10 Hidden Geologic Wonders of the U.S.

By. Scott H. Nyhof, P.G.

A “Top 10” list of the geologic wonders of the US would likely include several world-class National Parks including Yellowstone, Yosemite, Tetons, Glacier, and Grand Canyon.  But there are so many wonderful, unique geologic wonders on North America that do not necessarily include such a grand vista.  So in addition to the “wonder”, this list also describes the PROCESS by which each formed.

  1. Continental Glacier Terminal Moraine

The terminal moraine of the continental glaciers stretches from the end of Cape Cod, Massachusetts to central Illinois, along the Missouri River in South Dakota to the Rocky Mountains (see map, below).  This unique geologic wonder formed as a result of the southern-most advance of the most recent continental glaciation.

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Extent of Continental Glaciation, North America

 

About 3 million to 5 million years ago the Isthmus of Panama rose up, terminating equatorial flow between the Atlantic Ocean and the Pacific Ocean.  Afterward, there was a fundamental shift in paleo-currents.  Either coincidentally or as a direct result of this shift in ocean currents, climates began to oscillate between wet and dry, and warm and cool, and may have resulted in the onset of continental glaciation.

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Glaciers form when more snow falls than melts.  In the last 620,000 years or so, there have been several glacial “maximums”, listed below.  “Maximum” refers to periods of southern-most advancement of continental ice sheets, while “minimums” refer to periods when continental ice sheets have retreated and are either absent or nearly so.  We are presently in a “glacial minimum”, with retreating glaciers and ice caps worldwide.  The previous “minimum” appears to have been about 20,000 years long.  Website:  www.geologyclass.org

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  1. Arkansas Diamonds

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The world-renowned diamond markets in Africa are supported with lesser-known finds in Canada, Australia, and Russia.  And in Arkansas!  There is a small diamond mine outside of Murfreesboro, Arkansas (see map, left), where you can dig for diamonds and keep what you find.  There is even an Annual Diamond Festival to celebrate this place that is unique in the USA.  

Diamonds are probably more than 3 billion years old, and formed very deep, from about 60 miles to perhaps as much as 100 miles underground.  Typically, carbon is commonly incorporated into a carbonate mineral (i.e. calcite as CaCO3) and is also found as graphite, which is used in a standard No. 2 pencil.  The carbon found in graphite is set in a hexagonal lattice (see below, left), while the carbon in a diamond has been re-arranged into a cubic lattice (see below, right), which is much stronger.  Website:  www.craterofdiamondsstatepark.com

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  1. Great Basin on The Continental Divide

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Precipitation, usually in the form of rain or snow falls to one extent or another across the United States, with several distinct barriers, or “Divides” that separate where the subsequent streams flow.  Water and snow that falls to the west of the Rocky Mountains flows west, into the Pacific Ocean.  Rain and snow in North Dakota and a portion of Minnesota flow north into the Hudson Bay, and most of the precipitation between the Rockies and Appalachia flows through the Mississippi River into the Gulf of Mexico.  However, there is one oval-shaped area in southwestern Wyoming located ON the Continental Divide where precipitation doesn’t seem to know where to go.  This is The Great Basin (see red arrow).  Streams within the Great Basin are classified as “intermittent”, in that they do not contain standing or flowing water throughout the year.  The floor of the Great Basin is not completely flat or level; rather there are some low hills and valleys throughout.  Also, vegetation and wildlife in the Great Basin has adapted to prolonged periods of hot, semi-arid conditions.  Website:  www.wsgs.uwho.edu

  1. Michigan Basin

Michigan includes coastline along four (4) of the five (5) Great Lakes, including Lakes Superior, Michigan, Huron, and Erie.  There is also an “upper peninsula” that is connected to the larger portion of Michigan only by a 5-mile long 4-lane suspension bridge (Big Mac) that spans across the Mackinac Straits.  And then there is Detroit; also known as Motorcity USA, with international headquarters for both Ford Motor Company and General Motors Corporation.  So why are we just hearing about The Michigan Basin?

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The United States Geological Society (USGS) describes The Michigan Basin as “A giant incomplete bulls-eye is centered on the state of Michigan.  Extending into Illinois, Ohio, Indiana, Wisconsin, and Ontario, this annular pattern outlines the Michigan Basin, a bowl-shaped structure of uncertain origin that contains (about 4,900 m of) inward-dipping Paleozoic strata and a veneer of Jurassic sedimentary rocks.  This mysterious basin is located in the tectonically less active interior of the continent, between the Appalachians and the Rocky Mountains.  It subsided rapidly from Cambrian to Silurian time as it filled with shallow-water marine sediments, some of which host deposits of petroleum, coal, and salt.”

