The Birth of a Mountain Range
Graphic courtesy United States Geologic Survey (USGS)
How Did Denali Get So High?
Graphic courtesy NASA
One such tectonic plate, known as the Pacific Plate, forms the floor of the Pacific Ocean. It is slowly moving northward at about the rate that your fingernails grow. Oceanic plates are denser than continental plates. When an oceanic plate collides with a continental plate, the oceanic plate sinks below the continental plate in a process called subduction. In the diagram at left, the Pacific Plate is diving below the part of the North American Plate that holds Alaska's mainland. As the Pacific Plate moves northward, it carries chunks of land and pieces of other plates, some from thousands of miles away. These chunks and pieces are called terranes.
Its composition is another reason that Denali has grown to such a great height. It is composed mainly of the igneous rock granite. Denali's granite formed below the Earth's crust as part of a batholith. A batholith is a bubble or mass of magma within Earth's crust. Plutons are parts of batholiths, defined by their chemical composition. The chemical composition of the magma determines the type of rock that will crystallize. Other intrusive igneous rocks (that is rocks that cool within the crust rather than at the surface) include gabbro, diorite, and pegmatite, to name just a few. Granite usually happens to be less dense than much of the rock that surrounds it. Over millions of years, a granitic pluton will float slowly towards Earth's surface, as it has in the case of Denali. Denali sort of "popped" up to the surface, much like a cork held under water will pop up when released. Just remember that "popping up" can take millions of years in geologic time! Erosion of Earth's surface rocks also helped expose the granitic rocks that make up Denali.
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