The most likely intersection of a flat descending slab with the curved surface of the earth is a long, gentle curve or arc. Since the volcanic islands tend to form a few hundred miles "downslab" from the collision site (marked by a deep trench on the ocean floor), they usually form an arc-shaped group. The arc shape is easy to see in many examples: the islands of Japan, the Aleutian Arc extending from the Southwest Coast of Alaska almost to Siberia, and the Timor-Java-Sumatra chain in Indonesia. Consequently, these types of volcanoes are called "island arc volcanoes." What kind of eruptions do we get with island arc volcanoes? Remember, the lava forms because it contains a great deal of dissolved water and CO₂. Thus, when the lava reaches the surface, the gases are released, making very explosive eruptions. Some of the most violent historical eruptions are of the island arc type, such as Tambora (1815), Krakatoa (1883), and Mount Pinatubo (1992).

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. In the descending oceanic slab the same conditions for melting occur as before except this time as the liquid rock ascends through the continental crust, some of the granite is melted and mixed with the liquid lava. 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. The best example of this type of collision is the Himalayan Mountains in central Asia, which are the result of India crashing into Asia. Very little rock in these collisions is forced to great depths, so little melting occurs, and few volcanoes form. (Just in case you are wondering, there are only a few volcanoes in the Himalayas.)

So much for plate collisions. What about plate separations? In these cases no rock is forced down to where it can melt, so no volcanoes form, right? Wrong! Nature does it again! Where plates are separating, something must flow in to fill the gap or else we would have giant, open cracks extending hundreds of kilometers into the Earth's interior (talk about your ultimate skydiving experience!). The stuff that flows in is hot, soft rock from the upper mantle. This rock is originally under great pressure, but as it rises into the gap between plates, the pressure drops faster than the rock can cool. A small amount of basaltic rock melts and flows to the surface through cracks. There is little or no dissolved gas in this lava, so the eruptions are not explosive. The amounts of lava erupting at any given time are small, but the eruptions are continuous and occur along all the oceanic ridges, making this type of volcano the most common in the world. Iceland and the Kenya rift are among the very few places on land where this type of volcanism occurs.

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