GL 107 Earth Systems Science

K. Hannula

Discussion questions for 9/17/07

1.  What type of rocks form at divergent plate margins?

Basalt forms on the ocean floor; gabbro (mafic plutonic rock) forms underground as magma chamber cools.

2.  What type of rocks form at oceanic hot spots?

Basalt forms at the surface (gabbro forms underground).  Example:  Hawaii volcanoes.

3.  What type of rocks form at continental hot spots?

Both basalt and rhyolite can form at continental hot spots (such as Yellowstone).

4.  What causes melting at convergent plate margins?

Addition of water (which escapes from the downgoing plate) to the asthenosphere lowers the asthenosphere's melting point, causing melting.

5.  What types of rocks form in volcanoes at convergent plate margins?

Andesites are most common; rhyolite is also possible.

6.  At what type of plate boundary do the most destructive volcanoes form? 

Convergent plate boundaries.  (Continental hot spots can also be very destructive.)

Why do they form there?

Convergent plate boundaries erupt intermediate to felsic magmas (with a higher viscosity due to more Si), and the magmas typically contain more water.