volcanoes at plate boundaries

divergent plate boundaries

At divergent plate boundaries hot mantle rock rises into the space where the plates are moving apart. As the hot mantle rock convects upward it rises higher in the mantle. The rock is under lower pressure; this lowers the melting temperature of the rock and so it melts. Lava erupts through long cracks in the ground, or fissures.

midocean ridges

Volcanoes erupt at mid-ocean ridges, such as the Mid-Atlantic ridge, where seafloor spreading creates new seafloor in the rift valleys. Where a hotspot is located along the ridge, such as at Iceland, volcanoes grow high enough to create islands (Figure 1.3).

continental rifting

Eruptions are found at divergent plate boundaries as continents break apart. The volcanoes in Figure 1.4 are in the East African Rift between the African and Arabian plates. Remember from the chapter Plate Tectonics that Baja California is being broken apart from mainland Mexico as another example of continental rifting. Click image to the left or use the URL below. URL: The Cascade Range is formed by volca- noes created from subduction of oceanic crust beneath the North American conti- nent.


convergent plate boundaries

Converging plates can be oceanic, continental, or one of each. If both are continental they will smash together and form a mountain range. If at least one is oceanic, it will subduct. A subducting plate creates volcanoes. In the chapter Plate Tectonics we moved up western North America to visit the different types of plate boundaries there. Locations with converging in which at least one plate is oceanic at the boundary have volcanoes.


Melting at convergent plate boundaries has many causes. The subducting plate heats up as it sinks into the mantle. Also, water is mixed in with the sediments lying on top of the subducting plate. As the sediments subduct, the water rises into the overlying mantle material and lowers its melting point. Melting in the mantle above the subducting plate leads to volcanoes within an island or continental arc.

pacific rim

Volcanoes at convergent plate boundaries are found all along the Pacific Ocean basin, primarily at the edges of the Pacific, Cocos, and Nazca plates. Trenches mark subduction zones, although only the Aleutian Trench and the Java Trench appear on the map in the previous concept, "Volcano Characteristics." The Cascades are a chain of volcanoes at a convergent boundary where an oceanic plate is subducting beneath a continental plate. Specifically the volcanoes are the result of subduction of the Juan de Fuca, Gorda, and Explorer Plates beneath North America. The volcanoes are located just above where the subducting plate is at the right depth in the mantle for there to be melting (Figure 1.1). The Cascades have been active for 27 million years, although the current peaks are no more than 2 million years old. The volcanoes are far enough north and are in a region where storms are common, so many are covered by glaciers.

instructional diagrams

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volcanoes erupt at this type of convergent plate boundary.

a) ocean-ocean

b) ocean-continental

c) continental-continental

-->  d) both a and b

when two oceanic plates converge, it produces these types of volcanoes.

a) hot spot volcanoes

b) continental arc volcanoes

c) mid-ocean ridge volcanoes

-->  d) island arc volcanoes

iceland is part of the mid-ocean ridge.

-->  a) true

b) false

by what process do plates return back into the mantle, ultimately to make magma for volcanoes?

a) sea-floor spreading

b) transform motion.

-->  c) subduction

d) hotspots.

continental rifting does not produce volcanoes.

a) true

-->  b) false

volcanoes erupt at mid-ocean ridges due to

-->  a) magma rising to beneath the rift valley.

b) subduction of oceanic lithosphere.

c) subduction of continental lithosphere.

d) all of the above

lava erupts through long cracks in the ground called this.

a) volcanoes

b) faults

c) rifts

-->  d) fissures

during subduction the plate that goes under is

a) the less dense plate

-->  b) the more dense plate

c) both plates

d) none of the plates

these mountains are a chain of volcanoes made by convergent boundaries.

a) himalayas

b) hawaiian islands

c) appalachians

-->  d) the cascades

volcanoes at island and continental arcs are not active for very long, about 10,000 years on average.

a) true

-->  b) false

diagram questions

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