Transcripts of quizYourselfL6Review

My question to you was: Where is the Cascade Range on this drawing? It is a north-south line right about here and it extends farther off this map toward the south of course, but this is just a little bit of a cross-section. Where does that correspond to on the lower plate? It corresponds to this area right here where you can see that partial melt is being formed. And how does partial melt form? In a subduction zone setting you've got a basaltic oceanic plate being forced down under a big fat continental plate that is this tall. The basaltic oceanic plate goes under the granitic North American plate. At this particular depth where the partial melt is happening right here, the temperature and pressure of the mantle is such that volatiles that are in the crust of the oceanic plate are being driven off and they percolate upwards. As they percolate upwards they incorporate some of the material of the granitic overlying continental plate. What that means is by the time the lava comes out at the top it's more viscous than if it were just basalt. So this is not Hawaiian type lava. This is the kind of lava that forms composite volcanoes. They are steep-sided because the lava has more viscosity to it because it has some more silica in it than it would have if it had just erupted out of the ocean. And that's our review of Lesson 6!