andesite
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- n a dark grey volcanic rock
- Preliminary date shows the rock might have been formed more than a billion years ago by a volcano, since it appears to be made of a kind of andesite, the second most common type of .
- In the subduction volcano, the magma is composed largely of andesite, which is a melt relatively rich in silica, water vapor and other gases.
- Banderson says, "We believe its composition is similar to a rock on earth called andesite, but further analysis is needed to confirm this.