Sedimentary Rock Shale

Associated Smithsonian Expert: Catherine (Cari) Corrigan, Ph.D.

Cari Corrigan with two meteorites from the collection.

Photographed by Brittany M. Hance, Smithsonian Institution.

Dr. Cari Corrigan is a research geologist at the Smithsonian National Museum of Natural History who specializes in the study of rocks called meteorites that come from other planetary bodies in our solar system. While growing up in western Michigan, she often went hiking and exploring in the woods and lakes near her house. During her first two years in college, she took an astronomy class and a geology class, and wondered how she could combine the two. This led to her discovery of the field of planetary sciences, which she decided to try to to make her career. After earning her PhD from Case Western Reserve University studying Martian meteorites in 2004, she worked as a postdoctoral fellow at both the Smithsonian and at Johns Hopkins University Applied Physics Lab, and joined the Smithsonian permanently in 2008. In both 2001 and 2004, Corrigan participated in the Antarctic Search for Meteorites Program as a field member. These teams travel to Antarctica annually to collect meteorites from the Antarctic ice, where they have fallen and remained frozen since they fell to Earth. The program has returned over 20,000 meteorites for research and educational purposes since it began in 1976. At the Smithsonian, Corrigan curates this collection of Antarctic Meteorites, while also conducting research on both Antarctic and non-Antarctic meteorites from the Moon, Mars, and the asteroid belt.

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Sedimentary rock formation, southwestern Utah, USA
Courtesy of Mark A. Wilson, The College of Wooster

How Sedimentary Rocks Are Formed

At or near Earth's surface, sedimentary rocks form in two ways: by the accumulation of rock grains or by the formation of a solid from minerals dissolved in water. The fragments that go into making sedimentary rocks can be as big as boulders or as small as clay particles. Over long periods of time, the upper layers of debris compress the lower layers, squeezing out excess water or air trapped between the rock fragments. Under the pressure, individual fragments eventually dissolve and stick together, or the remaining fluid within the sediment brings in other substances that act as a cement, until the sediment has turned into rock. Scientists classify many sedimentary rocks based on the size of the particles that built the rock; mudstone and sandstone, for example, originally came from fine-grained mud and sand deposits that hardened over long time periods.

Overview of rock quarry in Kentland, IN, site of a large meteorite impact. Notice how layers on the left side of image are vertical, when they were originally deposited flat.
Photographed by Donald E. Hurlbert, Smithsonian Institution

About Kentland Quarry

Deep underneath the soil of Indiana lies some of the hardest rock around. Limestone was deposited as sediment at the bottom of an ancient ocean some 400 million years ago. Since then the ocean has disappeared, and the sediments turned into rock, and have been buried under hundreds of feet of more recent sediment. About 100 million years ago a large meteorite struck what is now northwest Indiana, creating an impact crater about 12 kilometers, or 7.25 miles, across - the fourth largest in the United States. The impact was large enough that the central area of the crater rebounded back upward. This created high ground that was not buried as deeply by the sediments that covered the rest of the crater and the Midwest.

Sandstone from Coconino formation near Meteor Crater, Arizona, USA
Photo by Smithsonian Institution, National Museum of Natural History, Department of Mineral Sciences

About Sandstone

Sandstone, a type of sedimentary rock, looks like sand frozen in place. When quartz, feldspar, and other silica-containing minerals and rocks break into fragments between 0.1 and 2 mm (0.004 to 0.08 inches) across, scientists call the pieces sand. In deserts, on beaches, and under bodies of water, layers of sand grains build up over thousands or millions of years, until the accumulated pressure from the weight of those layers compact the sand grains into solid rock, a process called lithification. The sand grains are commonly cemented together by fine-grained quartz and calcite. Sandstone makes up 10 to 20 percent of all sedimentary rocks on Earth because its ingredients are among the most widespread minerals. They are found worldwide and form under a wide range of depositional environments and conditions. Detailed examination of the mineral grains and rock fragments in sandstones is necessary to help geologists interpret the source rock and the environment in which the sandstone was deposited. For example, coarser sand grains in the rock could indicate that blowing wind or running water removed the smallest, finest granules before lithification took place.

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