Desscribe the origin of volcanoes on the ocean floor and along continental margins bordering ocean trenches...

Question

Ocean-floor volcanoes......


Volcanoes along continental margins bordering ocean trencehes......

Answer 1

Ocean floor volcanoes come from two different sources. There are seafloor spreading centers which are not really a volcano, but are similar in that they are areas where molten rock is being ejected from the interior of the earth creating new crustal rocks. The real volcanoes in the ocean come from geological "hotspots". These are places where a isolated area of the mantle is pushing hotter than normal material which melts the crust when when it reaches it. Since most of the earth's surface is oceanic crust these hotspots are usually found on oceanic crust, and form basaltic sheild volcanoes (like the big Hawaiian Island). These volcanoes can become VERY big because of their fairly gentle eruptions, and because the non-viscous lava spreads out making it possible for the volcano to be stable at much greater hight than a narrow stratovolcano. Mauna Kea was formed from a mantle hot spot, and it is the world's largest mountain measured from its base (about 33,500 feet tall). There are also areas where oceanic crust is subducting under other oceanic crust. When this happens you get a volcanic island chain like Japan or New Zealand.

Volcanoes along continental margins are usually formed because oceanic crust is subducting under the continent. The oceanic crust carries lots of water with it when it subducts under the continent. This water will lower the melting point of surrounding rocks when it is subducted, causing melting. Since this molten rock is less dense than the surrounding solid rock it migrates towards the surface either by flowing through cracks and other weak parts of the surrounding rock, or by melting surrounding rock. Since the magma is melting continental rock the magma becomes fairly viscous (felsic continental rock is more viscous than mafic oceanic rock) and as a result traps a lot of gas in the magma. This gas causes eruptions to be violent compared to the eruption of shield volcanoes. The violent eruptions form what are called stratovolcanoes (also called composite cones).