Plate Tectonics
Theory of Plate Tectonics:

the theory that Earth’s lithosphere is divided into tectonic plates that move
around on top of the asthenosphere.
Plate Boundaries:
o a place where tectonic plates touch
o can collide, separate, or slide past each other
o earthquakes can occur at each type of plate boundary

Types of plate boundaries:
o Convergent boundary: two tectonic plates collide

Continental-continental collisions: both plates buckle and
thicken, which pushes the continental crust upward

Continental-oceanic collisions: denser oceanic crust sinks into
the asthenosphere---subduction zone; old ocean crust gets
pushed into the ashtenosphere, where it is remelted an
recycled

Oceanic-oceanic collisions: one plate is subducted/sinks under
the other plate
o Divergent boundary: two tectonic plates separate; new seafloor forms
here; mid-ocean ridges are most common type of divergent boundary

Plates separate from each other, as they move apart, new
magma rises to fill the gap, cools and forms new sea floor
o Transform boundary: two tectonic plates slide past each other
horizontally (San Andreas Fault in California—where Pacific and North
American plates are sliding past each other)

Because tectonic plates have irregular edges, they grind and
jerk as they slide—produces earthquakes
Possible Causes of Tectonic Plate Motion:

Changes in density within the asthenosphere; density changes are
caused by the outward flow of thermal energy from deep within the
Earth; rock is heated, it expands, becomes less dense, and tends to
rise to the surface of Earth; as rock gets near the surface, rock
cools, becomes more dense, tends to sink
o Ridge Push: mid-ocean ridges, oceanic lithosphere is higher than
where it sinks into the asthenosphere; oceanic lithosphere
slides downhill under the force of gravity
o Convection: hot rock from deep within the Earth rises, cooler
rock near surface sinks; convection causes the oceanic
lithosphere to move sideways and away from mid-ocean ridge
o Slab Pull: oceanic lithosphere is denser than asthenopshere, the
edge of the tectonic plate that contains oceanic lithpshere
sinks and pulls the rest of the tectonic plate with it
Tracking Tectonic Plate Motion:

Very slow; can’t feel or see the movement; measured in centimeters
per year

Global Positioning Systems: system of satellites; radio signals are
continuously beamed from satellites to GPS ground stations—record
the exact distance between the satellites and the ground station;
over time, distances change slightly; recording the time it takes the
GPS to move a given distance, can measure the speed at which each
tectonic plate moves