 Beginning about 200 million years ago, tectonic pressure forced the sea floor beneath the American continental mass.As it sank, or subducted, beneath the western margin of the North American plate portions of the sea floor and overlying continental crust heated and melted, producing large molten masses (magma).Being lighter and hotter than the ancient continental crust above it, this magma forced its way upward, cooling as it rose to become the granite rock found throughout the Sierra Nevada and other mountains in California today.