Geology of the United States A slice of the Earth from the Pacific Ocean through the Pacific Northwest might look something like the adjacent image. Beneath the Cascades, a dense oceanic plate plunges beneath the North American Plate; a process known as subduction. As the oceanic slab sinks deep into the Earth's interior beneath the continental plate, high temperatures and pressures allow water molecules locked in the minerals of solid rock to escape. The water vapor rises into the pliable mantle above the subducting plate, causing some of the mantle to melt. This newly formed magma rises toward the Earth's surface to erupt, forming a
Sequoia National Park in the Sierra Nevada are composed of granitic rocks. These rocks, such as granite, diorite and monzonite, formed when molten rock cooled far beneath the surface of the earth. The molten rock was the result of a geologic process known as subduction. Powerful forces in the earth forced the landmass under the waters of the Pacific Ocean beneath and below an advancing North American Continent. Super-hot water driven from the subducting ocean floor migrated upward and melted rock as it proceeded. This process took place during the Cretaceous Period, 100 million years ago. Granitic rocks have a speckled salt-and-pepper appearance