When oceans close is the dangerous volcanoes because of subduction. More viscosious - dangerous, stuff comes out pyroclastic. Volcanic bombs or ash(is a misnomer - named. Iceland is very fluid not that dangerous, all gases come out. Pyroclastic broken by fire, rocks are shattered by eruptions and thrown up into air improperly: italy thought it was coal burning. The green areas subduction back into the mantle. The pacific ring of fire: tectonics of a closing ocean. Pacific rim - dense concentration of volcanes, major earthquakes, tsunamis. The stars marks earthquakes - the deepest earthquakes are 700km down. We know that slabs don"t melt till after 700km down. Magmatic arc a line of volcanoes directly above subduction zone. Oceanic crust made out of basalt. Silicic magma - sediment + seawater melted, reducing viscosiy of magma. Deep water trenches ocean floor going down into subduction zone. New zealand: small island, exposed tip of a much larger.