffs, too, but these are likely siltstones and sandstones. The differences in colors relate to the amounts of iron in the sediments and the oxygen conditions in the environment at the time of deposition, as related to atmospheric exposure in a river delta or floodplain setting. Generally, the more oxygen there is, the redder the sediments are. [Again, I am leaving out some details for the sake of brevity.]  So using these concepts, without knowing exactly which rock units (and ages) are present here, at least we can define some of the events that led to the particular landform that we see in this photo.

My Geologist "Wish List" (Perhaps a "Geo-Bucket List")

or at least part of it. [Yeah, this kind of stuff is only interesting to fellow geologists. I am trolling for more geology readers.] Maybe someday I can mark off at least a few of these things.

I may have alluded elsewhere to a few of thing things I still would like to see or do, relating to Geology (and other sciences).

As for collecting things, I would like the opportunity at some point to collect:

A decent zircon crystal, a decent topaz crystal, a complete trilobite, and a complete ammonite. A crinoid calyx with at least a portion of the column would be nice, too.

As for the minerals, I seek not gem-quality, but rather specimen quality, to be able to show kids what a topaz crystal (or at least a piece of topaz looks like, likewise with the zircon. I have collected a diamond in Arkansas; aquamarine in New Hampshire; green and pink tourmaline in Maine; golden beryl in Georgia; rhodonite, rhodochrosite, and huebnerite in Colorado; rutile and platinum in Georgia, yada, yada.

As for things I would like to see (and photograph) (taking my family, if they want into some of the back-country areas, in a 4 X 4):

Monument Valley, AZ/UT.  My parents went through there in the summer of 1980 and my Dad passed away in November of 1980.  I scanned some of his slides last year and for some of the Monument Valley photos, as I don't know the exact location and orientation of the photograph view, I do not know which mesas and buttes I am looking at.  Some of them I have been able to ID from the internet, but some are given different names by different photographers. I would like to be able to identify them myself. I would also like to take in the magnificent scenery seen in so many John Ford movies.  [Update: Made two brief visits during June and July, 2015.   A more thorough visit was planned for July, but heavy rains prevented exploring the "back country".]

Grants, New Mexico basalt flows.  As you drive on I-40 between Albuquerque and the Arizona line, in the Grants, NM area, there are basalt flows that seem to have "rolled up" to the edge of the freeway, when in fact, the flows were already there. As these flows are geologically young, in an arid climate, their features are well-preserved. I would just like to stop and get some close-up photos of these flows.  [Update: Made a brief photo-stop during July, 2015.]

The Jemez Mountains, NM.  In the Los Alamos, New Mexico area, there is a young, caldera-type volcano, which I visited on a field trip in 1985, but I misplaced my field notebook, so a few of my slides have left me wondering "what is that?". I would like to collect from some of the volcanic ash deposits along the roadway and get some more examples of pumice.

The Davis Mountains, Texas.  I visited the Davis Mountains several times as a UTEP grad student, but again, some of my slides are not labeled and I cannot identify the particular volcanic unit names nor the exact locations. You have to learn that your memory will not last forever. To a geology student, a neat photograph only has so much value if the subject is not well-defined.

Vinton Canyon, Franklin Mts., Texas.  I visited this locality several times as a grad student and it is a good locality for collecting Pennsylvanian-aged fossils and on the gradual slopes further to the west of the canyon's mouth, there are places to collect Permian-aged fossils. I would like to photograph the localities and re-identify which limestone and shale formations they are (this stuff contributes to the scientific value of fossils). And maybe collect a few more fossils.

The Eagle Mountains, Texas. I spent 10 weeks in the Eagle Mountains (a large caldera-type volcano) during the summer of 1978 and revisited the area in 1979 or 1980 for a weekend. I shot hundreds of good slides while there, but for a few of them, I can't remember exactly where in the mountains I was (of course I want to be there with topo maps. My field area was in a specific part of the mountains, which I remember well, it was when I ventured into other areas that my memory fails me. I would also like a few more close-up shots of some of the volcanic textures. There is also an area where volcanic ash and other eruptive debris was washed into a small lake and solidified as siltstone layers.  I would like to go back and walk the margins of those deposits and collect some more samples.  It is not often that you see sedimentary rocks deposited inside of a volcano.   Ideally, I would like to write a short article about this unusual occurrence.

