Wednesday, January 26, 2011

What a Geologist Sees - Part 33 Heavy Minerals [Original Post Date 10/29/10]

Heavy minerals have several definitions;
1) Specific Gravity (density) greater than quartz (>2.65 times the density of water).
2) A relative resistance to chemical weathering, greater than that of most silicate “rock-forming” minerals (except quartz).
3) In their original host rocks, they usually constitute trace quantities.

Some “common” heavy minerals and their respective Specific Gravities (vs. Quartz 2.65) include:

Zircon 4.6 – 4.8
Monazite 4.6 – 5.4
Tourmaline 3.0 - 3.3
Rutile 4.2 – 4.3
Ilmenite 4.5 – 4.7
Magnetite 4.9 – 5.2
Staurolite 3.7 – 3.8
Kyanite 3.5 – 3.7
Garnet 3.56 – 4.32
Diamond 3.5
Gold 15.6 – 19.3
Platinum 14 - 19

The process of releasing heavy minerals begins with the chemical and physical weathering of a given rock unit. In this Lithonia Gneiss saprolite, there is quartz, clays, altered minerals, and heavy minerals.

When the broken-down rock materials (clasts) are moved down-gradient - usually by water (erosion) and gravity - they are separated by density during high-energy water movement (floods).

The dark metallic colors of ilmenite, rutile, and magnetite help make placer concentrations of heavy minerals more easily seen.

Variances in water velocity help concentrate heavy minerals into "placer" deposits. Close to the source areas for mafic rocks, heavy minerals can include olivine, pyroxenes, and amphiboles, but these are unstable and will undergo degradation to clays during transport.

In the settings of a small to medium-sized creek, areas where the water velocity slows, such as the inside of a meander or on the upstream side of a boulder or other obstruction allow for the deposition of heavy minerals. If one is interested in panning for gold, the scoured area at the foot of a small waterfall might be a favorable site, also.

Another place where heavy minerals accumulate is in beach sands. In the southeastern portion of the United States, the white quartz sand beaches do contain small amounts of locally-concentrated heavy minerals in discrete stringers. In areas where there is little separation between the crystalline rocks and the shoreline (as in the northeastern United States), the heavy minerals present are much closer to their original source area.

The dark areas of this Jersey City, NJ urban beach are enriched with heavy minerals derived from the weathering of igneous rocks in the area and the erosion of glacial sediments. This heavy mineral portion of sands include a healthy quantity of garnet fragments. Other beach deposits in the NYC area - especially Montauk Point, Long Island - are even more garnet-enriched.

What a Geologist Sees - Part 32 [Original Post Date 5/27/10]

[Subtitled: Geophotos, Memories, and Hopes]

Within my geophoto database, the photos that are among my favorites include those from the Eagle Mts. (West Texas), the Bisti Badlands (San Juan Co., NM), and Monument Valley (UT/AZ). [The labeled photos have been used in some of my classroom Power Point presentations.]

Yeah, I got a bit carried away going down memory lane.

The Eagle Mts. (an Oligocene caldera) were the site of my originally-intended Master's Thesis work, during the summer of 1978. The 1st photo here was taken from the East Mill area, where we camped while we mapped the southeastern portion of the mountains. In the near foreground is a portion of Wyche Ridge, composed of Cretaceous sedimentary rocks, forming part of the margin of the caldera.

Eagle Flat is in the middleground and the Carrizo Mountains are in the background (and maybe the Beech Mountains and/or the Sierra Diablo in the far background, too). Alamo Springs may be visible from this location, also.

This was one of the few close-up photos of the pyroclastic textures that I took that summer. I guess I planned to get more in future trips, but I got "distracted" by events in my personal life and never finished this project. (I did start another thesis project in 1985 in the Aden Volcanic Field). I would, love the opportunity to take my son 4-wheeling back in the Eagle Mts., - maybe someday. To enjoy the quiet and get a few more photos and maybe find that rock hammer that I lost.

The Bisti Badlands in San Juan County, NM were the site of a 1979 summer job. I was hired to assist in a "fossil recovery project", locating Cretaceous vertebrate, invertebrate, and permineralized wood samples, prior to the opening of a coal mine. During the early part of my six weeks there, I took hundreds of slides, then unbeknownst to me, the shutter on my Miranda camera jammed.

It rained almost every day the first two weeks we were there and the clays in the Fruitland Fm. are like grease when they get wet. After that first two weeks, I don't recall anymore rain for the remaining four weeks of the project.

The primary goal of our project was to mark the location of every dinosaur bone in two and a half square miles, recover all loose bone fragments, then leave the removal of large pieces to the University of New Mexico. (Sometimes when I talk about being a Geologist to a bunch of kids, I tell them about the summer I got paid to look for dinosaur bones, that usually catches their attention.)

We were supposed to continue this same project in the summer of 1980, but the permits between the state and federal land didn't get resolved in time. I would have enjoyed another go-round in this area.

It has been years since I read any reports generated by this project, but I seem to remember my lead professor telling us that most of the bones we found were of hadrosaurs (duck-billed dinos). We also found turtle shells fragments, crocodile scutes, and a few fresh-water bivalves (the ecosystem had been an Everglades sort of setting).

It was always fun to find one of these areas just littered with permineralized logs, though they were not generally as colorful as the wood from the Petrified Forest.

I hope this stump was retrieved for a museum or at least given a place of honor outside of a college classroom building.

The site where I collected this "clinker zone" shale (actually outside of our study area), with the plant fossils is one of those places that I regret not having collected more samples from. I only picked up two pieces and gave one away during the intervening years. I wish I had filled a bucket.

Monument Valley is a place I have not yet visited, but someday hope to. My geophotos from Monument Valley are scanned slides taken in the summer of 1980 - by my Dad - when he and my Mom were on their last vacation together. He passed away in November, 1980.

I use these photos, along with my photos of Canyonlands NP and the Grand Canyon when discussing Colorado Plateau stratigraphy...

...and when discussing arid-climate weathering and erosion characteristics...

...and when discussing things like eroded volcanic necks.

Other stops on that trip included the Painted Desert/Petrified Forest,...

...and Dinosaur National Monument,...


...and the SD Badlands.

My Dad was not a geologist, but he did enjoy learning about new things. I will forever be thankful that he got me interested in photography.

What a Geologist Sees - Part 31 [Original Post Date 5/16/10]

OK, maybe it was because I was raised with both cats and dogs. Or there may be other reasons for my eccentricities. That can be discussed at another time and place.

