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).

Why are rocks in the Southwest red?

Dear Dr. Rock,

I do most of my traveling in the West, where I often see red rock and red earth. I have enclosed a photo I took at Red Rock Canyon near Las Vegas. 

In the gold rush country near Lake Tahoe I see a lot of red earth. Farther north, along Eagle Lake near Susanville, I came upon a stretch of highway where the pavement was red. I assume it is because red, volcanic rock was used to make it. Is that right? Of course, southern Utah has a lot of red rock, some of the most spectacular in Zion and Bryce Canyon National Parks. So my question: what causes rock and earth to be red?

Carl Weber

Sacramento, Calif.

Dear Carl,
There are fairly young cinder cone volcanoes around Eagle Lake, and it could be the highway department used some of their red rocks in the pavement you saw.

Bryce Canyon National Park

Red-colored rocks are usually the result of oxidation, or rusting, of iron-bearing minerals in rocks. Cinder cones are particularly rich in these minerals, especially olivine and pyroxene found in basalt lava. In southern Utah much of the rock is sandstone made of crystals and tiny fragments of rock deposited as ancient sand dunes or in seas. The sands were eroded from even older rocks that contained iron, so they rust when exposed to moisture and oxygen in the atmosphere. 

Sometimes the red coloring is only a stain on iron-poor rocks like limestone. The water runs down cliff faces from iron-rich rocks higher up. You'll see a lot of that at Grand Canyon.

Thanks for the photo from Red Rock Canyon. I used to do a lot of rock climbing there when I was a whipper snapper.

I really like to get your questions, so keep them coming! --Dr. Rock