Yellowstone National Park.  When I was there in 1974, I had an Instamatic camera and an undergrad's understanding of what I was seeing (which was very little).

The Snake River Plateau, Idaho.  Same 1974 trip, same deal.

Yosemite National Park.  Same 1974 trip, same deal.

Clayton-Raton Volcanic Field, NE New Mexico.

Sunset Crater, Flagstaff, AZ area.  Have been through the area several times, just haven't visited.  [Update: Visited area for a few hours in Mid-July, 2015.]

Central Wisconsin. While traveling the area in 1982, I forgot to reel in my completed roll of 35 mm film back before opening the back of the camera (Do'h!), whereby I lost all of my slides of glacial features.  My brewery slides were on other rolls, but I lost all of the geology stuff.

Aden/Afton Basalts, southern New Mexico.  I did my Master's Thesis in these young volcanics and I became sick of the "sameness" of the flows, while I was mapping some unusual craters.  I shot hundreds of slides and dozens of print photos. There are some other features that I saw, but did not photograph in other parts of the area, i.e., other volcanic features that one might have to otherwise see in Hawaii.  As they were not directly related to my field work, I said I would "get them later".

I guess the best thing I can do is pass along to current geology students (and others interested in nature photography/study) is - make good notes and label your photos/specimens. Despite your passion, you won't remember all of these details 10 or 20 years from now.

Hawaii.  Need I say more?

Mason, Texas area.  Topaz.

Calvert Cliffs, Maryland.  Fossils.

Coker Creek, Tennessee.  Gold Panning.

Arches National Park, Moab, UT area.  I have been to Arches NP twice, in 1977 and 1979.  Both times my camera shutter jammed, leaving me with zero slides.

Vicksburg, MS loess.   Just to touch it, just to feel it and its texture.  [Update:  Did this Late-July, 2015.]

To be continued...

What a Geologist Sees - Part 3

When molten rock is below the Earth's surface, we call it "magma". When it reaches the surface, depending on its mode of eruption, we call it "lava" or "pyroclastics".

Below the Earth's surface, magma bodies rise through the crust due to their bouyancy. In the lower part of the crust, because the crust is hot and "plastic" (pliable), magma bodies push aside and melt their way upwards.

When the magma bodies reach the shallower sub-surface, the existing rock is cooler and more brittle. This is when the process of "stoping" (rhymes with "roping") takes place. The rising magma body slowly fractures and forces aside the host rock(s). Often during this process, fragments of the host rock will break loose and become incorporated into the magma. If the magma is not hot enough to completely melt the incorporated fragment, it is preserved as a "Xenolith".

In the case of this xenolith, it is fairly small, with a maximum dimension of about 5 inches. The host rock is the Elberton Granite, from Elbert County, GA and the xenolith is a biotite gneiss.

The presence of the xenolith is referred to as an "inclusion" and using the Concept of Inclusions as articulated by James Hutton and Charles Lyell, without knowing the radiometric ages of either rock, we know that the xenolith is older than the host rock, as the xenolith was solid when it "fell into" the molten granite. [This is a brief glimpse into the concept of "Relative Age Dating", wherein we seek to establish a sequence of events without knowing radiometric ages.]

[As a further aside, at any give time, most magmas are probably not 100% molten, but rather a mixture of solidified minerals and still-molten minerals. The darker-colored minerals generally crystalize first, as the magma gradually cools. This progression is presented in the Bowen Reaction Series.]

When visible due to difference in color, sometimes individual crystals "xenocrysts" can be preserved in lava flows and solidified magmas ("plutons" or intrusions), having been "plucked" from older rocks as the lava/magma moved through.

[If nothing else, I have given you several new words to use in playing "Scrabble". "Xenolith" or "Xenocryst" would be worth a lot of points.]

Granites, being intrusive igneous rocks, only become "available" for study at the surface by way of faulting and/or erosion, as they form thousands of feet below the surface. The Stone Mountain Granite, of similar age to the Elberton Granite, is estimated to have been 10,000 feet below the surface when it solidified 300 to 325 million years ago. BTW, there are xenoliths exposed, in various places, on the weathered surface of Stone Mountain.

