Alan Stahler: Some rocks are older than others |

Alan Stahler: Some rocks are older than others

On their return to Earth, the crew of Apollo 11 looked back to make this image of the moon.
Courtesy NASA


With a forecast for clear skies, members of Nevada County Astronomers will set up scopes at the junction of Highway 49 and the old Downieville Highway, this Saturday, beginning 8 p.m. It’s a free event, and kids are welcome.

— Alan Stahler

Some numbers are hard to remember, but the age of the Earth — the number of years since Earth was born — is easy.

Just notice how one number follows another: Earth is 4.567 billion years old.

We’ve yet to find an Earth-rock that old, however, and likely never will. Rocks — on Earth, at least — don’t last forever. Exposed to the weather, they break down, becoming sand and clay and salt, and wash away.

Out beyond the orbit of Mars, a torus — a donut — of rocks circles the sun: the asteroid belt. Asteroids, it seems, are leftovers from the formation of the planets. In the vacuum of space, asteroids suffer neither wind nor rain.

Two spacecraft, one American, one Japanese, are right now working to bring samples of asteroids back to Earth. But we already have such samples. Meteors — “shooting stars” — are bits of dust and rock, blown or blasted off comets and asteroids, that have since fallen to Earth.

The solid bodies of the solar system, such as planets and asteroids, likely formed at the same time. The age of the oldest meteorites — 4.567 billion years — gives us the age of the asteroids, and, thus, the age of the Earth.

We don’t have to find any meteorites, though, to view rocks older than any of Earth. We only need to look up, after sunset tonight.

Earth did not always have a moon. But, it’s thought, some millions of years after Earth formed — some four-and-a-half billion years ago — something the size of a small planet smashed into Earth. The smash-up flung shattered rock out into space, into orbit around Earth. It was this material that coalesced to form the moon. Hot from the energy of impact, the young moon began as a molten mass of liquid rock.

The salt-and-pepper rock, common in the foothills, is granodiorite. It’s composed of two white minerals and a sprinkling of a black one. The black mineral, rich in iron, is hornblende.

Look closely at a chunk of granodiorite (it’s best to break one open, to see a fresh surface). It’s certainly solid, yet water has likely seeped into it, making it fairly easy to distinguish between the two white minerals. One white (maybe gray-white) mineral will be shiny — that’s quartz, which is very resistant to weathering. When granodiorite finally disintegrates, the quartz grains survive, wash downstream, and accumulate as sand.

The other white mineral — feldspar —is easily attacked by water, which destroys its shine, and makes it dull. Water turns feldspar to clay; mix in a bit of rusty iron, weathered out of the hornblende, and you’ve got red clay.

As the newborn, molten moon slowly cooled, minerals sorted themselves out. Feldspar floated to the surface, to crystallize as the bright rock of the moon, the background rock on which the dark man-in-the-moon is painted.

Looking up at the moon tonight, the bright feldspar is obvious. At four-and-a-half billion years old, it’s older than any Earth-rock.

With a forecast for clear skies, members of Nevada County Astronomers will set up scopes at the junction of Highway 49 and the old Downieville Highway, this Saturday, beginning 8 p.m. It’s a free event, and kids are welcome. Bring a sweater.

Al Stahler enjoys sharing nature with friends and neighbors. His science stories can be heard on KVMR-FM (89.5 MHz), and he may be reached at

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