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USGS CMG InfoBank: Convergent Plate Boundary Magma

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Comment: 14:51 - 16:24 (01:33)
Source: Annenberg/CPB Resources - Earth Revealed - 14. Intrusive Igneous Rocks
Keywords: "convergent plate", andesite, diorite, magma, basalt, lithosphere, marine, sediment, serpentine, melting, crust, felsic, "Bowen's Reaction Series", "David Sigurdson"

Our transcription: At convergent plate boundaries, andesite and its plutonic equivalent, diorite, are among the most common igneous rocks.
But geologists are uncertain about how their parent magma forms.
We think that much of the magma developed as a result of melting of basalt from the upper part of the descending lithospheric plate.
There may also be components that are derived from the marine sediments that may also be carried down.
These marine sediments may have interstitial water that can also contribute to and become dissolved in the magma.
There may be substantial amounts of serpentine, that are being brought down with the basalt.
These are hydrated minerals and could also contribute water.
Now, the presence of water is significant because it helps to bring about a fluxing of the remaining amount of materials.
It lowers the melting point, and it helps in the formation of magma.
Now, as this magma moves up toward the surface, is it basaltic in composition?
Is it more felsic?
We aren't completely certain of the nature of the magma.
As this magma moves up toward the surface, it encounters the base of the Earth's crust, mainly made up of felsic constituents.
These are lower melting materials.
As we look at Bowen's Reaction Series, we see that these minerals from the continental crust are really the low melting material, so they are very likely to become incorporated into the magma.

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