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Granite outcrops above Holcomb Creek (Section 11)

There have been many theories about how continental drift causes the formation of volcanoes. In the Pacific North-West it is thought that the Pacific Plate is descending beneath the Continental Plate. Water and gases from the porous oceanic crust are carried down and superheated, melting the surrounding rock to produce magma (the term given to molten rock, or lava, while it remains beneath the Earth’s surface). A combination of magma and highly pressurised gas can cause explosive volcanic eruptions, such as that seen when Mount St Helens exploded in 1980.

The mountains of Southern California and the Sierra Nevada are primarily composed of granitic rocks, formed about 80–240 million years ago when magma cooled and solidified below the Earth’s surface. Insulating layers of rock meant that that cooling process took place very slowly, allowing coarse-grained crystals to form. Continental drift has caused the rock to be lifted to heights above 20,000ft while the covering rocks have been eroded away, leaving granite as the predominant surface rock today. When granite is eroded, the large crystals tend to form the sandy and gravely soils that predominate in Southern California and the Sierra Nevada.