Thegeography of Iowa includes the study of bedrock, landforms, rivers, geology, paleontology and urbanisation of theU.S. state ofIowa. The state covers an area of 56,272.81 sq mi (145,746 km2).
Iowa's bedrock geology generally increases in age from west to east. In northwest IowaCretaceous bedrock is ca. 74 million years old; in eastern IowaCambrian bedrock dates to ca. 500 million years ago.[1]
Seventy-four million years ago, a large asteroid crashed into what is now southeastPocahontas county creating theManson crater. Nearly 22 miles in diameter,[2] it would have killed most animals within 650 miles, roughly an area from modern Denver to Detroit. This was originally thought to have been one of the causes of thedinosaur extinction, but recalculation of the impact's age indicates it occurred some 12 million years before the mass extinction. Althoughglaciation has erased all surface evidence of the impact, the bedrock associated with this impact is unique in Iowa.[3]
A much older meteorite strike created theDecorah crater during theMiddle Ordovician Period, 470 million years ago. The crater is estimated to be 3.5 miles (5.6 km) in diameter, covered by Winneshiek Shale.[4][5][6] There is no surface evidence of the impact, as the Winneshiek Shale is more than 50 feet below the bottom of theUpper Iowa River. The impact, equivalent to1,000 megatons of TNT,[5] did not appear to penetrate the Earth's mantle, but it did push down the underlying Ordovician and Cambrian bedrock several hundred feet.[7] It may be one of several Middle Ordovician meteors that fell roughly simultaneously 469 million years ago, part of a proposedOrdovician meteor event.
Buried deeply within Iowa's bedrock, theMidcontinent Rift System can be seen clearly in magnetic anomaly maps of Iowa. This is a billion-year-old tectonic plate scar that extends from Kansas throughLake Superior. This rift is not seismically active.[9]
No major activefault lines exist in Iowa, and Iowa is one of the mostseismically stable states in the U.S. With the exception of the1968 Illinois earthquake which caused the water tower atLineville to leak,[10] no injuries or significant damage has ever been caused byearthquakes in Iowa. Occasional small earthquakes occur nearFremont County in the far southwest, and theSioux City area can occasionally be shaken by nearby tremblors. Large earthquakes associated with theNew Madrid Fault of far southern Illinois and Missouri can occasionally be felt in eastern Iowa.[11][12] Recent earthquakes centered in Oklahoma have also been felt in Iowa, but have caused no damage.[13]
Historically, Iowa was a significantcoal producer, particularly theDes Moines River valley fromCoalville south. Much of the greater City ofDes Moines area was mined.Boone andWhat Cheer were important in the late 19th century. In the 20th century, the most important mines were farther south aroundAlbia,Centerville,Lucas andOskaloosa. Iowa coal tends to be too high insulfur for modern applications, and the last commercial mine closed in 1994.[14]
Iowa has very limitednatural gas andoil production.[15]
Although Iowa's bedrock is generally younger in western parts of the state, one exception is a small part of far northwest Iowa wherePrecambrianSioux quartzite bedrock is found in northwestLyon County. Siouxquartzite is a very hard rock of ruddy pink color used extensively in the region for road and railroad beds.[16] Sioux quartzite has been dated to have been laid between 1.64 and 1.76 billion years ago.[17]
Geodes are found in southeast Iowa and are the official state rock. They consist of grey to pink cobbles withinlimestone that when cut or smashed open reveal a hollow crystal-filled interior. Geodes are common aroundGeode State Park inHenry County.[18]
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Despite popular perception, Iowa is generally not flat; most of the state consists of rolling hills. Prior[19] divides Iowa into eightlandforms based onglaciation,soils,topography, andriver drainage:
Also known as theDriftless Area, this region of scenic, high relief landscapes includes such features as resistant, bluff-forming bedrockoutcrops, deep V-shaped valleys, caves, springs, andsinkholes. Glacial deposits andloess are thin or absent over most of the region.
Often called thePrairie Pothole Region, the Des Moines Lobe was glaciated up until 12,000 years ago during theWisconsin glaciation. The area is marked by rolling terrain and ridges.[20] Historically, this area was peppered with small interconnected swamps, most of which were drained for farmland. TheIowa Great Lakes occur along the western edge of the Des Moines lobe.
