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America's Volcanic Past
Nebraska

"Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994
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Map, Location of Nebraska

Volcanic Highlights and Features:
[NOTE: This list is just a sample of various Nebraska features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of Nebraska.]

  • Nebraska
  • Nebraska Regions
  • Nebraska's Volcanic Rocks
  • Agate Fossil Beds National Monument
  • Ashfall Fossil Beds State Historical Park
  • Volcanic Ash Deposits

Nebraska



Excerpt from:
   
Nebraska Regions

The Interior Plains:6
The Interior Plains is a vast region that spreads across the stable core (craton) of North America. This area had formed when several small continents collided and welded together well over a billion years ago, during the Precambrian. Precambrian metamorphic and igneous rocks now form the basement of the Interior Plains and make up the stable nucleus of North America. With the exception of the Black Hills of South Dakota, the entire region has low relief, reflecting more than 500 million years of relative tectonic stability.


   

Nebraska's Volcanic Rocks

Nebraska's Bentonite Beds:8
Silicified chalk, sometimes referred to as flint, has been found in large quantities in the Smoky Hill Chalk member of the Niobrara Formation of Cretaceous age that is exposed along the Republican River in south-central Nebraska. The fact that there are many bentonite (altered volcanic ash) beds in the Niobrara Formation suggests that the source of silica for these rocks was volcanic ash.

Nebraska's Blue Agate:8
Blue Agate, the Nebraska State Gem, has been found in place in wind-deposited claystones in the Chadron Formation of Oligocene Age in Sioux and Dawes counties. These gems have been found in colors other than blue and the large oval stone is a doublet with a blackened back to highlight the plumes in this material. The chalcedony probably originated from silica that was freed when devitrification (changing from a glassy to a crystalline state) of wind-blown volcanic ash took place. The chalcedony appears to have formed in or near sources of alkaline water.

Nebraska's Flint:8
Silicified chalk, sometimes referred to as flint, has been found in large quantities in the Smoky Hill Chalk member of the Niobrara Formation of Cretaceous age that is exposed along the Republican River in south-central Nebraska. The fact that there are many bentonite (altered volcanic ash) beds in the Niobrara Formation suggests that the source of silica for these rocks was volcanic ash.

Nebraska's Lake Superior Agates:8
The most popular gems from Nebraska's glacial deposits are probably the Lake Superior Agates. These originated in basalt flows of Late Precambrian age that are now exposed along the North Shore of Lake Superior. The agates actually derived their name from the Lake Superior Till, a glacial deposit that is exposed in much of eastern Minnesota and is known for both the quantity and quality of agates it has yielded to collectors.

Nebraska's Orbicular Jasper:8
Orbicular jasper forms when a silica rich rhyolitic ash flow cools quickly. Quartz and feldspar crystallize in spherulites, radial aggregates of needle like crystals, that provide the interesting structure seen in this kind of jasper. Better known examples of orbicular jasper are often seen offered as Poppy Jasper or Morgan Hill Jasper from California or Ocean Jasper from Madagascar. The Nebraska stone has a similar structure to these latter varieties.

Nebraska's Thunder Eggs:8
Thunder eggs are agates that have formed in welded ash-flow tuffs, whereas amygdaloidal agates have formed in tholeiitic basalts. Most of the Lake Superior Agates that have been found in Nebraska are the almond-shaped amygdaloidal agates, but occasional specimens are cores from thunder eggs which, are characterized by the squarish outline on the polished surface and raised ridges on the outer surface. Some thunder eggs may have originated near Grand Marais, Minnesota, where a bed of thunder eggs is known, and have been transported by Nebraska by glaciers of Pleistocene age.




Agate Fossil Beds National Monument

Agate Fossil Beds:4
About 19-20 million years ago a drought occurred in the plains of western Nebraska. Deprived of food, hundreds of animals died around a few shallow waterholes. Over time the skeletons were buried under silt, fine sand, and volcanic ash, carried by the wind and reworked by the streams. A large fossilized waterhole with hundreds of skeletons is preserved today in the Niobrara River valley at Agate Fossil Beds National Monument.

