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My Nevada 5: The Dramatic Events That Shaped Our Land
This piece comes from geology professor Steve Rowland. He studies the history of life on Earth as recorded in the fossil record, especially the paleontology of Southern Nevada and adjacent regions. His research ranges from the earliest (late pre-Cambrian) animal fossils, to Jurassic dinosaur tracks and those of co-existing animals in Red Rock Canyon and Valley of Fire State Park, to Ice Age fossils of the Tule Springs area. He also studies the history of geology, particularly 18th century Russian geology.
In the early 19th century, geology was a new science attempting to establish a firm foundation of first principles amid the shifting sands of ignorance and legend. A philosophical debate emerged, centered on the importance of catastrophic events. Some natural philosophers, led by a Frenchman named Georges Cuvier, argued that the most important processes in Earth history were catastrophes, such as floods and earthquakes.
Those on the other side of the debate, led by an Englishman named Charles Lyell, countered that the real story in Earth history was the action of slow, relentless processes such as erosion and deposition. For Lyell, floods and earthquakes were too 'biblical.' He didn't deny that such events occasionally occurred, only that they were not the main events to which geologists should pay close attention.
Lyell wanted to establish a methodology that called for the careful observation of ongoing, day-to-day, geological processes and then use those observations to unravel the details of Earth history. If observable, ongoing processes were not the fundamentally important processes in Earth history, Lyell argued, geologists wouldn't have anything to do - they may as well put their rock hammers away and read the Bible to learn about the history of the Earth. Lyell and his disciples won the day. They effectively buried the catastrophist philosophy in the graveyard of discarded ideas.
In recent decades, however, Cuvierian catastrophism has risen from the grave. Geologists now recognize that catastrophic events have indeed played an important role in shaping the Earth. Earth history is now seen as a combination of boring, gradual, ongoing processes, punctuated by rare, terrifying catastrophes. Both types of processes have profoundly influenced the geological history of Nevada, as well as its biological and cultural history.
Here are five events that helped shape the features of the Silver State. You can decide for yourself which of these are Cuvierian catastrophes and which are Lyellian gradualistic events. It largely depends on your sense of time.
1. The Great Nevada Meteor Smack Down
In the Devonian Period, more than 360 million years ago, Nevada was a low-relief continental shelf with a lot of marine creatures living in a warm, shallow sea. One day, like a bolt of lightning out of the blue sky, a large meteor traveling faster than 20,000 miles per hour smacked into Nevada, striking about 100 miles north of where Las Vegas sits today. Like a cannonball landing in a kiddie pool, the hypervelocity impact made a spectacular splash, launching a series of devastating tsunamis that rippled across Nevada, ripping up chunks of sea floor. The resulting layer of jumbled rock is called the Alamo Breccia. Ironically, ground zero was close to Rachel on Nevada State Highway 375 - nicknamed the Extraterrestrial Highway.
In the photo, the jumbled rocks and fossils of the Alamo Breccia in Lincoln County offer evidence of the tsunami; the quarter is lying on a fossil sponge. [Photo by Steve Rowland]
2. The "Big Squeeze" that Pushed the Seafloor 12,000 Feet into the Sky
A few tens of millions of years after the Alamo Breccia meteor impact, all of the continents assembled together to form a supercontinent that we call Pangea. This supercontinent remained intact for a while longer, but it eventually began to separate into the continents that we know today.
Many aspects of the geology of Nevada are associated with the disintegration of Pangea. As North America drifted away from the other continents, a portion of the floor of the Pacific Ocean was forcefully pulled beneath Nevada and eastern California, causing intense compression along the western margin of the North American continent. Huge slabs of rock, each thousands of feet thick, were shoved eastward on top of one another, like a load of plywood in a train wreck. Fossil-rich limestones that had been deposited on the shallow seafloor were lifted into the sky. Charleston Peak, for example, is composed of fossiliferous marine limestones that were thrust nearly 12,000 feet above sea level.
Pictured is Kyle Canyon and the crest of the Spring Mountains. Charleston Peak is the peak on the left, and Mummy Mountain is on the right. All of the rocks in view are fossil-rich, shallow-marine limestones that were shoved eastward and upward during the "big squeeze." [Photo by Steve Rowland]
3. The Eruption of the Hoover Dam Caldera
Fourteen million years ago a volcanic caldera violently erupted in the vicinity of present-day Hoover Dam, sending a column of ash tens of thousands of feet into the sky. At the same time -- and fulminating from the same orifice -- a ground-hugging cloud of dense, hot ash roared across the startled terrain at a speed of a couple hundred miles per hour. It killed every living thing in its path and buried the scorched landscape in a blanket of volcanic particles several hundred feet in thickness. The resulting layer of tightly welded volcanic ash forms the walls of Black Canyon, into which Hoover Dam is anchored. If that eruption had not occurred, the recent human history of Southern Nevada would have been much different.
The Nevada wall of Black Canyon is pictured just below Hoover Dam. The pinkish rocks are rhyolitic ash-flow tuffs that erupted from the volcanic caldera 13.9 million years ago. The disrupted dark band (composed of basaltic andesite) that cuts through the pinkish rock is called a sill. The dark rock is the result of magma being forcefully intruded into the ash. It is the product of another eruption, 1.2 million years later, from a nearby volcano. Multiple conspicuous faults subsequently sliced up the thick pile of consolidated volcanic ash. [Photo by Steve Rowland]
4. The Sierra Nevada's Pulse of Uplift
Southern Nevada lies within the Mojave Desert, while central and northern Nevada lie within the colder, higher Great Basin Desert. Both of these desert ecosystems owe their existence to the Sierra Nevada Mountain Range, pictured . As moisture-rich air masses travel eastward from the Pacific Ocean, they cross the high-elevation Sierra Nevada and lose much of their moisture. This Sierra Nevada 'rain shadow' formed about 3.5 million years ago, when the mountain range experienced a rapid pulse of uplift. If that uplift had not occurred, Nevada would be wet and forested today.
Pictured is the eastern face of the Sierra Nevada mountain range. This is Mt. Williamson (elev. 14,389 feet), with Mt. Whitney (elev. 14,505 feet) in the distance on the left, as viewed from the Owens Valley, near Independence, Calif. [Photo by Peter Starkweather]
5. The Extinction of Nevada's Ice Age Megafauna
During the Pleistocene Ice Age, which ended just 10,000 years ago, Nevada was populated by a diverse fauna of large animals - mammoths, camels, bison, horses, giant ground sloths, saber-tooth cats, and dire wolves - among other quadrupeds. The majority of these magnificent animals became extinct amazingly abruptly, over an interval of just a few hundred years, resulting in a dramatic transformation of North America's ecological fabric.
Columbian mammoths, in this illustration by Kelly Lance, and other large Ice Age mammals were probably important dispersal partners for some Mojave Desert plants, such as Joshua trees. The extinction of these animals disrupted the fabric of the Mojave Desert ecosystem in ways that scientists don't yet completely understand.
Human hunters arrived in North America shortly before most of the extinctions occurred, but we do not yet understand the degree to which these two events are linked, if they are linked at all.
Working its way through Congress right now is a bill that will create Tule Springs Fossil Beds National Monument along the northern edge of Las Vegas Valley. This unit of the National Park Service will be dedicated to the study and interpretation of Ice Age animals, providing an opportunity for all of us to learn about one of the most recent dramatic events that shaped the Nevada landscape.
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