The southern Iowa drift plain covers most of the southern half of Iowa. This is probably the most familiar landscape to travelers, since most ofInterstate 80 in Iowa runs through the SIDP. The classic Iowa landscape, consisting of rolling hills of Wisconsin-age loess on Illinoian (or earlier)till. The SIDP is some of the most productive agricultural land in the US.
Generally level areas of stream terraces, paleochannels, backwater sloughs, andoxbow lakes are found within the broadMississippi River valley.
TheLoess Hills consist of very thick deposits of loess in far western Iowa deposited during the Wisconsin and Illinoian periods. Highly eroded, leaving stark, beautiful "golden hills".
Northeast Iowa is covered with eroded Pre-Illinoian till with moderate loess formation, frequently in the form ofpaha ridges, muted relief except for steep rolling hills near river valleys, and deeper valleys. These picturesque hills are depicted in many of the landscapes ofGrant Wood.
Like the Iowan Surface, the Northwest Iowa Plains are rolling hills consisting of eroded soils developed since pre-Wisconsinan glaciation, but with significant amounts of loess.
Perhaps the only truly flat region of Iowa, the Missouri Alluvial Plain contains areas of terraces, sloughs, and oxbows. Its valley trench is not as deep as the Mississippi River system, and theMissouri River is contained in a much narrower channel. In Iowa, the eastern border of the Missouri Plains is the Loess Hills, forming steep rounded bluffs.
Iowa has ahumid continental climate throughout the state (Köppen climate classificationDfa) with extremes of both heat and cold. The average annual temperature at Des Moines is 50 °F (10 °C); for some locations in the north, such as Mason City, the figure is about 45 °F (7 °C), whileKeokuk, on theMississippi River, averages 52 °F (11 °C).[21] Snowfall is common, withDes Moines getting about 26 days of snowfall a year, and other places, such asShenandoah getting about 11 days of snowfall in a year.[22]
Spring ushers in the beginning of thesevere weather season. Iowa averages about 50 days ofthunderstorm activity per year.[23] The 30-year annual average of tornadoes in Iowa is 47.[24] In2008, twelve people were killed by tornadoes in Iowa, making it the deadliest year since1968 and also the second most tornadoes in a year with 105, matching the total from 2001.[25]
Iowa summers are known for heat and humidity, with daytime temperatures sometimes near 90 °F (32 °C) and occasionally exceeding 100 °F (38 °C). Average winters in the state have been known to drop well below freezing, even dropping below −18 °F (−28 °C). Iowa's all-time hottest temperature of 118 °F (48 °C) was recorded at Keokuk on July 20, 1934, during a nationwide heat wave;[26] the all-time lowest temperature of −47 °F (−44 °C) was recorded inWashta on January 12, 1912.[27]
Iowa has a relatively smooth gradient of varyingprecipitation across the state, with areas in the southeast of the state receiving an average of over 38 inches (97 cm) of rain annually, and the northwest of the state receiving less than 28 inches (71 cm).[28] The pattern of precipitation across Iowa is seasonal, with more rain falling in the summer months. Virtually statewide, the driest month is January or February, and the wettest month is June, owing to frequent showers and thunderstorms, some of which produce hail, damaging winds and/or tornadoes. In Des Moines, roughly in the center of the state, over two-thirds of the 34.72 inches (88.2 cm) of rain falls from April through September, and about half the average annual precipitation falls from May through August, peaking in June.[29]
| City | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Davenport[31] | 30/13 | 36/19 | 48/29 | 61/41 | 72/52 | 81/63 | 85/68 | 83/66 | 76/57 | 65/45 | 48/32 | 35/20 |
