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USGS Geology in the Parks

North Cascades Geology

From the Fiery Furnace: Volcanoes and Their Roots

For people who in the Pacific Northwest, the growth of the Cascade Volcanic Arc is the most dramatic and comprehensible act in the geologic drama presented in this website. The arc began erupting about 35 million years ago (Oligocene) and is still erupting.

Lavas representing the earliest stage in the development of the Cascade Volcanic Arc mostly crop out south of the North Cascades proper, where uplift of the Cascade Range has been less, and a thicker blanket of Cascade Arc volcanic rocks has been preserved. In the North Cascades, geologists have not yet identified with any certainty any volcanic rocks as old as 35 million years, but remnants of the ancient arc’s internal plumbing system persist in the form of plutons,which are the crystallized magma of chambers that once fed the early volcanoes.

Batholiths in North Cascades
Cascade Arc batholiths and associated plutons in Washington and nearby British Columbia. Heavy black lines show area covered by the geologic map

The greatest mass of exposed Cascade Arc plumbing is the Chilliwack Batholith, which makes up much of the northern part of North Cascades National Park and adjacent parts of British Columbia beyond. The batholith is actually made up of numerous separate, mostly granitic plutons, some large, some small, but all more or less contiguous. When the batholith was first named by Reginald Daly in the early twentieth century, the great range in ages of the individual arc root plutons was not known. Individual plutons range in age from about 35 million years old to 2.5 million years old. Based on their ages, we have divided the plutons of the batholith, and in the North Cascades in general, into three families: the Index Family, the Snoqualmie Family, and the Cascade Pass Family. Each family includes a wide range of igneous rocks. For more information on the families of the Chilliwack Batholith, see Families of the Arc Root Plutons.

The older rocks invaded by all this magma were affected by the heat. Around the plutons of the batholith, the older rocks recrystallized. This contact metamorphism produced a fine mesh of interlocking crystals in the old rocks, generally strengthening them and making them more resistant to erosion. Where the recrystallization was intense, the rocks took on a new appearance dark, dense and hard. Geologists call such rocks hornfels that is, horn rock. Many rugged peaks in the North Cascades owe their prominence to this baking. The rocks holding up many such North Cascade giants, as Mount Shuksan, Mount Redoubt, Mount Challenger, and Mount Hozomeen, are all partly recrystallized by plutons of the nearby and underlying Chilliwack Batholith.

Something extra: Making room for magma and Making granite and its relatives

On to Traces of the Last Arc

Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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