The Rock Elm Disturbance is an

impact crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...

in Pierce County,

Wisconsin Wisconsin ( ) is a U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest of the United States. It borders Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michig ...

, United States, roughly southwest of Menomonie. The disturbance is named for Rock Elm, Wisconsin, a nearby town.

Description

Blue Rock at Plum Creek in Nugget Lake County Park Panorama

The

meteorite A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...

that caused the impact crater is estimated to have been in diameter with a mass of and impact velocity of . The crater is in diameter, and fossils found in the rock filling the crater suggest it dates to the

Middle Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 41.6 million years from the end of the Cambrian Period Ma (milli ...

Period, about 455 to 430 million years ago. It may be one of several Middle

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

meteors that fell roughly simultaneously 469 million years ago, part of a proposed

Ordovician meteor event The Ordovician meteor event was a dramatic increase in the rate at which L chondrite meteorites fell to Earth during the Middle Ordovician period, about 467.5±0.28 million years ago, lasting for about 40 million years. This rate increase is ind ...

within the continental United States that includes the

Decorah crater The Decorah crater, also called the Decorah impact structure, is a possible impact crater located on the east side of the city of Decorah in Iowa, United States. It is thought to have been caused by a meteor about wide which struck during the Mi ...

in Iowa, the Slate Islands crater in

Lake Superior Lake Superior is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. Lake Michigan–Huron has a larger combined surface area than Superior, but is normally considered tw ...

, and the Ames crater in Oklahoma.

Crater characteristics

A raised area at the center of the crater 0.8 km (0.5 mi) wide by 2.4 km (1.5 mi) long suggests that the impact caused a major upheaval of lower-lying rock—

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

and

Mount Simon Sandstone The Mount Simon Sandstone is an Upper Cambrian sandstone and is found in many states in the Midwest such as Wisconsin, Minnesota, Iowa, Illinois, and Missouri. The Mount Simon formation is the equivalent of the La Motte Sandstone formation in t ...

, which lies beneath the surface and is much older than the rock layers in the area surrounding it. Additionally, Blue Rock, an exposed portion of faulted Prairie du Chien

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

lies at the south of the crater's edge, which can be viewed at Nugget Lake County Park.

Discovery of reidite

While studying the effects of

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

on areas of meteorite impact, researchers from the

University of Puerto Rico The University of Puerto Rico (Spanish language, Spanish: ''Universidad de Puerto Rico;'' often shortened to UPR) is the main List of state and territorial universities in the United States, public university system in the Commonwealth (U.S. i ...

discovered a rare high-pressure mineral, reidite, at the center of the Rock Elm impact site. Reidite is a dense form of

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...

(ZrSiO₄) that is formed by the intense heat and pressure as is caused by an impacting meteorite. The reidite found at the Rock Elm structure is the oldest known example of the mineral. It has been found in other impact sites such as the Xiuyan crater in China; the

Chesapeake Bay impact crater The Chesapeake Bay impact crater is a buried impact crater, located beneath the mouth of Chesapeake Bay, United States. It was formed by a bolide that struck the eastern shore of North America about 35.5 ± 0.3 million years ago, in the late Eoc ...

in Virginia, United States; and the

Nordlinger Ries Nordlinger is a surname. Notable people with the surname include: *Jay Nordlinger Jay Nordlinger (born November 21, 1963) is an American journalist. He is a former senior editor of ''National Review'', and a book fellow of the National Review I ...

crater in Germany.

References

External links

Article
from Geotimes.com, March 2004

from the

University of Wisconsin-River Falls A university () is an institution of tertiary education and research which awards academic degrees in several academic disciplines. ''University'' is derived from the Latin phrase , which roughly means "community of teachers and scholars". Uni ...

Craters in Wisconsin

Aerial exploration of the Rock Elm Crater
{{Impact cratering on Earth Impact craters of the United States Ordovician impact craters Landforms of Pierce County, Wisconsin