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Rocks from space reveal details of Earth’s birth

Given the primitive satisfaction that comes from lifting a rock to see what lurks beneath, imagine how satisfying it might be to somehow lift enough rock to reveal the matter lurking in the innermost parts of the Earth.

Most of our knowledge of the inner Earth is gleaned from vibrations triggered by earthquakes. Just as the sound of a bell can tell you whether it’s thick or thin, solid or cracked, seismologists (“students of shaking”) can interpret waves from the Earth to determine what sort of rocks they’ve passed through.

How satisfying it would be, though, to actually see what that deep rock and metal look like. Drilling is not the answer: The Earth is a good 4,000 miles thick, from surface to center; the deepest well goes down only a bit over seven miles.



If the deep Earth is far from our grasp, the early Earth is even farther … farther away in time, some 4.5 billion years. It was that long ago that a giant cloud of gas and dust collapsed on itself to form the sun, Earth and other planets. Atoms in the cloud joined to form molecules; molecules joined to form dust; dust to form sand; sand, gravel; gravel, cobbles; cobbles, boulders; boulders, planetesimals; and planetesimals joined to form planets.

The oldest rocks on Earth, however, are younger than this. Melted and buried by plate tectonics, the oldest rocks have disappeared from our planet.




Despite the distances, we can hold in our hands the same sort of rock and metal of which the inner Earth is made, and the primordial matter from which our planet formed long ago.

As the sun and planets were forming, conditions in some parts of the solar system caused the process to stall. Between the orbits of Mars and Jupiter, the gravitational tug of the planets prevented planetesimals from linking up. In a perpetual state of arrested development, these planetesimals, many of them miles across, circle the sun today as asteroids.

Jupiter continues to tug on the asteroids, altering their orbits, smashing them into each other. The collisions send shards flying in all directions; some asteroids are destroyed completely. Some of the fragments drop inward, toward the sun and the inner solar system.

As they’re flying through space, these bits of rock and metal are called meteoroids. Should they cross the orbit of Earth, gravity may haul them in. Careering through the atmosphere, they light up the air around them; we call them meteors, or “shooting stars.” Those that survive the fiery plunge and hit the ground become meteorites – samples of asteroids, primeval rock and metal, here on Earth.

One of the pleasures of collecting rocks is that the collection reminds one of the connections he or she has to various parts of the Earth. That pleasure extends to the collection of rocks that have fallen from space.

Beginning at noon Saturday, Sierra Friends Center invites readers of The Union family, friends and neighbors to an open house with music, food (yes, there is such a thing as a free lunch!), nature walks and more. If you come by the lab, you can join me in doing some simple experiments, and look at meteorites from my collection and from that of Wayne Watson of the Imaginarium.

Sierra Friends Center (formerly John Woolman School) is off Jones Bar Road. The open house lasts until late afternoon; at 8 p.m., we’ll regroup on the soccer field for a Sky Watch. Call 273-3183 for information and directions.

Trained as a biologist, Alan Stahler is also an amateur astronomer. He teaches biology and geology at Bitney Springs Charter High School. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).


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