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USGS CMG InfoBank: Anorthosite

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Comment: 22:45 - 24:21 (01:36)
Source: Annenberg/CPB Resources - Earth Revealed - 14. Intrusive Igneous Rocks
Keywords: anorthosite, plutonic, Moon, basalt, magma, crystal, differentiation, calcium, plagioclase, "San Gabriel Mountains", "David Sigurdson"

Our transcription: Anorthosite is another kind of rock that is relatively rare on Earth and almost entirely embedded in ancient plutonic belts cutting across the older portions of the continents.
Anorthosite may be rare on Earth, but it is the primary type of rock found in the highlands of our nearest neighbor in space, the Moon.
When the astronauts came back from the Moon, they brought back samples of a kind of rock called anorthosite.
Apparently it's very common on the surface of the moon, but it tends to be relatively rare here on Earth.
I have an example of anorthosite here.
This one comes from the San Gabriel Mountains, and it seems to represent a body of rock that was formed at great depth in the interior of the Earth.
It may have started out as a rock of basaltic composition, a magma of basaltic composition; but it was held at great depth for a long period of time, long enough that the crystals had an opportunity to grow very large.
I don't know if you can see, but this is made up of extremely large crystals.
It was evidently under tremendous pressure at that depth as well; and so this magma has undergone an extreme amount of differentiation, that is to say there was a separation of the crystals from the melt.
Apparently these crystals settled to the bottom of the chamber where they became almost purely made up of this one kind of crystal.
In this case, it's a calcium rich plagioclase; and a kind of rock made almost exclusively of this plagioclase is called, as I say, anorthosite.

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