Kent found a strange rock

Kent grew up near the upper Rio Grande. On one outing he found a strange rock.

While on a camping trip when I was fifteen years old, I came across a baseball-sized rock that I immediately realized was rather unusual. It was a jet-black, coarsely crystalline rock (unlike fine-grained basalt) with green patches and a high specific gravity. It showed none of the surface melting characteristic of a meteorite. The material was highly magnetic but it was not iron. Further investigation showed that the chief mineral was magnetite (magnetic iron oxide) and the green patches were chlorite. This was indeed a most unusual rock. I still don’t know exactly where it came from — but I’m confident its original home was close to the earth’s mantle.

In trying to understand where it came from, he presents an interesting geology lesson. His web log Trolling in Shallow Water entry Down to the Moho (05Ma14) gets into what we know of the structure of the earth and why.

How do we know this? There are two sources of information about the deep interior of the earth. The first is seismic measurements. Earthquakes produce seismic waves that speed through the earth’s interior like sound waves from a firecracker. (In fact, one kind of seismic wave, known as a p-wave, is essentially a very low-frequency acoustic wave.) The speed at which these waves travel depends on the properties of the material through which they are moving. Furthermore, while p-waves can travel through both liquids and solids, another kind of seismic wave — the s-wave — can travel only through a solid.

So, if you can handle terminology such as

“the Mohorovicic discontinuity, known affectionately as the ‘Moho.'”

“meteoritic evidence that a nearby supernova enriched the protostellar nebula with quite short-lived isotopes”

“A thin crust of relatively light silicates formed at the surface during the first half-billion years of the Earth’s history, the so-called Hadean Eon. This melting and differentiating process released additional energy, since heavy iron droplets released energy as they sank and light silicate droplets as they rose to the surface.”

The result is that the Earth today has a core of elemental nickel and iron, surrounded by a mantle rich in magnesium and iron silicates (ultramafic rock), topped by a crust dominated by aluminum, potassium, and sodium silicates (sialic rock). Convective currents in the mantle, driven by continuing radioactive decay, have piled the sialic rock into continents where the crust is typically thirty miles thick. The crust in the deep ocean basins is much richer in the heavier elements (mafic rock) and also much thinner, typically around six miles thick.

and what to learn a bit about the earth you inhabit, take a gander and see what you get trolling in shallow water.

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