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 Iowa volcanic features or events and is by no means inclusive.]
Excerpts from: |
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The Interior Plains:7
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Geologic History:1
Bedrock Outcrops:1
Iowa's Precambrian:1
Manson Impact Structure:1
Present Land Surface:1
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| Iowa's Volcanic Rocks |
Iowa's Ash Deposits from Yellowstone Caldera:5
Ash deposits
from these powerful eruptions
have been mapped as far away as
Iowa, Missouri, Texas, and even
northern Mexico.
Volcanic Rocks Beneath the Ground:1
Iowa's bedrock geology map shows rocks
from younger periods overlapping older rocks.
Most of the rock units are dipping gently to the southwest,
and this bedrock structure,
coupled with a long history of surface erosion,
contributes to the irregular bedrock surface
crossing rock units of different
ages.
Two small, but noteworthy features interrupt
this general bedrock pattern. The first is in
the far northwest corner of Iowa, where an ancient ridge of
silica-cemented sandstone pokes to the land surface.
At 1.6 billion years of age (Precambrian),
these scattered outcrops of hard, reddish Sioux
Quartzite are the oldest bedrock exposed anywhere in Iowa.
Elsewhere beneath the state, the Precambrian rocks
are usually igneous and
metamorphic types, and they lie deeply
buried by the thick sedimentary strata.
The second irregularity in the bedrock surface
is a 23-mile-diameter circular feature known
as the Manson Impact Structure, located primarily in
Pocahontas and Calhoun counties. Here,
a meteor impact 74 million years ago caused
massive disruption of the Cretaceous bedrock and older strata
beneath, an event so forceful that
it brought
deeply buried Precambrian granite rebounding
to the land surface.
The resulting crater and its highly
faulted, distorted rocks are covered with over
100 feet of glacial deposits, so are not visible on today's landscape.
| Des Moines - State Capitol Building |
State Capitol Building6
The state capitol building in Des Moines,
constructed from 1872-1884 from a
variety of building stones,
is a spectacular example of late
19th-century stone construction.
The granite base
was secured from Buchanan County boulders and quarries in Minnesota.
Limestone blocks comprising the foundation
and lower levels were quarried in Iowa
at locations in Johnson and Madison counties.
The bulk of the exterior was
constructed from sandstone blocks
derived from quarries in Missouri. Additional stone,
both local and imported,
was used in the interior construction,
including a number of decorative marbles.
| Glacial Erratics |
Glacial Erratics:2
Glacial erratics are boulders of igneous and metamorphic rock,
native to geographic regions well north of Iowa,
and carried into Iowa by glacial advances over 500,000 years ago.
They were concentrated at the land surface by later erosion,
which removed the fine-grained deposits once surrounding them.
Glacial erratics in Iowa are not difficult to identify.
The vast majority are igneous or metamorphic rocks,
rather than the usual sedimentary rocks of sandstone,
limestone, dolomite, and shale that constitute the
bedrock under most of Iowa. If you pick up a granite rock,
composed of interlocking crystals of
pink feldspar and glassy quartz,
you can be sure it is not native and that it
came from outside the state, most likely carried by glacial ice.
Glacial Erratics from Minnesota:4
Glacial erratics range in size from pebbles to giant boulders.
The greatest number of giant erratics are seen on the Iowan Surface of
northeastern Iowa. They were described in a 1970 Iowa Academy of Science
article by Drake University professors Richard Dirks and
Carl Busch, who noted that
80 percent of the giant boulders had a
similar composition, a light-colored, coarse-grained granite.
They
concluded from the boulders' composition and the direction of
glacial striations on the underlying bedrock surface that these erratics
probably originated in central and west-central Minnesota.
Glacial Erratics in Gull Point State Park:3
Large boulders are concentrated along
the shoreline at Gull Point State Park. These travel-worn "erratics," carried into Iowa by glacial
ice, are usually igneous or metamorphic rock
types, which are not native to Iowa.
Reddish quartzite, granite
and other crystalline rocks are
common, and they reflect the massive power
of glaciers and the northerly direction
from which they came. These boulders are an
impediment to agriculture, and in the
Gull Point vicinity, field stones
removed from crop ground often are seen piled in unused corners or
along fence rows. Field stones are also
used in building construction, and the
beautiful rustic lodge and shore patrol station
at Gull Point were built of glacial
erratics having various shapes and
mineral compositions by the
Civilian Conservation Corps after the
park was established in 1934.
Glacial Erratics at Nora Springs:2
Clearing farm fields of glacial erratics
is a necessary and frequent chore wherever
glacial deposits are cultivated. Over time,
seasonal freezes
and thaws work these rocks upward from below
the plow zone to the land surface. Smaller
glacial erratics can be hauled out of the fields; larger ones
are frequently blasted apart by dynamite
and the pieces hauled away;
while some of the largest are just left in
place and avoided. At the municipal
park in Nora Springs (Floyd County), an
adjoining city street actually narrows to
accommodate an erratic protruding into the right-of-way.
