Rock Elm Disturbance

The Rock Elm Disturbance is an impact crater in Pierce County, Wisconsin, United States, roughly 40 kilometres (25 mi) southwest of Menomonie.[1] The disturbance is named for Rock Elm, Wisconsin, a nearby town.

Rock Elm Disturbance
North American Middle Ordovician impact craters, which may be part of the Ordovician meteor event. Key: 1: Ames crater, 2: Decorah crater, 3: Rock Elm Disturbance, 4: Slate Islands crater
Impact crater/structure
ConfidenceConfirmed
Diameter6 km (3.7 mi)
Age430-455 Ma
Middle Ordovician
Exposed-
Drilled-
Bolide typeOrdovician meteor event?
Location
Coordinates44°43′N 92°14′W
CountryUnited States
StateWisconsin
DistrictPierce County
MunicipalityRock Elm
Rock Elm Disturbance is located in the United States
Rock Elm Disturbance
Location of the crater in the United States

Description
Rock Elm Disturbance as mapped by William S. Cordua in 1987. Outer line is extent of crater and inner line is central uplift.[2]
Blue Rock, a faulted portion of Prairie du Chien sandstone, is visible from the Underlook Trail at Nugget Lake County Park. Note that this sedimentary rock typically demonstrates horizontal stratification.
Panoramic context of Blue Rock faulted portion of Prairie du Chien sandstone.
The disturbance area as seen from County Highway HH and County Road CC near Plum City, Wisconsin

The meteorite that caused the impact crater is estimated to have been 170 m (560 ft) in diameter with a mass of 9×109 kg (2.0×1010 lb) and impact velocity of 30 km/s (67,000 mph).

The crater is 6 km (3.7 mi) in diameter, and fossils found in the rock filling the crater suggest it dates to the Middle Ordovician Period, about 455 to 430 million years ago.[3] It may be one of several Middle Ordovician meteors that fell roughly simultaneously 469 million years ago, part of a proposed Ordovician meteor event within the continental United States that includes the Decorah crater in Iowa, the Slate Islands crater in Lake Superior, and the Ames crater in Oklahoma.[4]

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 and Mount Simon Sandstone, 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 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 on areas of meteorite impact, researchers from the University of Puerto Rico discovered a rare high-pressure mineral, reidite, at the center of the Rock Elm impact site. Reidite is a dense form of zircon (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.[5] It has been found in other impact sites such as the Xiuyan crater in China; the Chesapeake Bay impact crater in Virginia, United States; and the Nordlinger Ries crater in Germany.[6]

References
  1. "Rock Elm". Earth Impact Database. Planetary and Space Science Centre University of New Brunswick Fredericton. Retrieved 2017-10-14.
  2. William S. Cordua, 1987. The Rock Elm Disturbance, Pierce County, Wisconsin in FIELD TRIP GUIDEBOOK FOR THE UPPER MISSISSIPPI VALLEY, MINNESOTA, IOWA, AND WISCONSIN. Edited by N.H. Balaban. Prepared for the 21st Annual Meeting of the Geological Society of America, North-Central Section, St. Paul, Minnesota.
  3. Peters, Christopher William, Middleton, Michael D., & Cordua, William S. (2002). "Paleontology of the Rock Elm Disturbance". Abstracts with Programs - Geological Society of America. 34 (2): 95. Archived from the original on 2012-02-11. Retrieved 2008-06-11.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. Vastag, Brian (18 February 2013). "Crater found in Iowa points to asteroid break-up 470 million years ago". Washington Post. Retrieved 19 February 2013.
  5. "ROCK ELM IMPACT STRUCTURE – Crater Explorer". Retrieved 2021-11-18.
  6. Oskin, Becky (November 3, 2014). "Rare Mineral Discovered in Ancient Meteorite Impact Crater". Live Science. Retrieved 2015-02-17.
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