As for Geology, I am one of those strange creatures that dwells within both "Soft-Rock Geology" and "Hard-Rock Geology". And for that reason, I am regarded with some suspicion by the zealots in either of these two camps.

For the normal folks out there, "Soft-Rock Geology" this is not an analysis of the type of music that we like, but rather an informal division of Geology that includes the study of sedimentary rocks, fossils, stratigraphy, geomorphology, weathering and erosion, Earth history as revealed in the sedimentary rocks, petroleum-related issues, and so on.

"Hard-Rock Geology" is the study of igneous and metamorphic rocks, minerals, mineral economics, structural geology, plate tectonics, mass wasting, and so on.

I enjoy all aspects of Geology, thus I have always seen Geology as a buffet, of sorts. This is reflected in my coursework and various jobs. As I enjoy a wide range of Geology, I have had no desire to become an expert at anything, rather a learned student about different geo-disciplines. As an opportunity presents itself, I pick from the Geological Buffet. It might be fossils this time, metamorphic rocks next time, tracing old river terraces another.

It has been my experience that, the more interests one has, the less likely one is to become bored with a situation. This also extends beyond Geology to the hobbies (and other science interests) that we have, in my case, Photography, especially Scientific Photography. Having a wide range of interests, I generally am able to find something to do, if the weather is good while visiting an area. I am not as likely to go "stir crazy" as an igneous or metamorphic petrologist would, if confined to Mississippi or Florida or Kansas.

In other words, I have less difficulty in finding a way to entertain myself, geologically speaking. I used to find river gravels boring, until I started noticing them on hilltops and began to think about "how this came to be". If I happen to see old gravels a half-mile (or more) from a present-day river, that immediately piques my interest.

I used to consider sands to be a tedious subject, until I started looking at them under a microscope, to look beyond the dominant quartz in most samples, to the accessory and trace minerals and what they mean.

If I happen to be in a place where I have already "scoped out" the local geology, I can go back for a more detailed look, just to find something "new". It always helps to have done a little study beforehand, online or by way of various geological publications, whether they be from governmental entities or private organizations.

I just wish I could convey the notion to my teenaged son that - you are only bored if you allow yourself to be. I wish I could engender that fascination with learning that I have come to value. That is one of the most valuable tools I have picked up along the way in my geo-journey. There is almost always something new to see, even when I revisit the same patch of woods for the 10th time.

So, herein this rambling prattle has been my attempt to explain my wide-ranging interests in Geology. An attempt to explain the "Method to my madness".

Wednesday, January 19, 2011

The Okie Dokie Diamond [Original Post Date 3/13/06]

Many people may not be aware of it, but Murfreesboro, Arkansas is the only place in the entire world where common people can pay a small fee and search for diamonds and keep everything that they find. All other diamond producing areas are owned/controlled by large corporations or governments.

From MSNBC News, an Oklahoma State Trooper, visiting the Crater of Diamonds State Park with his family for the first time, found a 4.21 carat, canary yellow diamond that is said to be flawless. Talk about beginner's luck!

The park is approximately 37 acres of ground that is periodically plowed. Visitors are allowed to crawl about on their hands and knees, that is how I found my small white diamond (.37 carats) on my first visit in 1973. There are other areas where people can dig and sieve sand and gravel in water to look for diamonds.

When I was there in the spring of 1978, a couple from Dallas was looking in the same area as I was looking. I left in the mid-afternoon to do some other things in the area. At dusk, I was parked along the road into town, looking for old beer cans in the woods when the Dallas couple recognized my truck and pulled over. They asked me to take a look at what they found and to tell them if it was a diamond. It was a 4 carat, brown diamond, not of gem quality, but with the classic octahedral diamond shape. I got to hold it and I was the first one to confirm that it was a diamond (the park office had closed for the day). I later saw a short newspaper article in a Dallas paper, wherein that diamond was valued at $4,000 because of its size, classic crystal shape, and it being an American diamond. And that was 1978.

The article mentioned that 84 diamonds have been found so far this year. When I was first there in 1973, they said about 250 diamonds per year were found by visitors. Most of them are not gem quality, but once in a while, someone finds a "blockbuster" of a diamond, worthy of faceting and mounting in jewelry. The three main colors at the Crater of Diamonds State Park are white (60%), brown (21%), and yellow (17%) - according to the linked site below. 383 diamonds were found in 2004 and 536 in 2005. The higher numbers than the 1970s may be partially a function of higher numbers of visitors and perhaps more serious methods of searching, perhaps more digging and less crawling.

As mentioned above, it is an Arkansas State Park, open to the public. The state of Arkansas has toyed with the idea of selling the property to mining company, but public pressure has so far preserved the status quo. I know that Libertarian/Conservative purists disapprove of government ownership of land, but this place is so unique, I think it should stay as a state park.

The first diamonds were found 100 years ago, when the area was a farm. The farmer, when dressing out chickens to eat, found shiny stones in their craws (not having teeth, some birds swallow small stones to aid in the digestion process and the shiny nature of the diamonds caught the eye of the chickens). The stones were identified as diamonds, but there were never enough to support a mining operation, so it became a tourist attraction. In 1972, it became a state park.

If you click on the Park link above, the middle-aged black man in the center "works" at the park. Every day the park is open, he is there to pay his fee and that is what he does all day, dig for diamonds. He was there the last time I visited the park in 1983 or 1984 and I talked to him briefly. He doesn't find a diamond every day, but he finds enough to scratch out a living. Some of his diamonds may be among the Arkansas diamonds for sale on this website.

If you ever go there, don't expect to find a diamond, but there is always a chance. There are other minerals of interest to kids, quartz crystals, amethyst, calcite, peridot, agate, conglomerate (a type of sandstone composed of rounded river pebbles) and other minerals. Just keep everything that might even look like a diamond, and the rangers at the park are more than glad to look over what you have found and tell you "what's what".

Only in America!

Gold! I Found Gold! [Original Post Date 2/20/06]

Naw, I ain't talking about the Olympics.

I found a little bit of gold in a small creek, on the west side of my hometown. It ain't enough to worry about, but I just had to tell someone!

To be honest about it, a local man told my dad 25+ years ago that he had found gold in this particular creek, but my dad was skeptical and he didn't get a chance to check it out before he passed away in late 1980. After I moved back here in early 1991, from time to time I thought about the creek, which is behind an office park. Finally, last summer I decided to check it out.

I "ran" a (gold) pan of sand from the creek and found a couple of tiny, tiny grains of gold. I went back last week and ran another pan and found three tiny, tiny grains. In fact, you need a hand lens to identify it as gold, but gold it is.

