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America's Volcanic Past
Iowa

"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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MORE America's Volcanic Past - Yellowstone Caldera Eruption


Map, Location of Iowa

Volcanic Highlights and Features:
[NOTE: This list is just a sample of various Iowa 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 Iowa.]

  • Iowa
  • Iowa Regions
  • Iowa - Brief Geologic History
  • Iowa's Volcanic Rocks
  • Des Moines - State Capitol Building
  • Glacial Erratics
  • Gull Point State Park
  • Manson Impact Structure
  • Volcanic Ash Deposits

Iowa



Excerpts from:
   
Iowa Regions

The Interior Plains:7
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.


   
Iowa - Brief Geologic History

Geologic History:1
Iowa's geologic history lies buried beneath the ground. The deeper, older and least frequently seen portions of this history consist mostly of sedimentary rocks such as sandstone, limestone, dolomite and shale, which are over 3,000 feet thick in places. These rocks originated as layers of loose sediment accumulating in shallow seas and along coastal and floodplain environments that occupied Iowa between 74 million years ago (Cretaceous) and 530 million years ago (Cambrian). With time, this sediment hardened into rock containing fossil remains of past animal and plant life.

Bedrock Outcrops: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. Fossil-bearing rocks are found among the abundant outcrops of northeastern Iowa, a topographic region called the Paleozoic Plateau. Also, bedrock is occasionally exposed along the state's river valleys, at roadcuts, and in quarries. Elsewhere across the state, the bedrock surface is covered with younger glacial-age materials. As a result, much of our information about Iowa's bedrock geology comes from rock samples brought up to the land surface during the drilling of wells.

Iowa's Precambrian:1
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.

Manson Impact Structure:1
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.

Present Land Surface:1
The present land surface across Iowa is dominated by loose materials much younger than the bedrock beneath. These materials consist of sediment originating from ice sheets, meltwater streams, and strong winds during a series of glacial events between 2.5 million and 10,000 years ago (Quaternary). This familiar "dirt" consists of pebbly clay, sand, gravel, and abundant silt, which over time have weathered into Iowa's productive loamy soils. These easily eroded "Ice Age" deposits account for the gently rolling appearance of much of the Iowa (and Midwestern) landscape.

   

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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01/28/03, Lyn Topinka