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