What's the difference between the Sierra and Cascade Mountains?

Dear Doctor Rock,

Why do we call the mountain range in California 'The Sierra Nevada' and the ones in Oregon and Washington 'The Cascades'? What is the difference?

Rita Bjornstein,
Topeka, Kansas

Dear Rita,
They are two geologically and geographically separate mountain ranges. The Sierra Nevada (the 'Snowy Range') are older than the Cascades, and were once volcanic. Between about 220 and 15 million years ago, there was subduction off the coast of California- the now-extinct Farallon Plate slid beneath the west-moving North American Plate, and a range of volcanic mountains rose where the Sierras are. The famous granite of Yosemite, Mount Whitney, and elsewhere in the Sierras was once the magma in the crust that fed those volcanoes. Subduction migrated northward up the coast of the continent, and the volcanic rocks that once covered the surface in the Sierras are mostly stripped away by erosion to expose the guts of the silenced volcanoes.
The Cascades are still an active volcanic chain and result from subduction of the Juan de Fuca Plate beneath North America. This volcanic chain is around 40 million years old. They extend from Lassen Peak in northern California through Oregon and Washington and then for another 125 miles north of Vancouver, British Columbia. (Yes, there are many volcanoes in British Columbia!) Subduction still occurs off the coast parallel to the Cascades chain, and as long as it continues, magma will be generated beneath the North American Plate and volcanoes will erupt in the Cascades.
Click here to see an animation about subduction and some more explanation. There is a great YouTube video showing the subduction of the Farallon Plate, the migration of subduction northward, and the end of subduction in California. The author, Dr. Tanya Atwater, is a pioneer is this geology.

Dr. Rock


The Cascade subduction zone. The thicker continental plate overrides the

ocean plate. The colored dots represent earthquakes. Volcanoes can grow once

the subducted ocean lithosphere (rocky crust plus strong upper mantle) have

descended far enough into the soft asthenosphere (weaker mantle).