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 |
Volcanic Highlights and Features:
[This list is just a sample of
various Illinois volcanic features or events and is by no means inclusive.]
The state of Illinois is underlain by approximately 80,000 cubic miles of
sedimentary rocks.
These include about 50% limestone, 25% sandstone
and the remainder consisting of shale, siltstone, and minor amounts of coal.
These rocks were deposited during the Cambrian, Ordovician,
Silurian, Devonian, Mississippian, and Pennsylvanian Periods of the Paleozoic Era.
There are a couple areas in the state where there are only
small deposits of Cretaceous age and younger rocks.
The Cambrian through Mississippian sedimentary rocks were deposited
in a shallow sea when the North American continent was near the
equator. The Pennsylvanian rocks were deposited in a more
terrestrial shoreline environment when the state was covered periodically with 1) a
large river delta, 2) swamps in which plant material
accumulated to great thicknesses and eventually turned to coal, and 3) shallow seas.
Volcanic Rocks Beneath the Surface:
Excerpts from: Illinois State Geological Survey Website, July 2001 |
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The Interior Plains:2
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Proterozoic:1
Cambrian:1
Ordovician:1
Silurian:1
Devonian:1
Mississippian:1
Pennsylvanian:1
Cretaceous:1
Tertiary:1
Quaternary:1
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| Illinois's Volcanic Rocks |
Volcanic Rocks Beneath the Surface:1
There is relatively little known about these rocks
from direct observations because they are not
exposed at the surface anywhere in Illinois.
Only about 35 drill holes have reached
deep enough for geologists to collect samples
from Precambrian rocks of Illinois.
From these samples, however, we know that
these ancient rocks consist
mostly of granitic and rhyolitic igneous,
and possibly metamorphic, crystalline rocks
formed about 1.5 to 1.0 billion years ago.
| Glacial Erratics |
Volcanic Rocks from Canada:1
Here and there in Illinois are boulders lying
alone or with companions in the corner of a
field or someone's yard, on a courthouse
lawn or a schoolyard. Many of
them -- colorful and glittering granites,
banded gneisses, and other intricately
veined and streaked igneous and
metamorphic rocks -- seem out of place in
the stoneless, grassy knolls and prairies of
our state. Their "erratic" occurrence is the
reason for their interesting name.
These exotic rocks came from Canada
and the states north. The continental
glaciers of the Great Ice Age scoured and
scraped the land surface as they
advanced, pushing up chunks of bedrock
and grinding them against each other or
along the ground surface as the rock-laden
ice sheets pushed southward. Sometimes
you can tell where the erratic originally
came from by determining the kind of rock
it is. A large boulder of granite, gneiss, or
other igneous or metamorphic rock may
have come from Canada.
| Hicks Dome |
Hicks Dome:1
Most geologists who have studied Hicks Dome
interpret it as the product of one or more
underground volcanic explosions.
Drilling into the core of Hicks Dome reveals greatly shattered
sedimentary rocks, intermixed with igneous material.
Small igneous dikes radiate from the
center of Hicks Dome on the surface.
Excerpts from:
1) Illinois State Geological Survey Website, 2001
2) USGS/NPS Geology in the Parks Website, August 2001
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