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Greenstone

Pictures of Metamorphic Rock Types

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Greenstone is a tough, dark altered basaltic rock that once was solid deep-sea lava. It belongs to the greenschist regional metamorphic facies. (more below)
Altered basalt

From stop 31 of the California subduction tour

Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)
In greenstone, the olivine and peridotite that made up the fresh basalt have been metamorphosed by high pressure and warm fluids into green minerals—epidote, actinolite or chlorite, depending on the exact conditions. The white mineral is aragonite, an alternative crystal form of calcium carbonate (its other form is calcite).

Rock of this kind is manufactured in subduction zones and is seldom brought to the surface unchanged. The dynamics of the Californian coastal region make it one such place. Greenstone belts are very common in Earth's oldest rocks, of Archean age. Exactly what they mean is still not settled, but they may not represent the kind of crustal rocks that we know today.

For more photos see the Metamorphic Rocks Gallery.

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