The most likely intersection of a flat descending slab with the curved surface of the earth is a long, gentle curve or arc. Remember the lava forms because it contains a great deal of dissolved water and CO 2 . Thus when the lava reaches the surface, the gases are released, making a very explosive eruptions. Now let's look at oceanic-continental collisions. In these cases, the oceanic crust always forms a descending slab beneath the continent because the granite in the continental crust is too light to sink into the mantle.
Granite contains lots of silica , so the lava becomes very viscous .
Consequently, when these lavas reach the surface, the resulting eruptions tend to be even more explosive than the island arc eruptions. The Cascade volcanoes in the American Northwest are the best studied examples of this type and include Mount Rainier and Mount St. Helens . A 3-D map showing the Cascades and the prominent volcanoes can be seen by clicking here . The third type of plate collision is continental-continental. The rocks on both sides of this type of collision are too light to sink into the mantle, so the edges simply crumble and fold into giant mountain ranges.