| Des Moines[32] | 31/14 | 36/19 | 49/30 | 62/41 | 72/52 | 82/62 | 86/67 | 84/65 | 76/55 | 63/43 | 48/31 | 34/18 |
| Keokuk[33] | 34/17 | 39/21 | 50/30 | 63/42 | 73/52 | 83/62 | 87/67 | 85/65 | 78/56 | 66/44 | 51/33 | 33/21 |
| Mason City[34] | 24/6 | 29/12 | 41/23 | 57/35 | 69/46 | 79/57 | 82/61 | 80/58 | 73/49 | 60/37 | 43/25 | 28/11 |
| Sioux City[35] | 31/10 | 35/15 | 47/26 | 62/37 | 73/49 | 82/59 | 86/63 | 83/63 | 76/51 | 63/38 | 46/25 | 32/13 |
| Climate data forDes Moines International Airport, Iowa (1991–2020 normals,[a] extremes 1878–present[b]) | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Month | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Year |
| Record high °F (°C) | 67 (19) | 78 (26) | 91 (33) | 93 (34) | 105 (41) | 103 (39) | 110 (43) | 110 (43) | 101 (38) | 95 (35) | 82 (28) | 74 (23) | 110 (43) |
| Mean maximum °F (°C) | 53.4 (11.9) | 58.7 (14.8) | 74.6 (23.7) | 83.9 (28.8) | 88.9 (31.6) | 93.1 (33.9) | 96.2 (35.7) | 94.4 (34.7) | 91.3 (32.9) | 83.3 (28.5) | 70.4 (21.3) | 57.8 (14.3) | 97.4 (36.3) |
| Mean daily maximum °F (°C) | 30.9 (−0.6) | 35.7 (2.1) | 49.2 (9.6) | 62.0 (16.7) | 72.4 (22.4) | 81.9 (27.7) | 85.6 (29.8) | 83.6 (28.7) | 76.9 (24.9) | 63.4 (17.4) | 48.3 (9.1) | 35.9 (2.2) | 60.5 (15.8) |
| Daily mean °F (°C) | 22.3 (−5.4) | 26.9 (−2.8) | 39.4 (4.1) | 51.3 (10.7) | 62.4 (16.9) | 72.2 (22.3) | 76.0 (24.4) | 73.9 (23.3) | 66.2 (19.0) | 53.2 (11.8) | 39.3 (4.1) | 27.7 (−2.4) | 50.9 (10.5) |
| Mean daily minimum °F (°C) | 13.8 (−10.1) | 18.0 (−7.8) | 29.6 (−1.3) | 40.6 (4.8) | 52.3 (11.3) | 62.4 (16.9) | 66.4 (19.1) | 64.2 (17.9) | 55.4 (13.0) | 42.9 (6.1) | 30.2 (−1.0) | 19.5 (−6.9) | 41.3 (5.2) |
| Mean minimum °F (°C) | −7.8 (−22.1) | −2.7 (−19.3) | 9.2 (−12.7) | 24.9 (−3.9) | 37.6 (3.1) | 50.2 (10.1) | 56.9 (13.8) | 54.8 (12.7) | 40.4 (4.7) | 26.8 (−2.9) | 12.6 (−10.8) | −1.2 (−18.4) | −11.4 (−24.1) |
| Record low °F (°C) | −30 (−34) | −26 (−32) | −22 (−30) | 9 (−13) | 26 (−3) | 37 (3) | 47 (8) | 40 (4) | 26 (−3) | 7 (−14) | −10 (−23) | −22 (−30) | −30 (−34) |
| Averageprecipitation inches (mm) | 1.08 (27) | 1.34 (34) | 2.17 (55) | 4.02 (102) | 5.24 (133) | 5.26 (134) | 3.82 (97) | 4.17 (106) | 3.18 (81) | 2.78 (71) | 1.91 (49) | 1.58 (40) | 36.55 (928) |
| Average snowfall inches (cm) | 9.4 (24) | 10.2 (26) | 4.4 (11) | 1.2 (3.0) | 0.2 (0.51) | 0.0 (0.0) | 0.0 (0.0) | 0.0 (0.0) | 0.0 (0.0) | 0.5 (1.3) | 2.7 (6.9) | 7.9 (20) | 36.5 (93) |
| Average extreme snow depth inches (cm) | 6.9 (18) | 7.4 (19) | 4.2 (11) | 0.8 (2.0) | 0.2 (0.51) | 0.0 (0.0) | 0.0 (0.0) | 0.0 (0.0) | 0.0 (0.0) | 0.3 (0.76) | 1.7 (4.3) | 4.9 (12) | 10.3 (26) |
| Average precipitation days(≥ 0.01 in) | 8.2 | 8.4 | 9.5 | 11.5 | 12.7 | 11.7 | 9.5 | 9.4 | 8.2 | 8.6 | 7.7 | 7.8 | 113.2 |
| Average snowy days(≥ 0.1 in) | 6.9 | 6.3 | 3.1 | 1.0 | 0.1 | 0.0 | 0.0 | 0.0 | 0.0 | 0.5 | 1.8 | 5.6 | 25.3 |
| Averagerelative humidity (%) | 71.0 | 71.3 | 67.9 | 63.2 | 63.0 | 64.8 | 67.7 | 70.0 | 70.9 | 66.5 | 71.0 | 74.6 | 68.5 |
| Mean monthlysunshine hours | 157.7 | 163.3 | 206.0 | 222.2 | 276.0 | 312.1 | 337.8 | 297.9 | 239.8 | 210.0 | 138.5 | 129.2 | 2,690.4 |
| Percentagepossible sunshine | 53 | 55 | 56 | 56 | 61 | 69 | 73 | 70 | 64 | 61 | 47 | 45 | 60 |
| Averageultraviolet index | 1 | 2 | 4 | 6 | 8 | 9 | 9 | 8 | 6 | 4 | 2 | 1 | 5 |
| Source 1:NOAA (relative humidity and sun 1961−1990)[36][37][38] | |||||||||||||
| Source 2: Weather Atlas (UV)[39] | |||||||||||||
