Formation of Volcanoes

How Do Volcanoes Form?

Volcanic activity is closely linked with plate tectonics (except for the theory of hot spot). As the Earth's huge plates move and interact in the lithosphere, or the planet's outer shell, they trigger a very gradual chain of events that can end with explosions.

Formation of Volcanoes
Taken from http://www.nationalgeographic.com/forcesofnature/forces/volcanoes.html

1. Island Arc

When two plates come together, one of the plates may slide under another in the process called subduction. Heat from deep in the Earth melts rock in the descending plate. An island-arc volcano is a type of subduction-zone volcano. In subduction zone, either island arc or volcanic arc can be formed. Island arc volcanoes occur when one oceanic plate subducts, or descends, under another oceanic plate. The crustal portion of the subducting slab contains a significant amount of surface water, as well as water contained in hydrated minerals within the seafloor basalt. As the subducting slab descends to greater and greater depths, it progressively encounters greater temperatures and greater pressures which cause the slab to release water into the mantle wedge overlying the descending plate. Water has the effect of lowering the melting temperature of the mantle, thus causing it to melt. The magma produced by this mechanism is usually andesite in composition. It buoyantly rises upward to produce a linear belt of volcanoes parallel to the oceanic trench. This action can then create a chain of volcanic islands.

Island arc formed by oceanic-oceanic collision
Taken from http://www.geology.sdsu.edu/how_volcanoes_work/Images/Diagrams/osubduction.gif

2. Hot Spots
About 5 percent of all known volcanoes form in the middle of plates, not at their edges. These intraplate volcanoes are caused by hot spot, unusually hot areas deep within the Earth. Magma rises from the hot spots and erupts as lava through crack in the Earth's surface, forming volcanoes. Mantle plumes appear to be largely unaffected by plate motions. As plates move across stationary hotspots, volcanism will generate volcanic islands that are active above the mantle plume, but become inactive and progressively older as they move away from the mantle plume in the direction of plate movement. Thus, a linear belt of inactive volcanic islands will be produced. A classic example of this mechanism is demonstrated by the Hawaiian seamount chain.

Hawaiian Seamount Chain model
Taken from http://www.geology.sdsu.edu/how_volcanoes_work/Images/Diagrams/hotspot_diagram_usgs_s.gif

3. Spreading Centres
Volcanoes called rift volcanoes arise in areas called spreading centres. In these zones plates are moving away from each other, spreading or splitting the Earth's crust. Basaltic magma, derived from the partial melting of the mantle, is injected into the fissures or extruded as fissure eruptions.

4. Continental Rift Zones
When spreading centres develop within continents, they form new plate boundaries and trigger volcanic activity. Spreading may have created East Africa's volcanic Great Rift Valley.

Oldoinyo Lengai, an active volcano in the East African Rift Zone.
Taken from http://pubs.usgs.gov/gip/dynamic/graphics/Oldoinyo_erupts.gif

5. Volcanic Arc
Volcanic arc occurs when one denser oceanic plate subducts under a less dense continental plate plate, leading to partial melting of the descending water-rich oceanic plate and some of the overlying mantle. This process produces low-density magma which rises onto the overlying continental crust, through faults and cracks in the lithosphere, where it will cool and crystallise at depth. The accumulation of magma on the continental plate over time would lead to the formation of volcanic arc.

Volcanic arc formed by oceanic-continental subduction
Taken from http://www.geology.sdsu.edu/how_volcanoes_work/Images/Diagrams/csubduction.gif

Andes Mountain, a volcanic arc formed by the convergence of the oceanic Nazca plate
and continental part of South American plate.
Taken from http://en.wikipedia.org/wiki/Andes