California's (and the World's) Oldest Rocks

I'm glad to see that Ben Burress, my colleague at KQED QUEST, was open to the thrill of deep time as he laid hands on some of California's oldest rock in Death Valley.

When I went to geology school, back in the ice ages, I brought with me the same normal, healthy fascination with extreme age—geological age. At that time there was still a great deal of mystery about the earliest times. To me, the most mysterious thing you could call a rock was "Precambrian," that is, rock dating from the time before the earliest hard fossils appeared, marking the base of the Cambrian Period. Precambrian time amounts to four billion years, nine-tenths of all Earth history. Unlike familiar, fossil-studded post-Precambrian time (I know that's a weird term: geologists call it the Phanerozoic Eon), the Precambrian was an endless succession of enigmatic, mashed-up rocks. Their story wasn't really a story but a pile of hints and fragments—mountain ranges rising and eroding, continents merging and separating, just one damn thing after (or before?) another in the dimness of deep time.

We have a better picture of the Precambrian now, but really, it's still pretty blurry. If you look at the geologic time scale, you'll see that the Precambrian time divisions are set at arbitrary even numbers of years, not significant geologic events. In California, our oldest rocks all originated around 1700 million years ago in the Paleoproterozoic Era, and they all sit in the corner of the state outlined on this geologic map.

The outline marks a segment of the North American continent's ancient foundation—the craton—called Mojavia for the Mojave Desert. It's a pretty young part of the craton, and it's all we've got.

The oldest basement rocks of Mojavia are all highly altered—squeezed and stretched rocks classified as gneiss or schist. And they're still on the move today as plate-tectonic interactions are both stretching western North America apart and, in California, yanking it northward along the San Andreas fault system. Just as Sierran granite (shown in red) has been pulled all the way up to the Bay Area, so has a big chunk of Mojavia making up the San Gabriel Mountains.

The westernmost outlier of those Paleoproterozoic rocks crops out in the San Emigdio Range, which forms the rim of the Central Valley southwest of Bakersfield. The quickest way to see them is to turn west off of Interstate 5 toward Frazier Park, then drive up either Frazier Mountain or Mount Pinos (Cerro Noroeste is also possible if you're ambitious). The gneiss shown at the top of this post is on Mount Pinos. It was turned into gneiss around 1450 million years ago, but the rock originated as something else, probably a sandy mudstone, around 1700 million years ago (source, USGS OF-02-406).

Elsewhere in California, you can repeat Ben Burress's experience in Death Valley by walking up the canyon at Badwater, but a more interesting canyon hike with the same Paleoproterozoic rocks starts about 2 miles north of Badwater. And blogger Garry Hayes describes more Paleoproterozoic rocks in the San Gabriels.

The really old rocks in America are found in Wyoming and the states around Minnesota. They date from the Paleoarchean Era and are more than 3 billion years old. One example can be easily seen in Washington, D.C., in a prominent spot between the White House and the Washington Monument: the twin Haupt Fountains, each one made from a 55-ton slab of Montevideo Gneiss from Minnesota. At the time they were made, this was considered the oldest rock in the country.

The reddish, scrambled-looking Morton Gneiss is a popular stone for buildings and gravestones, and at 3524 million years of age it's considered to be the oldest bedrock in the United States. You could travel to Minnesota to see it (there's even an EarthCache for it west of Minneapolis), but don't bother—you probably have samples right in your own town. David B. Williams, author of Stories in Stone, calls it the country's most beautiful building stone.

But you can go older still. There's a Canadian rock from a spot in Labrador called Nuvvuagituq said to be 4.28 billion years old, but the date is still not settled. Today the Acasta Gneiss, also from northern Canada, is the world's oldest firmly dated rock at 4.03 billion years. You can see that one at Rocklin, just up the road. Go to the campus of Sierra College, on the south side, and locate the excellent Earth History Rock Walk.

There, among the assorted amazing and instructive boulders, is a nice chunk of Acasta Gneiss.

But whenever I want to experience the deepest possible deep time, I reach for a meteorite from my collection and lick it. Nearly every common meteorite is older than Earth itself.