Fusion and Plate Tectonics
Plate Tectonics is the study of great rafts
or 'plates' of earth crust that move above
a layer of molten mantle deep inside
Earth. These plates are from 30 to 100
miles thick, slowly moving, carrying the
continents and ocean basins with them.
Sliding over a hot, semi plastic layer
below, the rigid plates grind and crush
together, causing earthquakes and
volcanic eruptions. This process, known
as sea floor spreading, works like this:
material from Earth's interior wells
upward through rift valleys, which are
located on the ocean floor. The ocean
floor moves steadily outward from the rift,
pulled or pushed [or both] across a less
rigid layer. Offsetting the steady creation
of new surface, other regions of the
ocean floor plunge down, or 'sub duct'
into Earth's mantle along deep-sea
trenches (1).
In Plate Tectonics theory, the oceanic plates transport the continents like a great slow conveyor belt.
Continental crust is estimated to be at least 4.5 billion years old. However, from data amassed by the
Deep Sea Drilling Project, the age of the oceanic crust ranges from essentially zero, along the
submarine ridge crests that mark sea floor spreading, to 180 million years in the eastern Pacific, the
part of the ocean farthest from a ridge. In other words, the oceanic crust rises at a ridge, moves
across the width of an ocean basin and descends along a trench that marks what is called a
subduction zone (2).

This steady creation of new ocean bottom
starts along a submarine ridge where
magma, or molten rock, swells or rifts up
to produce new oceanic crust.  This rifting
process from the interior of Earth begins
in the asthenosphere with the
establishment of a hot spot, otherwise
called a thermal anomaly, a hot spot is an
area in which the temperature is higher
than it is at an equal depth elsewhere.
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Age of
oceanic
crust;
youngest
(red) is along
spreading
centres