Igneous Rock Diabase

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.

Meet our associated expert

This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.
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Artist interpretation of rifting along the Eastern United States, leading to the creating of the Atlantic Ocean and rift basins that filled with sediments and volcanic rock.
Image created by Karen Carr Studio, Inc., 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.

Artist interpretation of rifting along the Eastern United States, leading to the creating of the Atlantic Ocean and rift basins that filled with sediments and volcanic rock.
Image created by Karen Carr Studio, Inc., used with permission

How People Build from Vulcan Quarry

The two rock types found at the Vulcan Quarry, igneous diabase and metamorphic hornfels, are used quite differently from each other because their properties are unique. Both rocks are dense, dark colored, and generally fine-grained with the diabase having a higher specific gravity, or density. Both rock types are used for construction aggregates in the production of ready-mix concrete and asphaltic concrete for construction of roads, bridges, schools, homes, and other structures. In the Washington, D.C. area, the crushed aggregates from the Manassas Quarry have been used in I-66, railway bed (ballast), and construction of the new Smithsonian National Museum of African American History and Culture. The larger-sized crushed stone is used in erosion and sediment control and storm-water structures. The hornfels have inclusions of certain minerals that make them vulnerable to breaking down. Because of this, the diabase is used for concrete, foundations, and roads while the hornfels is used primarily for fill.

Related Resources
Cripple Creek granite at Florissant Fossil Beds National Monument in Colorado
Photographed by Joseph Hall, U.S. National Park Service, Public Domain

About Intrusive Igneous Rocks

Molten rock, or magma, does not always reach Earth's surface. It may flow upward through cracks that end below the surface, where it gets trapped and cools slowly. Some intrusions, called plutons, are several kilometers or miles wide. During the slow cooling process, the magma freezes into crystals. Magma containing higher iron, magnesium, and calcium levels is the first to turn solid and forms dark, coarse-grained rocks such as gabbro. The lighter-colored granite comes from magma with high levels of silica (silicon dioxide) and relatively little iron and magnesium. Humans cannot witness the formation of intrusive igneous rock in the same way we see volcano eruptions. However, over millions of years, the crustal rocks above some intrusions wear away, leaving the solidified magma exposed to the environment in places such as the Sierra Nevada Mountains and Yosemite National Park in California.