There are other interesting minerals in the "black sand" that concentrates in the bottom of the gold pan. These minerals, more dense than quartz, are referred to by geologists as "heavy minerals" and are usually composed of metallic oxides and other unusual minerals and it this case it includes small garnet crystals and fragments of larger garnets and other colorful stuff.

Yeah, I am having a mid-life crisis and this gives me an excuse to play in a creek. Heh.

For Any Visitors...

The reasoning behind the reposting of old science material is that I am copying it from my original blog, to separate it from the political stuff that some folks don't like.

Interspersed will be new material, when time permits.

A Slip and a Fall [Original Post Date 1/23/06]

In the news is a story from Utah about a small hiking party, wherein one member slipped and knocked two other hikers about 60 feet down the mountainside, resulting in significant injuries to the two. They and the others have been rescued, so hopefully all will be well after the healing process.

It just reminded me of one of the few times I have really fallen while doing geologic work and how that fall might have changed geologic history (not my contributions, but someone much more important).

During the late 1970s or perhaps early 1980s, there was a geologic convention in El Paso, where academic papers were presented and field trips took place. One of the attendees was Dr. Preston Cloud (1912 - 1991), the 1977 recipient of the Charles Doolittle Walcott Award, from the National Academy of Sciences. [Dr. Walcott discovered the Burgess Shale fauna in 1909.] Dr. Cloud's NAS citation was for:

"In recognition of eminence and distinguished achievement in the advancement of sciences in pre-Cambrian paleontology and the early history of life on the primitive earth."

Here is one of Dr. Cloud's photographs along with an explanation of some of the material that he studied. Here from is a listing of references to Dr. Cloud in other science books. "The Garden of Ediacara" (in which there were nine references) is about the Precambrian Ediacaran fauna of Australia, the most important, discovered fauna from the time before the Cambrian Explosion. Here are a few more web links about Dr. Cloud.

One of the convention field trips was into the southern end of the Franklin Mountains to observe large, fossil algal structures (I know, only geologists dig this stuff) that had been studied by one of the UT El Paso geology professors. Dr. Cloud was one of the guests of honor on the field trip and I was tagging along with the rest of the geology grad students.

After observing the algal structures, we returned down the mountainside to the vehicles to go elsewhere on the field trip. On the way back down, Dr. Cloud paused at the top of a small cliff (perhaps only 10 - 15 feet high), to observe the scenic view of the Hueco Bolson and the Rio Grande River Valley southeast of El Paso. I was a few yards upslope and behind Dr. Cloud and when I saw him stop to take in the view, I attempted to stop my downslope steps. But instead, I stepped on some loose pebbles and started tumbling. I ended up on my hands and knees scant inches (4 to 6 inches) behind Dr. Cloud, scant inches from knocking a world-famous geologist off of a small cliff in the Franklin Mountains.

Maybe I am slightly over-dramatizing this event, but to this day I thank God that my legacy as a geologist wasn't written that day. True, the vertical distance wasn't that much, but below there were boulders and numerous cacti on the mountain slope. I am not even sure if Dr. Cloud knew how close I came to knocking him over the edge. I am not even sure that the lead professor knew and I never told him until I emailed the story to be part of his retirement party 3 or 4 years ago. He didn't reply, though I can imagine him slapping his forehead and saying "OMG" at the thought of what almost happened.

It was such an "OMG moment", I don't even remember the rest of the field trip. I don't even remember if I was driving one of the vehicles later or not.

Maybe a guardian angel stopped my tumbling or maybe it was shear dumb luck.

Monday, January 17, 2011

What a Geologist Does [Original Post Date 3/29/10]

Aside from my part-time Geology job and my part-time teaching...

What I am doing right now includes (when time permits, largely on weekends):

1) Retyping/rewriting my Master's Thesis (from 1989) and scanning the photos and related 35 mm slides. (It was probably one of the last theses typed on an electric typewriter). Because of the binding, scanning all of the text would be a hassle.

The reason I am doing this is to be able to send some info to a vulcanologist with the Hawaii Volcano Observatory. A while back, he contacted me with information relating Hawaiian volcanic shatter rings with the Quaternary "explosion-collapse" craters that I studied in the Aden Basalts, in southern New Mexico. In other words, he thinks that the five craters I described are probably "shatter rings".

My thesis advisor and I had scoured the literature available in the middle and late 1980s and found no references pertaining to these craters, characterized by an encircling rampart of boulders and a collapsed central floor. If I can secure his permission to reference his work, I may work up an abstract for a GSA meeting next year, if it doesn't conflict with a more substantial publication he has in the works.

2) Continuing the work on my science-photo CD. For the last 8 years I have been compiling a database of photos to use in my Geology and Environmental Science lectures. At this time, I am trying to fill in some missing categories. There are currently 900+ photos applicable to Geology, Biology, Weather (clouds), and Environmental Science.

I am greatly looking forward to going back to NJ and NYC this summer to get some photos of the glacial features of Central Park and maybe some of the terminal moraines on Long Island. Maybe I will get some good photos of the Palisades of the Hudson and some of the coastal features of New Jersey, including Sandy Hook. I would also like to collect some samples of the garnet beach placers on Long Island, i.e., heavy mineral sands dominated by garnet fragments.

3) Continuing work on a compilation of Cretaceous & Tertiary well logs from Burke County, GA. This began 10+ years ago while a co-worker and I were working on a state geologic survey project in the vicinity of the Savannah River. My friend is a well-known Gulf Coastal Plain stratigrapher and his detailed well-log descriptions were too voluminous to put in the original reports and our goal was to produce a separate report, which would hopefully resolve some of the stratigraphic nomenclature and correlation issues between this part of Georgia and adjacent South Carolina. [If memory serves me correctly, my friend logged about 13,000 feet of core for the Tritium Project.]

With my friend's retirement to Albuquerque a few years ago, it has hindered work on this paper (he was actually here a couple of weeks ago, looking at other Coastal Plain cores and rewriting well logs - once a stratigrapher, always a stratigrapher). If we ever get this paper finished, even if it doesn't get published, if we can print a few copies onto CDs and send them to some local colleges that might be interested, at least someone would have access to the descriptions to the cores for future projects.

4) Learning the "Ins and Outs" of Google Earth in tying GE images to visited sites and sample locations. As for the sample locations, my junior college is building a sand sample collection, i.e., various beach, river, and dune sand samples and I would like to be able to tie location maps (and descriptions of source area geology) to the individual sand samples.

