Metamorphic Rock Hornfels

Associated Smithsonian Expert: Timothy Rose, M.S.

Geologitst Tim Rose checks out volcanic rocks in Hawaiian lava tubes.

Photographed by Richard S. Fiske, Smithsonian Institution

Tim Rose is a geologist and manager of the analytical laboratories in the Department of Mineral Sciences at the Smithsonian National Museum of Natural History. His interest in fossils and rocks began at an early age, collecting fossils at Calvert Cliffs, Maryland and rock-hounding in the pegmatite quarries of the Black Hills, South Dakota. He took his first college-level geology courses at a local community college while he was a senior in high school. After receiving his undergraduate degree in geology at the University of Delaware in 1978, he worked briefly in the oil fields of Texas and Louisiana and off the coast of New Jersey in the Baltimore Canyon. Tim joined the Smithsonian in 1980 in the fledgling department of Automatic Data Processing (now Information Technology) and after changing to a sample preparation position, he went to the University of Maryland and received an M.S. in geology in 1991. Tim is currently involved in the study of Kilauea volcano’s explosive eruptive history as well as studies of ancient Mesoamerican artifacts.

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This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.
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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.

Detailed geologic time scale.
From the Geologic Society of America, used with permission

About Manassas Quarry

It is only chance that the United States' eastern coast is not part of Africa today. About 220 million years ago, the Atlantic Ocean began to form as Africa and North America separated along a long crack that is now the mid-Atlantic Ridge. The separation was not a perfect break and some inland basins, also known as rift valleys, formed during the separation on each side of the main oceanic rift. These rift basins, or grabens, were formed as the crust faulted and large crustal blocks moved downward. Sediments filled the basins, and magma was injected into the basin from below, sometimes erupting as lava flows. The magma became diabase, and the heat from it metamorphosed the surrounding sedimentary rocks, turning them in a rock called hornfels. Eventually as the Atlantic Ocean widened, the extensional forces lessened and activity ceased in the basins. Manassas Quarry is located within one of these basins along one of these bodies of diabase.