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Plate Tectonics
Plate Tectonic Theory
Introduction
• The lithosphere is broken into a number of
large pieces called plates.
• The plates completely cover Earth’s surface
and even extend under the ocean.
• Plates are made of continental and oceanic
crust.
Earth’s Moving Plates
• The plates move slowly – only several
centimeters per year.
• If the plates are moving, the continents are
“drifting”.
• Alfred Wegener’s hypothesis (early 1900s) :
over millions of years the continents collided to
form a great landmass called Pangaea. Then
after millions of years it broke apart again.
The Theory of Plate Tectonics
• 1960’s
• New Theory – Earth’s plates are in
constant slow motion, explains how
Earth’s plates form and move, how plates
interact, produce new volcanoes,
mountain ranges, earthquakes, and
features of the ocean floor.
How do the Plates Move?
• The Earth is a living, mobile
thing.
• The surface and the interior
are in motion.
• The mobile rock beneath
the rigid plates is believed
to be moving in a circular
manner.
• The heated magma rises to
the surface, spreads and
begins to cool, then sinks to
the bottom of the mantle
where it is reheated and
rises again.
Effects of Plate Tectonics
• Plate boundaries – the cracks between the
plates of the lithosphere
Oceanic Plates
• Collide (convergent boundary) – one plate sinks
below the other to form a deep sea trench
• Pull Apart (divergent boundary)– sea flooring
spreading occurs, lava erupts through the cracks
to form new rock for the ocean floor, forms the
mid-oceanic ridge
Continental Plates
• Pull Apart (divergent boundary) – forms a
rift valley (← →)
• Slide Past (transform boundary) - cause
frequent earthquakes (↑↓)
• Collide (convergent boundary) – plates
slowly fold and pile on top of each other
forming a mountain range (→ ←)
Divergent Boundaries
Convergent Boundaries
India and Asia to create the
Himalayas
Oceanic Nazca Plate is subducting
under the South American Plate to
create the Andes mountains and Peru
Chili Trench
Two oceanic plates colliding, one must
subduct under the other creating a
trench (Marianas Trench)
Creation of the Himalayan
Mountain Range
The Himalayas continue to rise
more than 1 cm a year—a growth
rate of 10km in a million years.
Transform Boundaries
San Andreas Fault
The San Andreas Fault slices through 2/3 or the length of
California. The Pacific Plate is moving horizontally past
the North American Plate at an average rate of 5 cm/yr.