5) Revisiting the trace fossils I found in the Permian Cloud Chief Formation in Southern Ellis County, OK. Originally found in July, 2007 and ID'ed as "Arthropod locomotion marks" by the Oklahoma Geological Survey, I recently did an internet search and determined that these are very likely Arthropleurid trackways. Only one side of each set was preserved, perhaps because the centipede-like creature was wider than the individual rock slabs. Though I hope to revisit the area again to do some more collecting and documentation, I doubt that it will be this summer.

What a Geologist Sees - Part 30 [Original Post Date 4/16/10]

These thin sandstone slabs are from the Permian Cloud Chief Formation from NW Oklahoma. They were collected from downstream of Lake Vincent in southern Ellis County.

The bulk of the formation is composed of siltstone and claystone redbeds.

(Click to enlarge images)

Pictured are the tops of the individual beds. The 1 cm bars are accurate, to show scale.

A few months after dropping some specimens off with the Oklahoma Geological Survey, they told me that there were probably "arthropod locomotion marks". Being busy with life in general, I accepted the answer without trying to determine "what sort of arthropods".

Recently, while looking at some of the slabs, I revisited a point of previous curiosity. Each trackway seemed to not have a companion for the opposite side of the critter. Perhaps because the critter was wider than the slab of rock.

So I started doing an internet search of Permian arthropods. An early result gave the word "Arthropleurid" as a likely suspect. An Arthropleurid was a large, centipede-like critter, in some cases up to 6 feet long. A later result gave this article, concerning the Late Pennsylvanian Cutler Group, in northcentral New Mexico.

If I get a chance to revisit the area, I will be looking for wider slabs, to hopefully find more-complete trackways and to document the occurrence a little better. Would like to maybe do a short paper/talk, maybe for a future GSA regional meeting. Or if I don't, maybe I will have somehow inspired someone else (in Oklahoma) to do so.

[The comments were generated with the original 2007 post.]

What a Geologist Sees - Part 29 [Original Post Date 10/02/09]

Shatter Ring on PKK Lava Tube (March 20-22, 2006) from this USGS site:

This video, shot by Tim Orr of the USGS, is of interest as I studied these Shatter Rings in the Aden Basalts of southern New Mexico. Not aware of the existence of other examples (20 years ago), we called them "Explosion-collapse" craters and I described 5 of them in my Master's Thesis.

Using the description from the USGS website, here is a description of the Shatter Rings:

"Shatter rings are circular to elliptical volcanic features, typically tens of meters (yards) in diameter, which form over active lava tubes. They are typified by an upraised rim of blocky rubble and a central depression."..."They form when lava pressure in the tube repeatedly exceeds the strength of the overlying rock. Repeated flexing of the lava-tube roof piles up rubble around the edges of the mobile area."

In the case of the shatter rings I studied, lava was extruded up through the shattered rocks and filled-in the bottom of the crater, creating a lava lake, which later collapsed after the lava below withdrew.

When I scan some more of my old slides and prints from my field area, I will write more about these features.

What a Geologist Sees - Part 28 [Original Post Date 9/13/09]

Shales. Mudstone. Claystone. Clay. They are the most common sedimentary rocks or sediments.

Yeah, sometimes they can be a bit boring if we only look at them from one dimension. While on vacation, I searched this Tulsa, OK outcrop for more than a half-hour and found nary a fossil. As I was pressed for time and saw no other outcrops nearby, I kept looking.

In other shale outcrops, splitting apart the layers can bring nothing or it can bring to light a fossil leaf, a fossil seed, a fossil shell, a trace fossil,...

In the case of the specimen at left, this is a sample of shale that has been baked by an underground "coal seam fire" sometime in the past. The heat baked the shale to a natural ceramic and preserved the Cretaceous-aged fossil leaves within. (I regret not having collected more samples from this site 30 years ago.)

Shale is a clastic sedimentary rock that consists of compressed clay. It differs from claystone or mudstone by being "fissile" as shown in the upper photo. Fissile refers to the property of splitting into thin layers, which is caused by the alignment of the microscopic, flat, hexagonal clay plates (visible only to scanning electron microscopes). In the fissile shales, bedding planes are often observed, presenting the planes of weakness that allow the splitting.

Claystone or mudstone, while being hard, do not have the same alignment of clay plates, thus they fracture in a more "massive" fashion with no visible bedding planes, often leaving a curving "concoidinal" fracture, as seen below in the kaolin sample.

When younger or less-compacted (and therefor softer), as on the Gulf or Atlantic Coastal Plains, we refer to it as "clay".

As defined by the Wikipedia entry, "Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations."

There are five groups (and other subgroups) of clay minerals, including some 14 minerals (one of which is not always considered a clay) - according to the Wikipedia entry.

Clays are derived from the chemical weathering of silicate minerals and rocks. Minerals such as micas, amphiboles, pyroxenes, but especially feldspars. In continental settings, such as exposures of highly-weathered rock, reddish colors (for the famous Georgia red clay) for some clays can come from iron-staining.

Once clays have been delivered to the river systems (including swamps and lakes) and then to the ocean, their eventual colors are a function of their environment of deposition. Due to the minute nature of the clay plates (less than .004 mm) and their buoyancy, quiet water conditions are needed for the clay flakes to finally sink to the bottom of the ocean (or other water body). Sometimes slight increases in water energy can result in silt (.004 mm to .063 mm) being deposited within the clay or as separated, interbedded layers. Silt is usually made up of minute silica (quartz) grains, but may include other minerals.

In the case of the Tulsa shale outcrop, this was probably an open-marine setting, where there was relatively good water circulation (and oxygen availability for bacterial degradation of any organics), usually yielding a light- to medium-gray color. The same is true for the shales interbedded with the thin limestones of this particular facies of the Ordovician Lexington Limestone. The alternating layers suggests fluctuations in the environment, due to changes in sediment supply, water depth, or other factors.

When you see reddish-colored shales or claystones (in a sedimentary setting), those were usually deposited in a continental or transitional setting, such as a river system, delta, or a tidal flat setting. The red color is due to the subaerial oxidation of the iron within the clay sediments. Usually the preservation of ripple marks suggests a certain amount of silt in the rock.

Currently, the most popular color of shale (for geologists) is dark gray to black. The dark colors are most-often due to the preservation of organics in stagnant (anoxic) conditions, where slow water circulation fails to replenish oxygen. So when organics drop to the bottom, the bacteria that would normally be there to "eat" them, are absent. This is the case in swamps (blackwater) or in restricted marine basins, e.g., the Black Sea or the deep part of the Gulf of Mexico.