So, when you walk past a stone building facade, if you see a distinctly different mass of rock surrounded by "matrix", this may well be a xenolith.

What a Geologist Sees - Part 4



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".]

Now if I try to explain this to local residents, i.e., if I point out to someone that river gravels underlie this local area 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.]

What a Geologist Sees - Part 5


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 gullies, as has happened at this construction site.

Differences in soil compaction on this slope may have facilitated the gully erosion seen in these three gulley examples. Though these gullies are short, they still illustrate how sediments are carried in flood conditions. While the storm water is in the channel (in this case the gully), 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 gullies in this photo, the one of the left has the best preserved alluvial fan. The middle gully 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 6

At right is a scanned slide that I took during a UTEP Geology field trip 30+ 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.

The Ducktown, TN Desert - Part I


[In 2007, as my son wanted to ride in a car with a working air conditioner (and he was tired from Scout camp), he declined the offer to roam around southeastern Tennessee and rode home with the mom of a fellow Scout. Sometimes he does like to get out and explore, but not this time.]

To the above-left is an aerial photo of a portion of the Ducktown, TN area, probably from the 1940s. The photo is part of the Ducktown Basin Copper Museum in southeastern TN.

Nary a tree in sight, from acid rainfall from the local smelters and deforestation to provide fuel for the furnaces.  Not to discount the environmental destruction pictured here, you can see what Geomorphologists/ Hydrologists refer to as a "dendritic stream pattern". Normally in the eastern United States, these patterns are obscured by trees.

The acid rainfall destruction was caused by the "roasting" of the iron and copper sulfide ores (pyrite, chalcopyrite, pyrrhotite,...). The museum curator mentioned that, in this decades-long process, five feet of topsoil was lost. The heating of the sulfide ores released sulfur dioxide (SO2) which, when mixed with rainfall produced sulfur acids, probably sulfurous acid (H2SO3).

The destruction of the local vegetation (over a 50 square mile area) wasn't done on purpose, it was one of those human screw-ups where we didn't understand the future environmental impacts of our actions.

The Ducktown, TN Desert - Part II



[2007] The photo to the right was taken from the grounds of the old Burra Burra mine in Ducktown, now the site of the museum.  The name Burra Burra is from a famous Australian copper mine.

To the lower left, you can see a circular pit, the result of a partial mine collapse, circa 1925.  Beyond and to the right of the pit are the areas that have been left "fallow", i.e., reclaimed by nature with no human assistance. This is to offer the viewer a perspective of the damage that was done in the late 19th and early 20th centuries.

Beyond the fallow area, there is a fenceline, beyond which are the human-reforested areas, the results of a project that began in the early 1900s.  Also useful in the process was the modifications made to the roasting process, by which sulfur dioxide (SO2) was captured and converted to sulfur trioxide (SO3), then sold for the production of industrial sulfuric acid (H2SO4).

The area definitely looks better than it did when I first visited in early 1976, while on a Geology field trip. It still may be a few decades before hardwood trees are ready to grow on this reclaimed ground. 

The last of the Ducktown Basin copper mines closed in 1987.  This was largely due to the importation of copper from other countries and the increased expenses of operating here.  If you happen to be in the area, check for the date of the yearly "miners homecoming" music festival.

Friday, January 9, 2015

115 Years Later, They Just Call it "The Storm"...

[9/2007] ...in Galveston, Texas. No other descriptions are needed. That is why as Katrina was barreling through the Gulf of Mexico, in 2005, Galveston had a fleet of school buses gassed and ready to go, just in case, while New Orleans' fleet of school buses was awaiting being idle and then flooded. Galveston will probably always be ready, as much as is possible, anytime a hurricane moves westward through the Gulf of Mexico.

Saturday was the anniversary, though I didn't find this Galveston Storm website, until tonight, by way of this Blue Crab Boulevard post.

To provide the context that others miss, we need to remember the death toll of this storm, as recounted on Blue Crab Boulevard:

..."at least 6,000 in Galveston proper and possibly as many as 12,000 in the immediate region."

