Geog Practice Question 1

Formation of Volcano

1) Cascade Range of USA, in which a linear belt of volcanoes can be found, is formed through the convergence of Juan de Fuca plate and North American plate, as shown in Figure 1. Explain how the convergence of the two plates leads to the formation of the volcanoes.

Figure 1
Source:http://vulcan.wr.usgs.gov/Imgs/Gif/PlateTectonics/Maps/map_plate_tectonics_cascades.gif

Answer:

• Juan De Fuca Plate is an oceanic plate, while North American Plate is a continental plate. Oceanic plate is denser than continental plate. Therefore, when Juan De Fuca plate converges with North American plate, it subducts underneath the less dense North American plate.
  
  
• As Juan De Fuca plate subducts, it is subjected to progressively higher temperatures and pressure. Water trapped in the sediments is probably squeezed out 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. As the hybrid magma, derived partially from the overlying mantle rises, it reacts extensively with the overlying crust to produce a magma that is even more complex. This magma may contain components derived from oceanic sediments, from metamorphosed oceanic basalt, from the peridotite in the mantle wedge and from the overlying crust. On the way to the surface, the magma also may mix with other batches of magma.
  
  
• Having lower density than the surrounding, the magma buoyantly rises through the overlying mantle to form a linear belt of volcanoes parallel to the oceanic trench between the Juan de Fuca plate and North American plate. This is how the linear belt of volcanoes in the Cascade Range may be formed.
  
  
• This diagram can be drawn in the answer. (Other diagrams with the same idea are also accepted)
  
  

Source:
http://vulcan.wr.usgs.gov/Imgs/Gif/PlateTectonics/Maps/map_plate_tectonics_cascades.gif