This preserved organic material becomes the interpreted source of oil and/or natural gas, depending on the type of organics and/or the temperature conditions. An increasing amount of our domestic natural gas is being produced from Paleozoic and Mesozoic dark shales, such as the Marcellus, Barnett, Woodford, Eagle Ford, and other shales, due to our ability to "frac" (fracture) the shales using hydraulic pressure to shatter the shale and proppants (sand or minute ceramic spheres) to prop open those fractures. Without this process (or natural fractures), the shale is generally too "tight" to produce much of anything. Generally, the shales with a little bit of silt-sized silica are a little more brittle and frac more easily.

Another aspect of clay is that when compressed, the alignment of the flat clay plates makes the semi-impervious (or impermeable), i.e., they don't pass water or other fluids very well. This is why we use clays in ceramics. Layers of clay or shale can serve as "confining beds" to separate layered aquifers or as "caprock" to trap hydrocarbons.

There are other characteristics of clays that make them both a bane and a boon to humans. Some clays swell when wet and this can be useful when adding clay pellets to seal the annular space in a water (or other) well, but can play havoc with heaving (and cracking) of roads and foundations with wet/dry weather cycles. The brick steps to my back porch are a testament to this characteristic of rising and falling with wetting and drying, as they have broken loose from the foundation of the porch.

Clays also can act as absorbents for pollutants, whether in your cat's litterbox or in dealing with oil or other spills. They can be used in filtration settings, as filler material, as the "binder" for Kaopectate,...

To go any further would require re-writing "War and Peace" in a geological sense. I hope you get the picture that clay is more than just hardened mud.

For other "What a Geologist Sees" posts, click on the Tag below.

What a Geologist Sees - Part 27B [Original Post Date 5/04/09]

...or would like to see.

Somewhat related to the last numbered What a Geologist Sees post (#27), here are the Top 15 places I would like to visit or revisit, now that I have a digital camera - after #1 in no particular order. [I may expand this to 20, later.]

1) Arches National Monument - I have been there twice (1977 and 1979) and both times my 35mm film camera crapped out.

2) Monument Valley, AZ and UT

3) Antelope Canyon, AZ

4) Valles Caldera, near Los Alamos, NM and some of the nearby Rio Grande Rift features, including some south of Albuquerque.

5) The basalt flows along I-40 near Grants, NM

6) Mount Saint Helens

7) Hawaii - primarily the big island

8) Zion National Park

9) Portions of Wisconsin, where there are glacier-related landforms. When I was there in 1982, I inadvertently opened the back of my 35 mm without reeling the exposed film back into the cartridge. D'oh! My brewery photos were safe on another roll, but I lost all of my geology slides.

10) Glacier National Park

11) Aden Basalt Field, southern New Mexico - I learned to hate it when I was working on my Master's Thesis 20+ years ago, but I actually miss those monotonous flows. There are many things to enjoy and photograph while doing a walkabout.

12) Yellowstone National Park - last time I visited was 35 years ago.

13) Devil's Tower, NE Wyoming

14) Exposures of the Niobrara Chalk in Kansas, Castle Rock and Monument Rocks. Better still, somewhere where collecting fossils from the chalk is legal.

15) Yosemite National Park - again, last visit 35 years ago.

16) I would like to replicate my 1974 trip across the western U.S. with a non-geologist friend. This time, I would insist on stopping to take some more photos. As I was still an undergrad at that time, I would have a much better idea of what I waas looking at this time. (I will update more later.)

What a Geologist Sees - Part 27 [Original Post Date 2/27/09]

100 Things a Geologist Should See or Do

For the source of this list, see this link at Geotripper.

To see the entire list, visit the link. Printing the entire list is too long, so I will list the things I have done or seen and the things that I consider in the realm of possibility of doing sometime in the future.

Been there/done that:

3. See an active geyser... such as those in Yellowstone
6. Explore a limestone cave. Carlsbad Caverns in New Mexico, Luray Caverns, VA, Raccoon Mt., TN; Cumberland Caverns, TN; Mammoth Cave, KY
7. Tour an open pit mine,... a copper mine in Santa Rita, NM and a uranium mine in Sierra Peña Blanca, Chihuahua.
8. Explore a subsurface mine - a coal mine in Mexico.
13. An exfoliation dome, such as those in the Sierra Nevada - or Stone Mt., GA.
16. A gingko tree, which is the lone survivor of an ancient group of softwoods that covered much of the Northern Hemisphere in the Mesozoic. - Got one in my side yard.
17. Living and fossilized stromatolites (Glacier National Park is a great place to see fossil stromatolites - or the Franklin Mts., El Paso area while Shark Bay in Australia is the place to see living ones) - done 1/2 of that
19. A caldera - Valles Caldera, Los Alamos, NM, several calderas in West Texas
26. A large sinkhole - Silver Springs, FL
33. Petrified trees Bisti Badlands, San Juan County, NM (see this post)
34. Lava tubes Aden Crater, NM
35. The Grand Canyon. All the way down. And back. 1/2 of this, I have been on the South Rim of the Grand Canyon 4 times.
36. Meteor Crater, Arizona, also known as the Barringer Crater, to see an impact crater on a scale that is comprehensible - 1978
58. The Carolina Bays, along the Georgia coastal plains
62. Yosemite Valley - 1974
63. Landscape Arch (or Delicate Arch) in Utah - camera crapped out on both visits 1977 & 1979
80. The Black Canyon of the Gunnison in Colorado - 1977, 1979
84. Find a trilobite (or a dinosaur bone or any other fossil) - found plenty of fossils, including dino bones, but haven't found a complete trilobite, yet.
85. Find gold, however small the flake - numerous times in GA and CA
88. Experience a sandstorm - First spring in El Paso, 1977 and other times
90. Witness a total solar eclipse
95. View a great naked-eye comet, an opportunity which occurs only a few times per century
96. See a lunar eclipse

So, it looks like I have only done 21 of these things (or been 21 of these places). That is not to say I haven't seen a countless number of interesting things, but they might not be interesting enough to put on a Top-100 list.

