sample quiz question #9 : LUSENET : Introductory Geology, Oswego State : One Thread

why cant a divergent boundary be recognized by a volcanic island arc?

-- erika eldred (, January 31, 2001


Volcanic island arcs form at subduction zones, not divergent boundaries. Can anyone explain why volcanic island arcs don't form at divergent boundaries?

PS If you want me to answer, say so (by responding to Erika's original question.

-- Sharon Gabel (, January 31, 2001.

It states on page 16-17 of our text, that in divergent boundaries the plates are moving apart. As they begin to pull apart, magma rises between the newly formed ridge and "fills it in". Volcanic activity can take place, but the volcanic isl. arc's take place when the plates subduct (which is the opposite of the movement that is happening here, so it's not possible).

-- Terry McCall (, January 31, 2001.

Vulcanism is a response to subduction, the movement of one plate (usually oceanic) under another (usually continental). As the subducted plate drives deeper beneath the surface, it begins to melt, creating magma. This seeks to rise (hot magma being relatively less dense that its overlying solids) and finds outlet on the surface as a volcano. Sometimes it promotes granitization instead. As subduction is a product of convergent plates, vulcanism is not consistent with divergence. Igneous activity associated with divergent place tends instead to be non explosive, horizontal flow of basalt, i.e. in spreading oceanic centers or continental rift valleys.

-- peter matt (, October 08, 2004.

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