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Shri Ashtapad Maha Tirth - II
Sea Floor Spreading
When a rift occurs in a plate carrying ocean floor, lava from the mantle rises up and hardens into new crust. Thus over time the sea floor spreads as new crust is created at the rift. The rock of this new crust takes on the characteristic magnetic polarity of the earth at the time of the rock's formation. Because this polarity shifts over time, bands of alternating polarity form on either side of the rift. This deep-sea record allows the dating of the ocean floor.
Plate Collisions
continental crust
Oceanic
Ancient Tibet
crust
MANTLE
mountain building.
MANTLE
continental crust
continental crust
continental crust
290
oceanic crust
MANTLE
oceanic crust
MANTLE
continental crust
continental crust
When a plate carrying oceanic crust collides with a plate carrying continental crust, the ocean floor bends beneath the continental plate, and is remelted in the deeper, hotter layers of the earth. This molten rock then rises, forming volcanoes along the edge of the continental plate. As the oceanic crust on a plate is consumed in this way, ocean between continents shrinks. This brings land masses into collision and builds mountain ranges as continental plates
meet.