Might go there/do that someday:

1. See an erupting volcano - I would like to visit either Iceland or Hawaii
2. See a glacier
4. Visit the Cretaceous/Tertiary (KT) Boundary. Possible locations include the San Juan Basin, NM.
5. Observe (from a safe distance) a river whose discharge is above bankful stage (I have watched a rather intense flash flood near Hillsboro, New Mexico, I don't know if that would qualify or not)
11. A slot canyon. Many of these amazing canyons are less than 3 feet wide and over 100 feet deep. They reside on the Colorado Plateau. Among the best are Antelope Canyon, Brimstone Canyon, Spooky Gulch and the Round Valley Draw.
14. A layered igneous intrusion, such as the Stillwater complex in Montana or the Skaergaard Complex in Eastern Greenland.
15. Coastlines along the leading and trailing edge of a tectonic plate (check out The Dynamic Earth - The Story of Plate Tectonics - an excellent website).
18. A field of glacial erratics
20. A sand dune more than 200 feet high
22. A recently formed fault scarp
23. A megabreccia
24. An actively accreting river delta
25. A natural bridge
27. A glacial outwash plain
28. A sea stack
29. A house-sized glacial erratic
30. An underground lake or river
31. The continental divide
32. Fluorescent and phosphorescent minerals
39. The Waterpocket Fold, Utah, to see well exposed folds on a massive scale.
40. The Banded Iron Formation, Michigan, to better appreciate the air you breathe.
44. Devil's Tower, northeastern Wyoming, to see a classic example of columnar jointing
46. Telescope Peak, in Death Valley National Park. From this spectacular summit you can look down onto the floor of Death Valley - 11,330 feet below.
50. The Goosenecks of the San Juan River, Utah, an impressive series of entrenched meanders.
51. Shiprock, New Mexico, to see a large volcanic neck
54. Mount St. Helens, Washington, to see the results of recent explosive volcanism.
59. The Mima Mounds near Olympia, Washington
61. The moving rocks of Racetrack Playa in Death Valley
64. The Burgess Shale in British Columbia
65. The Channeled Scablands of central Washington
66. Bryce Canyon
67. Grand Prismatic Spring at Yellowstone
68. Monument Valley
69. The San Andreas fault
75. A catastrophic mass wasting event
76. The giant crossbeds visible at Zion National Park
77. The black sand beaches in Hawaii (or the green sand-olivine beaches)
78. Barton Springs in Texas (will try to do that next time I am in Austin)
79. Hells Canyon in Idaho
82. Feel an earthquake with a magnitude greater than 5.0.
83. Find dinosaur footprints in situ
86. Find a meteorite fragment
87. Experience a volcanic ashfall
91. Witness a tornado firsthand. (Important rules of this game). (We were in our basement at 1 AM when we got hit by a tornado in 1998, it is probably not the same thing as watching one cross the plains of Oklahoma or Kansas)
92. Witness a meteor storm, a term used to describe a particularly intense (1000+ per minute) meteor shower
93. View Saturn and its moons through a respectable telescope.
94. See the Aurora borealis, otherwise known as the northern lights - (I was in Wisconsin in the summer of 1982, but I was enjoying the local beer and I forgot to look for the Northern Lights at night).
97. View a distant galaxy through a large telescope
98. Experience a hurricane
99. See noctilucent clouds
100. See the green flash

I would add a couple more things:

101. Go to the Crater of Diamonds State Park in Arkansas and stay there until you find a diamond. I found one my first trip there in 1973.
102. Stand on the platform of an operating oil drilling rig. I have sort of done this, we visited a couple of drill rigs in SE New Mexico on a Geology field trip in 1982, both were operating rigs, but they had suspended drilling for safety reasons while we were there (or else some maintenance was going on).

What a Geologist Sees - Part 26B [Original Post Date 2/25/09]

Just a few more photos from my 1979 job in the Bisti Badlands of San Juan County, New Mexico.

The uppermost photo is from a clinker zone, which I may have explained in the previous post on this subject. It is basically baked shale from adjacent to a burned coal seam. On the left is a stem of some sort and on the right is a leaf fragment.

The second photo is of a permineralized (petrified) stump. I hope it somehow got hauled of to a museum or a geology department. It was way too heavy for me to move, though I would love to have something like this in my front yard.

The third photo is of one of the areas rich in permineralized logs. We were to collect samples from each of these and mark them on the map. I hope that the University of New Mexico gathered up these logs before the mine opened.

Sunday, January 16, 2011

22 Different Drivers in the Driver's Seat [Originally Posted 2/4/2010]

Climate Drivers are those inputs (influences) that affect the Earth's weather and climate, all are of different magnitudes and some (especially the first four) are on different cycles (or are subject to random events). Some of these cycles and drivers may exaggerate each other (Synergy) or they may partially (or wholly) cancel each other out (Antagonism).

Geologist H. Leighton Steward has produced a chart with 18 different climate drivers (or climate forcers). He acknowledges that there might be more and I have included several more myself.

