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Plate Tectonics
What is Plate Tectonics?
Earth’s surface is made of rigid slabs of rock
called plates that move with respect to each
other.
Alfred Wegener
Nearly 100 years ago, a German scientist named
Alfred Wegener proposed a radical idea. He
proposed to his colleagues that the continents
were not fixed to the earth’s surface, but rather
they were in constant motion which he called
Continental Drift.
Wegener’s Proposal cont.
In addition, Wegener also proposed that millions
of years ago, all the continents were part of a
supercontinent which he named
Pangaea
Pangaea
Evidence
As you may guess, no one believed him. He
need proof. So he began his investigation to
prove his theory. He researched three different
areas: Climate, Rocks, and Fossils.
Climate
Wegener suggested, at the time of Pangaea,
parts of South America, Africa, India, and
Australia were located closer to Antarctica and
would have been much colder and possibly
covered by glaciers.
Rocks
Wegener analyzed sediment deposits and
glacial grooves cut into the rock from each
continent. He found similarities in sediment
deposits and grooves carved by glaciers that
helped support his theory.
Evidence of Glaciation
Rock Evidence
The Caledonian and Appalachian Mountains appear to
be a single mountain chain composed of the same
rock type, age, and structure.
Fossil Evidence
Glossopteris
An ancient plant that lived in a warm wet
climate. The fossils have been found in
South America, Africa, India, and Australia.
Today, these continents are separated by vast
oceans and it would impossible for seeds to
travel that far.
Glossopteris
Mesosaur
Distribution of Glossopteris and Mesosuar
More Fossil Evidence
Separation of the Supercontinent
120 million years ago Pangaea split into two
giant land masses, Gondwanaland and
Laurasia
Gondwanaland
Gondwanaland remained in the southern
hemisphere and consists of Africa, South
America, Arabia, India, Australia, and
Antarctica.
Laurasia
Laurasia moved to the north and was
composed of North America and Europe and
Asia
The Breakup of Pangaea
How the Continents Move
Lithosphere
The cold, rigid outermost layer of the Earth is
known as the lithosphere. The lithosphere is
thicker below the continents and thinnest at
the mid-ocean ridges. The lithosphere forms
the floating plates. The largest plate is the
Pacific plate and the smallest is the Juan de
Fuca plate.
Asthenosphere
The layer below the lithosphere is the
asthenosphere. This layer is so hot that it
behaves like a plastic material that flows.
Plate Boundaries
Plate boundaries are located anywhere that two
or more plates interact. There are three types
of boundaries: Divergent, Convergent, and
Transform.
Divergent Plate Boundaries
A divergent plate boundary forms where two
plates separate.
Rift Valley
A rift valley is a divergent plate boundary that
occurs in the middle of a continent.
Rift Valleys
Mid-Ocean Ridges
Mid-ocean ridges are mountain ranges in the
middle of the oceans.
Seafloor Spreading
Seafloor spreading is the process by which new
oceanic crust forms along a mid-ocean ridge. As
new crust is formed, older oceanic crust moves
away for the ridge.
Seafloor Spreading
Convergent Plate Boundaries
Convergent Plate boundaries form where two
plates collide. The denser plate sinks below the
more buoyant plate in a process called
subduction. The are three main types of
convergent boundaries: Ocean-to-Continent,
Ocean-to-Ocean, and Continent-to-Continent
Ocean-to-Continent
Ocean-to-Continent
Mount Rainier in Washington State
Ocean-to-Ocean
The Marianas Trench
The Marianas Trench is the deepest location on
the planet at 36,200 feet, (6.9 miles), deep. At
that depth the pressure on a body is
8 tons or 16,000 lbs!
Mariana Trench
SCARY CREATURES IN MARIANAS TRENCH
Continent-to-Continent
Continent-to-Continent
The Himalaya mountains are created by the
collision of the Eurasian and Indian Plates
Forces Causing Plate Movement
Convection Currents
Convection current is the circulation of material
caused by differences in temperature and
density. Convection currents in the mantle are
created by the decay of radioactive elements
such as uranium, thorium, and potassium
heating the surrounding rock.
Convection Currents cont.
The heated rock becomes less dense, rises to
the crust, cools and sinks back down to be
heated again. These convection currents cause
three forces to act on the crust: Basal Drag,
Ridge Push, and Slab Pull
Basal Drag
Convection currents in the asthenosphere
circulate and drag the lithosphere similar to a
conveyor belt.
Ridge Push
Mid-oceanic ridges have a greater elevation
than the surrounding seafloor. Because the midocean ridges are higher, gravity pulls the
surrounding rock down and away from the
ridge. The rising mantle material at the midocean ridge creates the potential for the plates
to move away from the ridge with a force called
ridge push.
Ridge Push cont.
Ridge push moves the lithosphere in opposite
directions away from the mid-ocean ridge.
Slab Pull
When tectonic plates collide, the denser plate
will sink into the mantle along a subduction
zone. This plate is called a slab. Because this
slab is old and cold, it sinks, pulling on the rest
of the plate with a force called slab pull.
In the end, Hess’s work with sonar mapping
proved Wegener's basic idea right and clarified
the mechanism that broke the once-joined
continents into the seven we know today and so
began the modern Theory of Plate Tectonics