This was out of a Galveston population of approximately 37,000. Approximately 1 out of every 6 persons died. In contrast, the estimated death toll for the 1906 San Francisco earthquake was 3,000. Adding the death toll from the 1889 Johnstown, PA flood still does not equal the Galveston death toll.

The hurricane changed the course of Texas history. Prior to the hurricane, Galveston was a major economic powerhouse. After the hurricane, economic activity shifted inland to Houston, which was protected from the direct storm surge.

The story of the storm was recounted in the book "Isaac's Storm", named after the U.S. Weather Bureau meteorologist on duty in Galveston, Isaac Cline. The paperback edition of this book, by Erik Larson, was published by Random House, ISBN 0-375-70827-8.

From the Random House website, here is a portion of the narrative on the book:

"'An absurd delusion,' is how Isaac Cline, a dedicated and highly trained first-generation employee of the new U.S. Weather Bureau, characterized the fear that any hurricane posed a serious danger to the burgeoning city of Galveston, Texas. Based partly on Cline's expert opinion, Galveston dismissed a proposal to erect a seawall, claiming it a needless, wasteful expense."...

..."At the turn of the century, Galveston was booming. It was the nation's biggest cotton port, its third-busiest port overall, and the second-most-heavily-traversed entry for immigrants arriving from Europe, nicknamed the "Western Ellis Island." The city had more millionaires, street for street, than any other in America. The nation, too, was bursting at its borders with optimism and confidence."...

Not to minimize the death toll from Hurricane Katrina, but if you hear the characterization of Katrina as "the worst", please endeavor to politely remind the speaker of the 1900 Galveston hurricane, as the human death toll is a measure that should never be forgotten in favor of monetary damages. But to some, what is highlighted depends on the political benefits.

What a Geologist Sees - Part 7


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 8



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 - "conchoidinal 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 Sees - Part 9


The photo at top is of an embankment of saprolite, which is literally (and 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.

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 10 (During drought conditions in 2007)

At Laurel Park in Hall County, GA, 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 original 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 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 10B (During drought conditions in 2007)

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] A possibility is that this chunk of rock fell into the river from a nearby (now eroded) bluff or rolled down a slope into the river.

[2] 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 dislodging 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.

[3] 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 to carry it.

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

I have proposed three 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, scientists should understand that our initial opinion may not be the correct one.

Just to Give the Claustrophobes - Including Me - a Little Chill... (Late 2007)

This is my son crawling the "wild tour" of Cumberland Caverns, TN last year at age 12. [His Scoutmaster took the photo.]

I was bravely holding down a picnic table in the commons area of the caverns at the time. I know my limits. One of the dads that did the crawl admitted afterwards that he started to panic a little, but kept it under control.

On one of these tours, you cain't turn around and go back. It ain't like an outdoor hike where you can turn around and go back after 1/2 mile if you decide you ain't up for it. Or if your knee is getting gimpy.

So to all those that can do this, more power to you. Maybe because Cumberland was drier than Raccoon Mt., there was less slipping and banging of knees. My son said he preferred this dry "wild tour" of Cumberland Caverns to the muddier, more slippery Raccoon Mountain Caverns tour.

I guess I should have taken some crawling photos in Raccoon Mt., but when I am doing that and pushing a duffel bag, I am thinking "I want out", not "Gee, this is a Kodak moment".

What a Geologist Sees - Part 11



[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 Formation 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 - Part 12

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 Georgia Geologic Survey Tritium Project) never subjected this particular well to a detailed examination by geophysical logging, it probably "bottomed out" and was drawing water from one of several formations in the unconfined Upper Three Runs Aquifer.  These could include the Late Eocene Utley Limestone Member of the Clinchfield Formation, Griffins Landing Member of the Dry Branch Formation, 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 the 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, 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 tritium 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 13

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).

An Update...

I am reposting "geophotos" (with explanations) from another blog, which is slated for retirement next month. Scroll through as you wish.

What a Geologist Sees - Part 14


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 15


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 other posts in this series. When the contrasting piece occurs within a sedimentary rock, it is an inclusion.

With the upper slide, it is in 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.