After each listed Driver will be the Principle Influence, Impact, and Comments.
  1. Solar Heat & Solar Magnetic Field - Solar heat and Magnetic shielding - Strongest - Heat retained by Earth influenced by other drivers. There are numerous solar cycles of different time intervals, that can affect quantity of heat, light, and magnetism being emitted by the Sun.
  2. Orbital Eccentricity - Determines distance from Sun - Strong - Distance affects amount of solar heat received, influenced by gravitation pull of Saturn, Jupiter, and other planets.
  3. Earth's Axial Tilt - Determines seasons and amount of heat received by higher latitudes - Strong - Additional tilt can affect polar ice melting (or growth).
  4. Earth's Axial Wobble (precession) - Determines Earth's seasons closest to or farthest from the Sun, caused by the unequal distribution of land masses - Strong - Can be positive or negative feedback.
  5. Water Vapor - Greatest quantity of all of the Greenhouse Gases - 90 - 95%, affects clouds, precipitation volumes, albedo, and vegetation, "thickens" the air - Strongest of Greenouse Gases - Highly variable, may not be included in computer models for this reason.
  6. Water Droplets & Ice Crystals - As components of clouds, affects amount of visible light reaching the Earth's surface and traps rising heat from Earth's surface - Strong - Highly variable, may not be included in computer models for this reason.
  7. Carbon Dioxide - Captures infrared heat rising from Earth's surface, reradiates some heat - Strong at low saturation - Generated by ocean releases, volcanic activity (including hot springs), animal/bacterial respiration, and combustion (natural and human), Greenhouse Effect non-linear, usually follows temperature changes, currently 0.0385%.
  8. Methane - Captures infrared heat rising from Earth's surface, reradiates some heat - Moderate (low quantity in atmosphere) - Generated by wetlands, by animals, and industries, currently 0.00018%.
  9. Ocean Currents - Distributes heat from Tropics to higher latitudes, can change quickly or slowly - Strong - Largest reservoir of surface heat.
  10. Plate Tectonics (seafloor spreading) - Causes volcanism, releases carbon dioxide, sulfates, chlorine, and other gases, results in mountain uplifts, earthquakes - Strong, long-term - Affects position of continents, sea level, volcanic ash and sulfates affect atmospheric chemistry & atmospheric CO2.
  11. Location of Continents - Affects major ocean currents and distribution of heat - Strong to weak - Land over poles increases glaciation, position of continents affects rising infrared heat from surface.
  12. Elevation of Land Masses - Higher elevations promote glaciation and affect local wind currents; moderate elevations promote rainfall (through Orographic Uplift) & chemical weathering of rocks (see below) - Moderate - Affects regional climates, monsoons, and locations of deserts, especially North American deserts.
  13. Chemical Weathering - Releases elements and compounds from minerals, affects chemistry of water bodies - Weak, long-term - Little short-term effect on climate.
  14. Vulcanism - Constants sources of CO2, sulfates, ash particulates - Moderate to strong, short-term - May affect ocean chemistry, builds new islands, affects atmospheric chemistry.
  15. Extraterrestrial Impacts - Immediate fires, then colder temperatures for a few years/decades - which affect plant communities and the food webs built thereon - Strong, very short-term - May affect atmospheric and oceanic chemistry.
  16. Albedo - Determines amount of Solar Energy reflected or retained (absorbed) - Moderate to strong - Constantly changing, affected by other drivers.
  17. Flora & Fauna (plants, animals, bacteria) - Affects albedo, oxygen, carbon dioxide, and methane content of atmosphere - Moderate - Terrestrial ecosystem diversity and richness affected by atmospheric moisture, temperature, and chemistry; Aquatic ecosystem diversity and richness affected by temperature, water energy, water chemistry.
  18. Atmospheric Circulation - Vertical and Horizontal winds distribute heat and moisture and affect land surface and upper ocean circulation patterns - Moderate - Affects weather systems and distributes nutrients to oceans, affecting oceanic flora, fauna, and chemistry.
  19. Cosmic Rays - Evidence suggests that cosmic rays produce particulates that seed low-level clouds - Impact to be determined - More research needed to determine impact.
  20. Earth's Magnetic Field - May affect quantity of cosmic rays reaching the atmosphere - Impact to be determined - More research needed to determine impact.
  21. Changes in Land-Use Patterns - Includes deforestation for logging and farming, growth of Urban Heat Islands, variable local and regional effects - Impact to be determined - More research needed to determine impact.
  22. Carbon "Soot" and other Particulates - In some cases, particulates may reflect sunlight, in other cases they may absorb sunlight, may also serve as condensation nuclei for clouds and rainfall, some particulates are generated by combustion and human disturbances to the soil, i.e., farming, construction, and other human activities, in addition to natural sources - Impact to be determined - More research needed to determine impact.

My point here was to illustrate the complexity of the atmospheric interactions of these drivers (and any others yet-to-be-identified). In other words, it is a bit premature to say "the science is settled", as some political charlatans have said. Mother Nature is wild and we mere humans will never totally understand her.

The human influences on some of these could be ameliorated, but not if our economy (and the economies of other developed countries) are hobbled by unnecessary taxes and regulations, administered by un-elected bureaucrats driven by resentment of our freedom and prosperity.

Saturday, January 8, 2011

A Lost Civilization Beneath the Waters of the Persian Gulf?

From the website comes this fascinating article concerning ancient Middle Eastern history and human culture.

[Image below courtesy of the above-linked article. Click here for an enlarged image of this map.]

From the article:

"Veiled beneath the Persian Gulf, a once-fertile landmass may have supported some of the earliest humans outside Africa some 75,000 to 100,000 years ago, a new review of research suggests.

At its peak, the floodplain now below the Gulf would have been about the size of Great Britain, and then shrank as water began to flood the area. Then, about 8,000 years ago, the land would have been swallowed up by the Indian Ocean, the review scientist said."...

In the article, the flooding of the fertile valley is attributed to rising sea levels following the end of the last major Pleistocene ice age. There is, however, another plausible event that may have contributed to the flooding of this valley - Plate Tectonics.

On the opposite side of the Arabian Plate from the Persian Gulf, lies the Red Sea, which is part of the East African Rift. The activity of the rift is pushing the Arabian Plate to the northeast, where it is colliding with the Eurasian Plate, uplifing the Zagros Mountains of Iran. Where you have the collision of two continental plates, it is common to have the uplift of a linear mountain range, e.g., the Himalayas. Adjacent and parallel to this mountain range is commonly a Foreland Basin, of which the Persian Gulf is an example.

[Image from the What On Earth blog.]

Continuing from the original article:

..."The Gulf Oasis would have been a shallow inland basin exposed from about 75,000 years ago until 8,000 years ago, forming the southern tip of the Fertile Crescent, according to historical sea-level records.

And it would have been an ideal refuge from the harsh deserts surrounding it, with fresh water supplied by the Tigris, Euphrates, Karun and Wadi Baton Rivers, as well as by upwelling springs, Rose said. And during the last ice age when conditions were at their driest, this basin would've been at its largest.

In fact, in recent years, archaeologists have turned up evidence of a wave of human settlements along the shores of the Gulf dating to about 7,500 years ago."...

Aside from the rising sea levels related to the post-Pleistocene ice-cap retreat, is possible that the gradual sinking of this Foreland Basin contributed to the flooding of the area.

Prior to this being a Foreland Basin, it was a part of the ancient Tethys Seaway - a Mesozoic - early Cenozoic seaway between portions of the separating supercontinent Pangea - as partially-described in a World Oil website post. [Scroll down to see Figure 4 and the text above the figure.]

From this World Oil post, in describing the geology behind the 151 giant oilfields in the region:

..."They are concentrated in a large foreland basin formed during the Late Cenozoic collision of the Arabian Peninsula with Eurasia. Downward flexure of the Arabian Peninsula beneath the Zagros Mountains of Iran/Iraq was caused by the northeastward consumption of the Tethys Ocean at the Zagros suture zone. Additional causes of this flexure were the eventual Cretaceous-recent convergence and collision of the Arabian plate against the Eurasian plate. This protracted convergent event has created the Persian Gulf and Mesopotanian [sic] lowlands as a sag in the foreland basin, as well as formation of the Zagros Mountains, with a culmination of fold-thrust deformation in Miocene and Pliocene time."...