The Role Of Volcanic Ash at the Agate Fossil Beds:4
Millions of years before the drought took place, sediments from eroding mountain ranges to the west were laid down to form the bed of a shallow sea during the "Age of Dinosaurs". About the time of the extinction of dinosaurs, the Rocky Mountains were developing into ranges we know today, the ocean receded, and tropical lowlands occupied the region that today is the Great Plains. As time passed, the climate of North America became cooler and drier, and volcanic activity in the western United States produced enormous amounts of volcanic ash that was flown eastward. Ash-mantled plains were home to great herds of plant-eating mammals and their predators. Like the savannas of east Africa today, the rich volcanic soils supported grasses, which, together with small trees and bushes growing along shallow streams, were a ready food source for grass- and leaf-eaters. As time passed, however, the climate became more arid. To the west, the Rocky Mountains continued to rise, and the flow of moisture-laden air from the west was interrupted. With less rain came plants that could survive on less water. Drought became commonplace. Hundreds and thousands of some species died, littering the area around and in the waterholes with their remains. In time, the rains returned, the streams filled, and the process of burial began. Silt, sand, and ash covered the remains, burying them under several feet of wind- and stream-transported sediment.




Ashfall Fossil Beds State Historical Park

Ashfall Fossil Beds State Historical Park:7
Some 10 million years ago, hundreds of rhinos, three-toed horses, camels and other animals died and were buried by volcanic ash around the edges of a watering hole in what is now northeast Nebraska. Still locked in their death poses, the amazingly well-preserved skeletons of these prehistoric beasts lay undisturbed, wrapped in a blanket of jagged glassy particles, until the 1970s, when scientific study of the fossilized remains began. Located 6 miles north of U.S. 20 between Royal and Orchard in northern Antelope County, Ashfall Fossil Beds State Historical Park is a joint project of the Nebraska Game and Parks Commission and the University of Nebraska State Museum. The park offers a fascinating and educational experience for the entire family a chance to step back in time and see what Nebraska wildlife was like long before modern man ventured onto the Great Plains. Visitors are invited to watch the ongoing excavation of this unique "time capsule." A 2,000 square foot "Rhino Barn" protects part of the deposit, where skeletons are uncovered and displayed exactly where they are found. Walkways give visitors a close-up view as paleontologists carefully brush away the volcanic ash from the massive skulls of native American rhinos and the delicate side hooves of tiny ancestral horses.

Volcanic Eruption in Idaho (?):7
Sweeping across the plains like a gray blizzard, the sudden fall of volcanic ash must have devastated the landscape. Confused and choking, the animals began to die. Scientists believe that the ash that killed and eventually buried the animals at the park blew eastward from an incredible volcanic eruption in the Rocky Mountains, probably in what is now southwestern Idaho. Part of the great cloud of abrasive dust settled out to a foot or so deep over much of northern Nebraska, then it began to blow around like fresh snow. Eventually the high ground was blown free of ash, but low-lying areas like the marshy pond at the Ashfall site were filled to depths of eight feet or more.




Volcanic Ash Deposits

Nebraska's Volcanic Ash Deposits:1
Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits. A thick ash deposit sandwiched between layers of sandstone in Nebraska, the massive granite peaks of the Sierra Nevada mountain range, and a variety of volcanic layers found in eastern Maine are but a few of the striking clues of past volcanism.

Volcanic Ash from Idaho:5
Eleven million years ago (Pliocene), the Bruneau-Jarbridge eruption south of Hagerman deposits ash as far east as Nebraska. (See Ashfall Fossil Beds State Historical Park above)

Volcanic Ash from Long Valley:3
About 760,000 years ago a cataclysmic volcanic eruption in the Long Valley, California area blew out 150 cubic miles of magma (molten rock) from a depth of about 4 miles beneath the Earth's surface. Rapidly moving flows of glowing hot ash covered much of east-central California, and airborne ash fell as far east as Nebraska.




Excerpts from:
1) Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication
3) Hill, et.al., 1996, Living With a Restless Caldera -- Long Valley, California: USGS Fact Sheet 196-96
4) U.S. National Park Service Website, Agate Fossil Beds National Monument, 2000
5) U.S. National Park Service Website, Hagarman Fossil Beds National Monument, 2001
6) USGS/NPS Geology in the Parks Website, 2001
7) Nebraska Game and Parks Commission Website, 2001
8) Conservation and Survey Division, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln, Website, 2002

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04/26/07, Lyn Topinka