Uncategorized – Joshua R Benton

August 15, 2014Earth Science, Geology, Uncategorized

Grand Canyon – The Precambrian

The Earth is very very very old. Roughly 4.54 billion years old, or 4,540 million years old, or 4,540,000 thousand years old (however you would like to think of it). Prior to the development of radiometric dating to give us numbers and absolute dates we mostly used fossils and other principles to obtain relative dates of rocks. So all of our subdivisions of time are based upon the fossil record. The Precambrian refers to an immense span of time (4,540 million years ago to 542 million years ago) where the rocks contain very few fossils due to the fact that organisms never developed hard parts that could be fossilized until later in Earth’s history.

Zoroaster Granite and Vishnu Schist

At the bottom of the Grand Canyon we find rocks that formed during the Precambrian, two of which are the Zoroaster Granite and Vishnu Schist. In general a granite is defined as an igneous rock that forms from the crystallization of magma while a schist is a metamorphic rock that forms when shale is subjected to heat and pressure.

These rocks formed roughly 1,700 million years ago when a tectonic collision occurred between an older section of our continent that stretches from Southern California to Wyoming (dubbed Wyomingland) collided with a volcanic island arc. This collision of Earth’s plates provided the heat and pressure to morph the shale that formed the Vishnu Schist while producing the magma that formed the Zoroaster Granite. The Zoroaster literally intruded into the Vishnu as magma before cooling and solidifying to become a rock itself. We came across the first exposures of these rocks within the inner gorge of the canyon along side Bright Angel Creek:

This is a spectacular example where you can see huge sections of the Vishnu schist that broke off into the magma chamber before the magma cooled and solidified locking them into the place they are now.

By observing that there are inclusions of the Vishnu inside of the Zoroaster you can determine which rock is older than the other. The Vishnu must have been there first for the magma to intrude into it and break off pieces.

The Great Unconformity

Further upstream we got some impressive views of an interesting natural phenomena called an unconformity. In fact, this is known as the Great Unconformity which is represented here by an angular unconformity. The Great Unconformity was first identified in the 1800s by John Wesley Powell, a one-armed civil war vet who was the first person to lead an expedition down the Colorado River and later became the second director of the USGS. Here is a picture of the unconformity:

Can you spot it? Does it help that Kristi is pointing directly to it? Clearly there are beds of tilted rock dipping towards the right but are truncated at the top by horizontal bedding of a different rock. What in the world would cause this to happen? These are two different types of sedimentary rocks sitting right on top of each other at different angles. Would natural processes deposit the original sediments like this right on top of each other? The answer is no. The two rocks are not conformable. The “line” separating the two is an erosional surface which represents a gap in time that we call an unconformity. The rock unit on top is known as the Tapeats Sandstone that’s roughly 520 million years old while the unit on the bottom is known as the Dox Formation that’s roughly 1,120 million years old so there’s somewhere around 600 million years of missing time in between the two.

Here’s how this works:

Pretty cool right? Unconformities are found throughout the Grand Canyon and all throughout the world. Sometimes they are hard to identify but this one stands out as a prominent feature along the rim of the inner gorge.

April 7, 2014Uncategorized

Monday Mineral – Carnotite

Carnotite (Uranium Vanadate) – K2(UO2)2(VO4)2·3H2O

Credit: University of Wisconsin - Dept. of Geosciences — Credit: University of Wisconsin – Dept. of Geosciences

I’m going kick off this new mineral themed weekly blog post by talking about the mineral Carnotite (Named after the chemist M. A. Carnot (1839-1920)). This is a bright yellow radioactive ore composed primarily of uranium, vanadium, and oxygen. What causes a mineral to be defined as an ore is determined by whether or not you can mine it at a profit. Therefore a mineral’s status as an ore can change depending upon economic shifts and changes within the local and global market.

Carnotite had it’s “claim to fame” in The United States during the Cold War when the Atomic Energy Commission started to make high demands for uranium, one of Carnotite’s primary constituents, . Why would the federal government want to stock up on uranium? The answer might be quite obvious. Some Nuclear fission weapons, like the one dropped on Hiroshima, require the use of the isotope Uranium-235 (enriched from the more commonly occurring crude Uranium-238). The Uranium atom has the highest atomic mass therefore it releases more energy whenever the atom is split in a nuclear reaction.

The Little Boy Uranium Bomb. Credit: Carey Sublette - University of North Carolina — The Little Boy Uranium Bomb. Credit: Carey Sublette – University of North Carolina

Reading Edward Abbey’s autobiographical book “Desert Solitaire” inspired this blog post. Carnotite commonly occurs as a dust in sandstone and is found in abundance around the Colorado River and in the canyon lands of Utah where Abbey worked as a Park Ranger during the 50s. In the book he speaks in great detail about how the presence of Carnotite led to a mining boom in the region surrounding Moab, Utah during the Cold War as prospectors and corporations poured into the region hoping to get rich off of the land’s natural resources. By the end of the Cold War the demand for Carnotite also dwindled leaving the town’s larger population without the industry to support it. This summer I’ll be visiting Moab so I hope to learn more about this subject then and maybe catch a glimpse of one of the larger mines built to extract the once precious ore.

