plate tectonics revision.
plate tectonics revision.
destructive plate boundary:
example: African and Eurasian plate
Hot spots: where a plume of magma travels up through a fault in the middle of a plate. creating a volcano.
young fold mountains: as the plates come together the pressure builds up within the continental. Forcing the plate upwards. this creates folds in the land.
old, cold and dense oceanic plate subducts under a neighboring continental plate and forms a deep ocean trench.
The subducted part of the oceanic plate melts in the asthenosphere.
seismicity: the path of the subducting zone is indicated by a zone of dipping earthquakes called the beioff zone.
the type of volcano that most commonly forms at a destructive plate boundaries is a composite volcano. the higher the silica content the greater the gas pressure within the magma. therefore, high silica magma produces more explosive eruptions.
conservative plate boundary
At a conservative plate boundary, the plates slide past one another and this movement causes friction. Energy builds up between the two plates and eventually this is released as an earthquake. they rarely cause volcanoes. there Is not subduction or constructive zone at a conservative plate boundary. movement of the plate can be erratic. this can therefore result in earthquakes as powerful as anything caused at a destructive plate boundary.
example: the Pacific plate moving past the North American plate. this plate boundary is the cause of the San Andreas fault.
the two plates either slide past one another in different directions or in the same direction but at different speeds. this makes it appear as if the plates are moving in different directions.
faults: faults are cracks in the rock caused by forces that compress or stretch a section of the earths crust.
earthquakes occur at a conservative plate boundary. when the plates slip past one another they can lock for years at a time. when this friction is eventually overcome, the shockwaves created produce shallow and powerful quakes.
transform faults:a strike-slip fault occurring at the boundary between two plates of the earth's crust that stems 90 degrees from the fault line.
constructive plate margin
gravitational sliding:movement of weak rock. plates are moving away releasing molten rock. this magma solidifies when it reaches the surface creating new land.
ocean ridges: underwater mountain range which uplifts the ocean. ocean ridges are formed by numerous eruptions of basaltic lava.
the crust thins out (stretching and extending) creating a rift valley. an example of this would be the African rift valley. rift valleys are when the crust is moving apart it creates brittle fracture- it then drops downward between two parallel faults.
volcanoes that form here are shield volcanoes and they are formed by basaltic lava.
paleomagnetism: this is the study of the earths past magnetic field. solidified magma is rich in iron that is found within the magnetic field. this is evidence to prove the theory of plate tectonics.
The North pole has shown to be reviewing the changing movement of the field - causing oceanic crust to move and enlarge.