 Geologically, it appears that the basin has in-filled with a sequence of sedimentary rocks that were deposited relatively-shallow water.  As such, it appears that the Basin generally appears to have deepened slowly over geologic time, and was in-filled with the sedimentary blanket that has not been disturbed.  The petroleum (oil and gas) reserves are reported to be most prevalent along the northern portion of the Basin.  Website:  www.tapestry.usgs.gov

  1. Snake River Plain, Idaho

Looking at a Geologic Map of the (lower 48 States) of the United States, there is a long, broad “smile” that extends toward the southwest from Yellowstone National Park, and is about 300 miles long by up to 80 miles wide (see figure, below).  The rocks that are exposed in this area typically consist of many forms of basalt.  Website:  www.imhn.isu.edu

This unique geologic feature is believed to be a series of calderas that formed during the slow, persistent passage of the North American tectonic plate as it passes toward the east-northeast over an essentially stationary “mantle plume”.  Radio-isotope dating of rhyolites taken from the Snake River Plain generally show regularly-spaced calderas, represented by the red circles on the map above.  This trail of calderas leads toward the northeast, terminating with the caldera we see today in Yellowstone National Park.  These eruptions appear to have occurred about 0.63 M, 1.3 M, 2.0 M, 5.4 M, and 8.5 M years ago.  See: http://volcanoes.usgs.gov

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  1. Gold Mines – Placer vs. Lode

Since ancient times, the value and usefulness of gold (chemical symbol Au) cannot be under-stated.  Gold is malleable, conducts electricity, can easily be heated and cast, does not oxidize or corrode, and is usually said to be very pretty to look at.  Gold is used for jewelry, dental devices (crowns), as a foundation for financial workings of countries, for key elements in high-end electronics, and as a skirt on the Lunar Excursion Module (LEM) during the Apollo missions in the 1970’s.  The catch is, there isn’t very much gold to be found world-wide.  Arguments can be made that it was the early Spanish explorer’s search for gold that let European monarchies fund early expeditions that led to the discovery of North America and South America.  Website:  www.blm.gov www.wells.entirety.ca/lode

 Generally speaking, there are two (2) types of gold deposits that are mined, those being a “placer” deposit, which consists of “nuggets” that are usually found within loose, unconsolidated sand/gravel deposits, and “lode” deposits, which are found as “veins” in hard rocks.

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Placer gold deposits have been worked commercially in many states including Colorado, Arizona, California, and most famously – Alaska.  Hard rock, or “lode” mining has been successful on a commercial basis in fewer states including Alaska, Arizona, Nevada, South Dakota, California, and others.  Gold mining has inspired many to take leave of their senses, and lifetimes searching.  Alaska was purchased from Russia for $7.2 million dollars in 1867.  Gold was first discovered in Alaska in the 1872.  Although modern-day production data, along with the corresponding values are tough to come by, the total estimated placer gold production from Alaska between 1880 and 1957 is about 28.9 million ounces, or about 903 tons. With a modern-day value of about $1,750 per ounce, that is worth $49 billion.

 And gold mining has worked its way into our modern-day vernacular, in terms of “searching for the Mother Lode”, for instance, which is now generally referred to as an area of gold deposits at the base of the Sierra Nevada range, covering parts of seven (7) counties in California.

  1. Adirondack Mountains, New York

The Adirondack Mountains are located in north-central to northeastern New York, are about 150 miles in diameter (see orange area in figure, below).  Rather than having a linear orientation, much like the Rocky Mountains, the Alps, or Himalaya, the Adirondacks are referred to as having a “dome” shape.  Although they are located along the inland (western) side of the Appalachian Mountains, the Adirondacks have little in common with them.

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Collectively, the rocks that are exposed in the Adirondacks first formed more than 1 billion years ago, long before the Atlantic Ocean formed between North America and Europe.  Although the rocks themselves cooled from deep magma chambers, the uplift that pushed them above the surrounding plains and plateaus happened about 5 to 10 million years ago.  New York State describes the Adirondacks as “new mountains from old rocks”.

Website:  www.adirondack-park.net

The central “core” of the mountains is made of several versions of gneiss, a hard, dense metamorphic rock that formed by the re-heating of sedimentary rocks at a depth of about 10 to 15 miles.  Modern-day weathering includes “exfoliation” where sheets of rock have been exposed by the release of pressure.  The Adirondack Mountains have also served as a source rock for glacial erratics found to the south.  Lastly, the rocks in the center of the Adirondacks are comprised of anorthosite, which is also found extensively on the Moon.

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  1. Western Interior Seaway

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During Cretaceous time, which extended from about 66 to 144 million years ago (Ma), there was a long, relatively shallow inland seaway that extended to the north from the modern-day Gulf of (see figure), and is generally referred to as the Western Interior Seaway.  The stratigraphic sequence of rocks included a thick massive sandstone that holds fossilized bones of the reptilian megafauna – Dinosaurs.

