What is Plate TectonicsWhat is Plate Tectonics
• The Earth is broken into large Plates.
• The size and position of Plates change.
• Plate edges colliding - geologic activity
• Cause of Earthquakes & Volcanoes
• Creates Mountains
• Deep ocean vents – site of creation of life
Earth has 3 LayersEarth has 3 Layers
Outer Layers of the EarthOuter Layers of the Earth
(Greek, Lithos for
Stone) is the rigid,
outermost layer of
outer crust and
This makes up the
“Plate” of Plate
The Asthenosphere is
the solid part of the
mantle that “Flows” in
a process called
Plastic behavior is
much like toothpaste
moving in a tube.
Continental DriftContinental Drift
Continental Drift theory was
proposed by Alfred
Wegener in 1912. He used
continental fit, distribution of
fossils, ancient climates,
and the wandering of
Earth's polar regions to
support his Theory. He
stated the present
continents were part of a
super continent – Pangaea.
Sea-Floor SpreadingSea-Floor Spreading
Harry Hess proposed that new ocean floor is
formed at the rift of mid-ocean ridges. The ocean
floor, and the rock beneath it, are produced by
magma that rises from deeper levels. Hess
suggested that the ocean floor moved laterally
away from the ridge and plunged into an oceanic
trench along the continental margin.
As new ocean floor is
created by sea-floor
spreading at the mid-
oceanic ridges it is
consumed at the
• When the leading edge of
an oceanic crust collides
with continental crust.
• Oceanic crust is denser –
it is subducted, or forced
under the less dense
• A deep oceanic trench
forms along a subduction
• When two leading edges of continental crust
• Neither crust is conducted – they have the same
• Colliding edges are crumpled and uplifted,
• A collision between oceanic crust and another
• A deep ocean trench forms when one of the
oceanic crust is subducted.
• Part of the subducted crust melts to form
• Molten magma rise to form volcanic islands.
Convergent BoundariesConvergent Boundaries
Places where plates crash
or crunch into each other.
All the folding and bending
makes rock in both plates
break and slip, causing
earthquakes. Rock deep in
the Earth melts, builds up
Continental CrushContinental Crush
An ocean floor will always slide under the land
mass. This is because the land mass is more
buoyant, or lighter, than the ocean floor. When two
land masses meet neither will slide under the other.
Instead, the two crush together at what is known as
a collisional boundary. They crumple and fold.
Some pieces of land are thrust over
or under other pieces. The result is
a mountain range.
Divergent BoundariesDivergent Boundaries
Places where plates come apart are called
divergent boundaries. When Earth's brittle surface
layer is pulled apart, it breaks along parallel faults
that tilt slightly outward from each other. As the
plates separate along the boundary, the block
between the faults cracks and drops down into the
soft, plastic interior. The sinking of the block forms
a central valley called a rift. Magma (liquid rock)
seeps upward to fill the cracks. In this way, new
crust is formed along the boundary.
Transform BoundariesTransform Boundaries
Places where plates slide past each other are
called transform boundaries. The most famous
transform boundary in the world is the San
Andreas fault. Los Angeles will not crack off and
fall into the ocean as popularly thought, but it
will simply creep towards San Francisco at
about 6 centimeters per year. In 16 million
years, the plates will have moved so much that
Los Angeles will be north of San Francisco!