Rock Elm

Rock Elm Disturbance

The Rock Elm Disturbance is an impact crater situated in Pierce County, Wisconsin, United States. Located approximately 40 kilometers southwest of the city of Menomonie, the site is named after the nearby community of Rock Elm. The impact event is estimated to have occurred during the Middle Ordovician Period, between 455 and 430 million years ago.

Description and Characteristics

The crater itself measures approximately 6 kilometers (3.7 miles) in diameter. The celestial body responsible for the impact is believed to have been a meteorite around 170 meters in diameter. The impact caused significant geological upheaval, creating a raised area at the crater's center composed of breccia and Mount Simon Sandstone, rock types that lie beneath the surface and are considerably older than the surrounding geological layers. An exposed section of faulted Prairie du Chien sandstone, referred to as Blue Rock, is visible at the southern edge of the crater, accessible at Nugget Lake County Park.

Geological Significance

One of the most significant discoveries at the Rock Elm site is the identification of reidite, a rare high-pressure mineral. Researchers identified the oldest known example of reidite here, a dense form of zircon (ZrSiO4) created by the extreme heat and pressure associated with meteorite impacts. This finding places Rock Elm among other notable impact sites worldwide where reidite has been found, including locations in China, the United States, and Germany. The crater is also considered potentially part of a broader Ordovician meteor event that may have occurred across the continental United States, possibly including the Decorah, Slate Islands, and Ames craters.