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This was a land and time of sauropods, triceratops, and the tyrannosaur.  Ichthyosaurs swam and pterosaurs soared.  To correct a popular misconception about “Jurassic Park”, these large dinosaurs had not yet evidently developed into their body size or range at that time.

This ecosystem was preserved in series of sedimentary rocks along the modern-day Utah – Colorado border, along the Green River.  To preserve a dinosaur, a specific series of events needs to occur in a proficient, timely manner.  Following the death of a dinosaur, the soft tissue (muscle and internal organs) decomposes.  Usually scavengers use this opportunity to feed and typically a skeleton would be disassembled.  But not every time.  If the animal died in a location where sediment builds up before it is discovered, the skeleton may remain intact.  As sediments accumulate, the pressures at depth will rise.  Eventually the sediments undergo a lithification process, and turn into rock.  Certain types of sedimentary rock preserve skeletons better than others.  Shale, for example formed from mud, and has a very fine texture, while sandstone forms from lithified sand grains.  During lithification the bones and teeth undergo mineralization when the calcium is replaced with new, typically harder minerals.  So far this process had taken place at depth.  But for the skeleton to be discovered in modern times, it has to be exposed, typically through uplift and erosion.  What we know about dinosaurs is a result of the natural process as much as it was that someone was lucky enough to find it.  What is shown in the photo below is an assemblage of bones that came apart shortly after the dinosaur died.  This type of event probably happened after a catastrophic storm and/or mudslide, either above ground or under water that both killed and buried the animal.

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Quarry at Dinosaur National Monument, Utah (photo dated 1959)

The Time of the Dinosaurs came to a very abrupt end, apparently with the impact from a large meteorite on the modern-day Yucatan Peninsula that altered the climate drastically.

Website:  www.fossilmuseum.net

  1. North America’s Impact CratersScreen Shot 2014-06-27 at 12.34.51 PM.png

Since the proto-earth cooled and formed around the Sun, asteroids, comets, and meteorites have been running into us, leaving scars, cracks, and marks on every continent, with North America being no exception (see map, right).  One of the best-preserved impact craters is in the desert southwest, near Flagstaff, Arizona (below, left).  The crater measures about 550 feet deep by 3,900 feet wide.

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Barringer Crater, Flagstaff, Arizona Chesapeake Bay, Virginia

About 3,900 feet wide | About 20 miles wide

During the impact of the meteor that formed the Barringer Crater, so much energy was released that the quartz sand grains in the bedrock melted and re-formed with mis-aligned mineral lattices.  “Stishovite” is found ONLY around impact craters, and is not believed to be associated with the object, but rather re-formed ejecta from the bedrock beneath the crater.

Websites:  www.nasa.gov     www.meteorcrater.com     www.webmineral.com/data/stishovite


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The larges impact crater yet found is in Quebec, Canada, and was so large that it was first discovered after the first space-based photographs of the earth were processed (false-color image, left).  The Manicougan Crater is about 40 miles in diameter, and appears to have deformed the crust of the earth in such a way that drainage patters were altered and repeated sequences of continental glaciation have not yet erased the crater from view.  It also appears to be one of, if not the oldest known impact craters, having formed about 215 million years ago, when the Atlantic Ocean had not yet opened.

  1. La Brea Tar Pits, California

The La Brea Tar Pits are located in western Los Angles, California, between Inglewood and Beverly Hills, a very urban setting for such a geological wonder.  They were first discovered by early Spanish explorers and settlers in the 1700’s in a large plain at the southern end of the Santa Monica Mountains.  Rather than pools of spring water, these pits have pools of heavy, black dried crude oil that resembles asphalt.  But the real intrigue is what is in the asphalt – BONES, and lots of them.  Keep in mind that “brea” is Spanish for “tar”.

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About 650 species have been identified at La Brea including plants and animals.  But what most people are intrigued by are the skeletons from the large mammals from the Pleistocene, which extended from about 12,000 to 40,000 years ago, during the last glacial period.  Fossils from smilodon (saber-tooth cat, previous page), ground sloth, dire wolf, camel, steppe bison, mastodon (below, left), which are all extinct, along with rodents, antelope, bear, and horse have all been found here, along with song birds, vultures, shore birds, eagles, and owls.  Unexpectedly, there have also been several human bones and primitive tools encountered in the excavations.

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The source of the “tar” is a crack, or “fissure” in the ground surface where crude oil oozes up to the ground surface.  Lighter fractions in the crude oil evaporate, leaving behind a thick, gooey pool of asphalt that from a distance looks like water.  Over time, hundreds of thousands of animals have become stuck in the oil, and die.

Websites:  www.tarpits.org http://science.howstuffworks.com.

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Thank you for taking the time to learn more about the earth and the geological processes that happen here each and every day.

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