Glacial Erratics at
Owa Lakeside Laboratory, West Okoboji Lake6
Glacial deposits across Iowa contain an abundance
of boulders and cobbles of
igneous and metamorphic rocks transported
via glaciers from Minnesota.
In areas where
the bedrock is deeply buried, these easily accessible
field stones have been utilized for buildings, principally house
and barn foundations.
Large Glacial Erratics:2
Glacial erratics range in size from
pebbles to giant boulders. The
greatest number of giant erratics
are seen on the Iowan Surface of northeastern
Iowa. They were described in a 1970 Iowa Academy
of Science article by Drake University
professors Richard Dirks and Carl Busch, who noted
that 80 percent of the giant boulders
had a similar composition,
a light-colored,
coarse-grained granite.
They concluded from the boulders'
composition and the direction of glacial
striations on the underlying bedrock surface
that these erratics probably originated
in central and west-central
Minnesota.
In another Iowa Academy of Science article in 1961,
geologist Charles Gwynne of Iowa State University
described the fate of a large Black Hawk
County erratic near Waterloo. It originally
measured 30 feet long, by 20 feet wide,
by 27 feet high and was broken up in 1891; the pieces were used
to construct the Boulder Church which housed
the congregation of the First Presbyterian Church in Waterloo.
This building was used as late as 1961
by the Salvation Army.
The 1916 Annual Report
of the Iowa Geological Survey described a boulder
in Floyd County, about three miles west of
Nashua, as the largest erratic remaining in Iowa.
Its dimensions then were 50 feet long by 40 feet
wide by 11.5 feet above the ground, with a nearby
fragment measuring 17 feet by 7 feet
by 1.5 feet apparently broken from the larger rock.
In 1961, this same erratic was listed as being 40 feet by 30
feet by 12 feet. Other large erratics that can
still be seen are St. Peter's Rock four miles
southeast of Alta Vista in Chickasaw County, a granite
specimen five miles west of Cedar Falls in
Grundy County, and a granite boulder in
Grammer Grove Park in Marshall County.
Glacial erratics are an
easily observed piece of Iowa's geological history.
Each one has a story to tell about its
original composition, its point of origin,
its journey to Iowa,
and its final resting place.
Find some for yourself and see what they tell you.
| Gull Point State Park |
Gull Point State Park:3
Large boulders are concentrated along the
shoreline at Gull Point State Park.
These travel-worn "erratics," carried into Iowa by glacial
ice, are usually igneous or metamorphic rock
types, which are not native to Iowa.
Reddish quartzite,
granite and other crystalline rocks are
common, and they reflect the massive power of glaciers and the northerly direction from which they came. These boulders are an
impediment to agriculture, and in the Gull Point vicinity, field stones removed from crop ground often are seen piled in unused corners or
along fence rows. Field stones are also used in building construction, and the beautiful rustic lodge and shore patrol station
at Gull Point were built of glacial erratics having various shapes and mineral compositions by the Civilian Conservation Corps after the
park was established in 1934.
| Manson Impact Structure |
Manson Impact Structure:1
An irregularity in the bedrock surface
is a 23-mile-diameter circular feature known
as the Manson Impact Structure, located primarily in
Pocahontas and Calhoun counties. Here,
a meteor impact 74 million years ago caused
massive disruption of the Cretaceous
bedrock and older strata
beneath, an event so forceful that it brought
deeply buried Precambrian granite rebounding
to the land surface.
The resulting crater and its highly
faulted, distorted rocks are covered with over
100 feet of glacial deposits,
so are not visible on today's landscape.
| Volcanic Ash Deposits |
Ash Deposits from Yellowstone Caldera:5
Ash deposits
from these powerful eruptions have been mapped as far away as
Iowa, Missouri, Texas, and even
northern Mexico.
Excerpts from:
1) Jean Cutler Prior, Geology of Iowa: Iowa's Earth History Shaped by Ice, Wind, Rivers, and Ancient Seas,
Iowa Department of Natural Resources Website, 2001
2) Iowa Department of Natural Resources Website, 2001
3) Lynette S. Seigley and Deborah J. Quade, Gull Point State Park: A Glacial Legacy,
Iowa Department of Natural Resources Website, 2001
4) Raymond R. Anderson and Jean Cutler Prior, Glacial Boulders in Iowa,
Iowa Department of Natural Resources Website, 2001
5) Kious and Tilling, 1996,
This Dynamic Earth: The Story of Plate Tectonics:
USGS Special Interest Publication
6) Brian J. Wirzke, Geologic Sources of Historic Stone Architecture in Iowa,
Adapted from Iowa Geology 1996, No. 21, Iowa Department of Natural Resources,
Iowa Department of Natural Resources Website, 2001
7) USGS/NPS Geology in the Parks Website, 2001
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