January 22, 2014Uncategorized

The Conococheague Formation

During a small window of nice weather earlier this week I decided to drive around and observe some of the various limestone formations exposed within the Shenandoah Valley, while at the same time getting to see the Virginia State Arboretum for the first time (because who doesn’t like arboretums?). My first stop was at Mulberry Run just north of Strasburg to view an exposure of the Conococheague Formation along Route 55. This is an early Cambrian deposit of limestone that formed roughly 520 million years ago.This outcrop has a lot of interesting features. It has interbedded dolostone (light gray) and limestone (medium gray).

The limestone contains spherical grains called ooids. Ooids form in shallow warm tropical seas like the Bahamas under the influence of wave action. Here is a close up of these ooids looking through my hand lens:

This is truly a window into a different world! The Appalachian mountains have not yet come into existence and the south east portion of our continent is engulfed in a shallow sea. Within this sea are mats of cyanobacteria that form large dome shaped colonies called stromatolites. Here is one of the stromatolites you can find at Mulberry Run:

Here’s a closer look: Below is a picture that I borrowed from sharkbay.org of living stromatolites in Western Australia:

December 27, 2013Uncategorized

Hebgen Lake Earthquake (1959)

On Aug 17, 1959, Hebgen Lake was hit by one of the largest and most destructive earthquakes in US history. This magnitude 7.5 quake caused 28 fatalities and millions of dollars in property damage; most of which was the result of a massive landslide triggered by the sudden movement of the Earth’s crust.

The Northern Rockies are famous for their superb fishing destinations and beautiful scenery. So much so that when I traveled there earlier this year I found myself to be surrounded by fisherman on the plane, hearing stories of giant trout and stunning mountain vistas before I even touched ground in Montana. The person sitting next to me, a middle aged businessman from New Jersey, explained to me that August and September are the BEST months to go fly fishing in Montana as the weather is cooling down and the fish are returning to shallower waters. I suddenly thought this may be one of the reasons the 1959 quake was so deadly, because on the night of the quake dozens of people were camped along the Madison river just west of the lake, many of whom were fisherman just like my friend here on the plane.

Hebgen Lake sits directly northwest of the Yellowstone caldera close the border of Wyoming and Montana. It is fed by the Madison River, a tributary of the Missouri, which flows towards the east.

[googlemaps https://maps.google.com/maps/embed?pb=!1m18!1m12!1m3!1d12678665.111120671!2d-99.11762847812518!3d39.12064664263646!2m3!1f0!2f0!3f0!3m2!1i1024!2i768!4f13.1!3m3!1m2!1s0x5351b4a13c99d331%3A0x61c352040faebc79!2sHebgen+Lake!5e0!3m2!1sen!2sus!4v1388167881891&w=600&h=450]

Using seismograph data from the University of Utah I placed a marker showing the approximate location of the epicenter of the fatal earthquake.

Soon after the earthquake, a section of rock 3 quarter miles wide slid from the south wall of the Madison Canyon into the river immediately displacing all the water and air in its path. The incredible momentum of the landslide sent 30 foot tall waves and gale force winds roaring up and down the canyon. Below is a picture we took of landslide scar looking towards the south standing on top of the natural dam.

Immediately after the landslide completely dammed the Madison River water began to rise upstream forming another lake called Earthquake Lake.

You may be wondering by now, what caused a massive earthquake in the middle of the Rocky Mountains? Hebgen Lake is surrounded by a system of active normal faults that are part of the Basin and Range Extension, the literal stretching of the Earth’s lithosphere in the western United States driven by plate tectonics. The fault movement in this area is also thought to be connected to a larger seismic chain originating from the Yellowstone Hotspot (Yellowstone geology will have to be covered in a different post).

A “normal fault” is a fault where one side moves in a downward motion relative to the other. This type of motion is responsible for much of the landscapes you see out west, often creating vertical cliffs on the Earth’s surface called a fault scarp. Here is a picture of the scarp that appeared after the Hebgen Lake Earthquake:

scarp — Credit: McGregor, K. Abston, Carl, U.S. Geological Survey

Prior to the 1960’s there were no significant attempts in measuring seismic activity in the Hebgen Lake area, however; this changed after the 1959 quake. Since then there has been seismographs setup to record data, along with surveys and mapping done to further understand the structural geology of the region. Events like this should be a stark reminder to us that the Earth’s surface is a constantly changing environment and fault movement in North America is nothing new but a process that has been ongoing for millions of years.