America's Volcanic Past -
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| "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 |
MORE America's Volcanic Past - Crater Lake Caldera (Oregon) Eruption
MORE America's Volcanic Past - Hagerman Fossil Beds (Idaho) Eruption
MORE America's Volcanic Past - Long Valley Caldera (California) Eruption
Volcanic Highlights and Features:
[This list is just a sample of
various Nebraska volcanic features or events and is by no means inclusive.]
Excerpt from: |
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The Interior Plains:6
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| 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 Crater Lake:2
The caldera now filled by Oregon's
Crater Lake
was produced by an eruption that destroyed a volcano the size of
Mount St. Helens
and sent volcanic ash as far east as Nebraska.
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
2)
Wright and Pierson, 1992,
Living With Volcanoes, The
U.S. Geological survey's Volcano Hazards Program:
USGS Circular 1973
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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