Geo 105, Main points for Volcanos for Jan. 27

Volcanic Hazards

• Local: explosive blast, tsunami (in a few cases), lava rivers
• Regional: fall-out of rock debris from explosion
• Global: fine rock dust causes global changes to atmosphere

Tectonic setting of volcanos

• Spreading centers: mostly under the ocean, not directly hazardous
  Special note: quite beneficial to the bio communities that live there
• Hot spots: NOT a plate boundary setting, magma "burns" through the oceanic lithosphere. Classic example is Hawaii, several other examples in the world's ocean, a few have been identified in continents (e.g. Yellowstone)
• Subduction Zones: most of the volcanos -- that stick above sea level -- are found in subduction zones. Although we still do not have a complete understanding of these volcanos, nearly all of them occur above the point where the top of the subducting plate reaches a depth of 100 to 125 km.

Rock types

• Basalt: the rock type at spreading centers, which then explains why the oceanic crust is basalt. Also, hotspots found in oceans (e.g. Hawaii) erupt basaltic magma. Basaltic volcanos have a mound shape ("shield" volcanos).
• Granite (or Andesite): the rock type for Subduction Zone volcanos. Compared to basaltic magma, granitic magma is more viscous. This changes the eruptive style and appearance of the volcanos (steep sides, "strato" volcanos). Examples: Mt. Fuji, Mt. St. Helens, Mt. Ranier, Krakatoua.