As in most of the U.S., surface water in Iowa is never safe to drink untreated, contamination by agricultural runoff includingnitrates,herbicides,pesticides, andanimal waste is common, though theClean Water Act has helped. Municipal water supplies are typically heavily chlorinated, this chlorine, combined with high nitrate levels, often give municipal water a strong smell, and the limestone bedrock in much of the state causes hard water.[40] Some communities, such asIowa City resort to additional carbon filtration andlime softening coagulation-sedimentation to make the water more palatable.[41] Water treatment can be surprisingly effective; Des Moines' advanced filtration system has led to water quality ranked among the nation's best.[42]
The Jordan Aquifer is the largest source of groundwater, extending from northeast Iowa to south central Iowa, and is ultimately the source of much of Iowa's agricultural and industrial water. In addition to pollution threats, the aquifer is threatened by overuse in well-source irrigation,ethanol production, and the diminishment of resupply caused by extensive field tilling. The aquifer has dropped by as much as 300 feet since the 19th century, resulting in dry wells, the disappearance of natural surface springs, and the diminishment of water quality.[43][44]
TheNRCS divides Iowa into 23 soil regions. In general, soils of southern, eastern, and western Iowa are loess-derived, while soils of northern and central Iowa are till-derived. Most level areas of Iowa have soils highly suitable for agriculture, making Iowa one of the most productive farming regions of the world.[45]
Like most Upper Midwest and Plains states,radon is a common problem in Iowa, especially in areas with clay-rich soils.[46]Radon is the highest in the southern and the western parts of Iowa.
Floods in 1993 washed away all the soil and unconsolidated bedrock along the spillway of theCoralville Lake Dam inJohnson County, exposing a rich collection ofDevonian-agefossils. This area has been transformed into a visitors' center, where hikers can tour the bedrock. Unfortunately, looters have illegally removed many of the better fossils.[47] The June 2008 floods expanded the fossil bed floor, and removed some of the weathered overburden.[48]
Jurassic andCretaceous bedrock in western Iowa have potential to contain dinosaur remains, and in nearby parts of NebraskaHadrosaurid (“duck-billed”)ornithopod dinosaur remains have been recovered in Cretaceous bedrock similar to that of Iowa. The deep loess that covers much of western Iowa typically conceals the bedrock, limiting opportunities to finding dinosaurs to mining and quarrying operations.[49]
Archaeological andpaleontological sites in Iowa have produced an extensive collection ofPleistocene andHolocene animals; these have been used to reconstruct past environmental conditions in the Midwest.[50] Some of the earliest paleoclimatic reconstructions of the midcontinent were made from Iowa collections, such as theCherokee Sewer Site.[51]
A Pleistocene giantsloth is under excavation along West Tarkio Creek nearShenandoah, Iowa. Three individuals ofMegalonyx jeffersonii, or Jefferson's Ground Sloth, have been identified so far, including one adult and two juveniles of different ages.[52][53]
Samuel Calvin (1840–1911) was Iowa's first systematic geologist, who helped to make the first bedrock and landform maps of Iowa, as well as lead geological research throughout the state.[54] Calvin Hall at theUniversity of Iowa is named for him.Clair Cameron Patterson (1922–1995) developed the lead-lead dating and calculated an age for the Earth of 4.55 billion years; a figure far more accurate than those that existed at the time and one that has remained unchanged for over 50 years.Charles Rollin Keyes was also an early Iowa geologist who helped map the soils and bedrock of southeast Iowa; he, with Calvin, was a founder of the Iowa Geological Survey.
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