It would be fascinating to read a detailed weaving-together of Old Testament Biblical History (and of other ancient texts) with geologically-recent Plate Tectonics events in the Middle East. Including the eruptions of Mt. Etna, Mt. Vesuvius, and the Santorini explosion/tsunami.

Returning to the second-cited source, the What on Earth blog, and the tectonic map, if you notice along the western edge of the Arabian Plate is a transform fault zone, which is the source of the Dead Sea Basin, as a very deep "pull-apart basin" (see the What on Earth January 29, 2009 post, same link).

But that is for another discussion.

Monday, January 3, 2011

A Follow-up to the Mineral Videos [Original Post Date 01/04/09]

on minerals, just a few additional words on the subject to review clarify this issue for non-geologists (normal people). [Being on the road when those two videos were posted, I didn't have time to watch much of either, so if I repeat any of what was said, it is because what I wrote below is just part of my standard opening to the Physical Geology chapter on minerals.]

By definition, minerals are:

Naturally occurring, solid (at normal temperatures), inorganic (though they may be formed by organic processes), they have a definite chemical composition (or range), they have an orderly internal structure, and they have definite characteristics, e.g., crystal habit, cleavage, hardness, color (though color may be unreliable because of trace elements).

Minerals are important because they are the "building blocks of rocks". Most rocks are composed of two or more diffrent minerals, though there are a few rocks that are only composed of a single mineral, e.g., pure marble, pure quartzite, pure limestone...

Geology students generally learn to recognize individual minerals first, then they usually learn to recognize them in igneous rocks, as igneous rocks are the original source of most minerals, including the minerals that make up sedimentary and metamorphic rocks.

Some of the most common minerals that we come in contact with are salts - Halite (NaCl) being the most common of these and Sylvite (KCl) if you use Morton Lite Salt. There are other salts - potassium iodide (KI), etc. that are used for various reason in foods to deliver various trace elements that we need (or that enhance flavors).

Other minerals we encounter are quartz (the most common mineral on Earth), diamond (the hardest), perhaps some other gemstones, gypsum, and for anybody that still uses black-and-white camera film, some silver salts (I am clueless as to the chemistry of color film emulsions). [Sadly, IMHO, film photography is slipping further into history, which some folks will regret as digital images themselves are lost over time.]

BTW, for the beautiful crystals that some folks like to marvel over, for those nice crystals to form, they have to have "room to grow", perhaps into a fracture zone, or some other cavity or open space, or they were among the first minerals to crystallize in a cooling magma. Sometimes those growing crystals include (surround) other minerals as the crystal grows or in the case of gypsum (or other salts) in sediments associated with salt lakes, sometimes the crystals will include small rock fragments, sand grains, and other stuff.

Usually, geologists are not lucky enough to have good, well-shaped crystals for the purpose of identification. That is why we learn the other characteristics of individual minerals. There are high-tech ways of analyzing rocks, but they take time and cost money, so field geologists are still required to make a quick-and-dirty assessment of what minerals are present in a rock and uses the proportions of major minerals to define the rock itself.

[As I think of other examples, I may include them.]

Minerals - Part 2 of 6 [Original Post Date 12/30/2008]

YouTube poster: mineguy101

Part 2 of a 1976 Series.

Mineral Video - Part 1 of 6 [Original Post Date 12/30/2008]

From mineguy101.

This is part 1 of a 1976 series. If it is good, will add more.

What a Geologist Sees - Part 25 [Original Post Date 11/16/2008]

Oh, the stuff we can see at construction sites and quarries!

[Disclaimer: I only enter construction sites on Sunday, when there is no activity, I stay away from the equipment and any obviously dangerous places and if there are any "No Trespassing" signs, then I don't go in.]

One "treat" at a construction site is to be able to see the effects of erosion and deposition in the exposed materials. In the uppermost photo, you can see the gulley erosion in the soft, graded soil. Just downslope from the gulley is a small "alluvial fan", where the eroded material was deposited. Larger examples of alluvial fans are seen at the mouths of mountain canyons.

In the second photo, in a sand pile at a quarry, as sand is removed from below, it triggers miniature slumps and landslides in an attempt to bring the slope back into equilibrium. In larger settings, slumps and landslides generally happen on slopes that have become destablized due to construction and heavy rainfall.

As one would expect, in a construction site, rocks are exposed that we usually wouldn't see at the surface. In the third photo, road construction has exposed a portion of a diabase (basalt) igneous dike that was most likely intruded during the Triassic or Jurassic Period. The iron-rich silicate minerals in the diabase are susceptible to weathering (by oxidation) in this humid climate, thus these blocks from the shallow sub-surface show a "rind" of oxidized material, with fresher rock material within the block.

In the fourth photo, we see "saprolite" that has been exposed during the construction of a drugstore. Saprolite is called "rotten rock" by some, it is rock that has been chemically weathered to the point that its structural integrity has been lost and the material can be easily crushed by hand. The "parent rock" - exposed nearby - is a biotite gneiss, similar to a granite, and in the case of the saprolite, the feldspars, micas, and other minerals (except for quartz) have been altered to clays. If not covered over quickly, this sort of material would wash into a nearby creek, resulting in "silting up" of the stream (and a probable EPA/Ga EPD fine).

The fifth photo, of another sand pile, shows how gravity, with the help of the wind, attempts to stablize the slope of this sand pile. Unconsolidated (loose) materials have a defined "angle of repose", which is the maximum angle-of-slope that particular sized material can sustain. If a slope is "oversteepened", miniature landslides and slumps carry material downslope in an "attempt" to establish equilibrium at the angle of repose, which generally varies between 25 and 35 degrees, depending on the size and angularity of the particles.

In the final photo, in this pile of mixed sand and gravel, rainfall has induced "rill erosion" (small erosion channels) on the slopes and small alluvial fans at the base of the slope.

[All of these photos were taken in the greater Atlanta area.]

In Geology, we term the mass downslope movement of material to be "mass wasting". [Yeah, I know Geologists can get mass-wasted after too many adult beverages, but that is another story.] Mass wasting occurs when gravity overcomes cohesion and internal friction. Water can be a facilitator of this process, as well as earthquake, traffic, and construction vibrations (as suggested above).

What a Geologist Sees - Part 24b [Original Post Date 10/31/2008]

Just a couple of photos from the City of Rocks State Park in Grant Co., NM, as referenced in the previous post. It is a neat place to camp, but one time I stopped there at night to show it to a friend as we were traveling through the area. In the moonlight, the "hoodoos" were just a little too spooky.

If memory serves me correctly, these pyroclastic ash flow tuffs were erupted from the very large Emory Caldera.