High pressures and temperatures within the Earth's crust can change rocks from one type to another. Scientists call this process metamorphism, which comes from the Greek words for "after" and "form." At the junctions where the Earth's crustal plates collide, pushing up mountain ranges, or where one plate slides underneath another, the original rocks, or protoliths (from the Greek words meaning "first" and "rock") undergo reactions that change the chemical or crystal structure of the rocks with little or no actual melting. The grains of minerals within many metamorphic rocks are aligned in parallel due to the forces pushing on them. The pressure-temperature combination causes some sedimentary rocks to lose water molecules or become anhydrous.

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.