Where am I? Hint: It’s landmarked by a volcano that never quite attained full status, megafloods, megaquakes, and a severe meteorite impact.
The answer is...Illinois! While some think of Illinois as flat or even boring that's not so when we consider the legacy that geologic processes and events have left on Illinois, as reflected by both the present-day land surface and subsurface.
Illinois' 'volcano'
The “volcano” is called Hicks Dome. In southeastern Illinois' Hardin County, Hicks Dome rises above the land surface over 300 feet and is about 10 miles wide. Its rocks are uplifted about 4,000 feet, with older rocks (Devonian) in the center, concentric rings of younger rocks (Pennsylvanian being the youngest), and many faults within. It has been interpreted as resulting from explosions beneath the surface and therefore is referred to as a “cryptoexplosive” or “cryptovolcanic” feature. It formed during the Permian, about 270 million years ago. The region of Hicks Dome is rich in fluorite and other minerals.
Megafloods
The Great Flood of the Illinois and Mississippi Rivers in 1993 resulted from months of persistent heavy rain, but this event was nothing in comparison to megafloods that occurred as glaciers were melting and receding northward. The Kankakee Torrent exhibited massive amounts of water flowing down the rivers about 19,000 years ago; this resulted from glacial melting, lakes forming behind moraines, and then moraines becoming breached. Similar massive flooding also occurred about 4,000 years later. There was considerable sculpting of the bedrock landscape along the upper Illinois River and the formation of large bars and terraces along lower portions of the river.
Megaquakes
Illinois' megaquakes are the well-documented New Madrid events that occurred between Dec. 16, 1811, and March 15, 1812. Three very large seismic events shook the region; more than 200 moderate to large aftershocks also occurred. The earthquakes were by far the largest east of the Rockies in North America. The December 16th event had a magnitude estimated to be as great as 7.5, and ground shaking was felt as far away as the East Coast. The cause of the earthquakes is not well understood but is thought to be related to movement along the faults beneath the Mississippi River alluvial plain.
Meteorite Impact
The feature resulting from a large meteorite impact is known as the Des Plaines Disturbance. This Cook County feature underlies the northeastern portion of O’Hare International Airport. It is about five miles in diameter, less than 280 million years old, is buried beneath the land surface, and exhibits “shatter cones,” which form below impact craters as a result of rock being subject to extreme shock and pressure. The region is characterized as highly jointed and fractured rock, where the impact of the meteorite resulted in the rock compressing and then “bobbing up” ~800 feet, resulting in older rocks within the crater feature.
