The Rock Elm Disturbance is an

impact crater An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters ...

in Pierce County,

Wisconsin Wisconsin () is a state in the upper Midwestern United States. Wisconsin is the 25th-largest state by total area and the 20th-most populous. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake M ...

, United States, roughly southwest of Menomonie. The disturbance is named for

Rock Elm, Wisconsin Rock Elm is a town in Pierce County, Wisconsin, United States. The population was 504 at the 2000 census. The unincorporated communities of Exile and Rock Elm are located in the town. The unincorporated community of Waverly is also located parti ...

, a nearby town.

Description

The

meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...

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. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. Th ...

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. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

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. This is indicated by abundant fossil L chondrite meteorites in ...

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 in central North America is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. and the third-largest by volume, holding 10% of the world's surface fresh wa ...

, 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 rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

and

Mount Simon Sandstone The Mount Simon Sandstone is the basal sandstone of the Potsdam Sandstone. It was deposited in a nearshore environment, unconformably overlying Precambrian basement. It is overlain by the Eau Claire Formation or Ordovician strata. It is presume ...

, 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 sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

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 water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...

on areas of meteorite impact, researchers from the

University of Puerto Rico The University of Puerto Rico ( es, Universidad de Puerto Rico, UPR) is the main public university system in the U.S. Commonwealth of Puerto Rico. It is a government-owned corporation with 11 campuses and approximately 58,000 students and 5,3 ...

discovered a rare high-pressure mineral,

reidite Reidite is a rare polymorph of ZrSiO4 created when zircon experiences high pressure and temperature. Reidite is denser than zircon and has the same crystal structure as scheelite. All natural occurrences of reidite are associated with meteorite i ...

, 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 the r ...

(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 Eoce ...

in Virginia, United States; and the Nordlinger Ries 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 higher (or tertiary) education and research which awards academic degrees in several academic disciplines. Universities typically offer both undergraduate and postgraduate programs. In the United States, the ...

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