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.

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.

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.