GEOGRAPHY

Constructive Plate Boundaries

• Volcanoes and Earthquakes occur on constructive boundaries
• Constructive boundaries occur where two plates move apart
• Mantle under pressure from plates above and after movement this pressure is released
• The mantle melts because of this pressure producing magma
• Magma is less dense than plate so it rises above it to erupt and form volcanoes e.g. Helgafjell in Iceland
• EXAMPLE: Eurasian plate and North American Plate are moved apart at Mid Atlantic Ridge. Iceland has been formed because of the rising magma
• Some plates move faster than others which causes pressure to build up. Too much pressure causes fault lines. This fault line then causes an earthquake e.g. 2008 Iceland Earthquake
• At conservative boundaries, two plates get locked together which causes pressure to build up and therefore crack (fault is formed)
• E.g. the Pacific Plate is moving past the North American Plate resulting in the San Andreas Fault in California. Loma Prieta Eartquake 1989.

Destructive Plate Boundaries

• Volcanoes and earthquakes also occur at destructive plate boundaries
• It is where two plates move together
• Continental crust and oceanic crust move towards each other and the more dense oceanic crust gets subducted, in what is called a subduction zone
• The oceanic crust is then heated by friction. This friction heats the upper mantle which turns into magma
• The magma then rises to the surface to form a volcano e.g. Mount Pinatubo 1991
• EXAMPLE: Pacific plate is being subducted beneath the Eurasian plate at the Japan trench and the Japanese island arc has been formed. Same process has happened with the formation of the Indonesian Islands
• When one plate is being subducted they can get stuck, which causes pressure to build up.
• When this pressure gets too much, the plates jerk past each other causing an earthquake e.g Kobe Earthquake 1995 and the Kashmir Earthquake 2005