When oceans close is the dangerous volcanoes because of subduction. Iceland is very fluid not that dangerous, all gases come out. More viscosious - dangerous, stuff comes out pyroclastic. Pyroclastic broken by fire, rocks are shattered by eruptions and thrown up into air. Volcanic bombs or ash(is a misnomer - named 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 down. We know that slabs don"t melt till after 700km down. Magmatic arc a line of volcanoes directly above subduction. Silicic magma - sediment + seawater melted, reducing magma. Deep water trenches ocean floor going down into subduction zone. Red boundary - pacific plate and austrailian plate. The southern alps mt. cook in new zealand. Killed the gate keeper that protected the volcano.