About 27,000 years ago, lava flowed on top of the Navajo Sandstone, forming black “ropey” basalt in Snow Canyon State Park, Washington County.
Utah is peppered with volcanoes and other signs of the hot molten rock (magma) that forms within the crust beneath us. Magma, less dense than the surrounding solid rock, rises where it can and intrudes into adjacent rock to form laccoliths, batholiths, dikes, and sills upon cooling. It can also extrude onto the surface as lava, sometimes explosively ejecting lava fragments and ash. The resulting igneous rocks can vary widely in appearance.