•Plate Tectonics
What 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 Layers
•Core
•Mantle
•Crust
Plate tectonics
• Plates are driven by cooling of Earth (convection)
• Gravity provides additional force to move plates.
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Modified from USGS Graphics
Convection is like a boiling pot. Heated soup rises to the surface, spreads and begins to
cool, and then sinks back to the bottom of the pot where it is reheated and rises again.
What are the tectonic plates?
AKA: Lithospheric plate
• The ~100-km-thick surface of the Earth;
• Contains crust and part of the upper mantle;
• It is rigid and brittle;
• Fractures to produce earthquakes.
Outer Layers of the Earth
•The Lithosphere
(Greek, Lithos for
Stone) is the rigid,
outermost layer of
outer crust and
uppermost mantle.
This makes up the
“Plate” of Plate
Tectonics.
USGS Graphics
What is the asthenosphere?
Asthenosphere:
• Is the hotter upper mantle below the lithospheric
plate;
• Can flow like silly putty; and
• Is a viscoelastic solid, NOT liquid!!
Outer Layers of the Earth
•The Lithosphere
(Greek, Lithos for
Stone) is the rigid,
outermost layer of
outer crust and
uppermost mantle.
This makes up the
“Plate” of Plate
Tectonics.
Three Basic Types of Plate Boundaries
Divergent
Using hands to show relative motion
Transform
Convergent
USGS Graphics
Three Basic Types of Plate Boundaries
Divergent
Transform
Convergent
USGS Graphics
Tectonic Plates
There are a dozen large lithospheric plates (smaller plates not shown).
Some plates have continents; some don’t. All are in motion.
Question: What evidence is there for these plate boundaries?
Earthquakes
There are thousands of small earthquakes every day
“Strong” earthquakes (~M7) occur once a month. >M8 occur about once/year.
Where are the deepest earthquakes?
For earthquakes of the past 2 weeks, go to http://www.iris.edu/seismon/
Earthquakes & Plate Boundaries
Notice that the earthquakes coincide with plate boundaries,
and the deepest quakes (blue) are in subduction zones.
Question: Where would you expect to see volcanoes?
Create your own maps at http://www.iris.edu/quakes/maps.htm
Modified from USGS Graphics
Volcanoes & Plate Boundaries
This map shows that locations of volcanoes (ones above sea
level) also tend to occur along the plate boundaries
Modified from USGS Graphics
Tectonic Plates
How fast are the plates moving?
Plates move 1-10 centimeters per year (≈ rate of fingernail growth).
Fingernail growth plotted: http://jclahr.com/science/earth_science/thumbnail/index.html
Modified from USGS Graphics
Continental 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.
Continental Drift
How fast are the plates moving?
Plates move 1-10 centimeters per year (≈ rate of fingernail growth).
Source: http://www.tectonics.caltech.edu/outreach/animations/index.html
Pangaea
Sea-Floor Spreading
Sea-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.
Seafloor Spreading
Source: http://www.tectonics.caltech.edu/outreach/animations/index.html
Subduction
•As new ocean floor
is created by seafloor spreading at the
mid-oceanic ridges it
is consumed at the
subduction zones
where the
lithosphere sinks
under the
asthenosphere.
Convergent Boundaries
• Oceanic-Continental Convergence
• Continent-Continent Convergence
• Oceanic-Oceanic Convergence
Oceanic-Continental
Convergence
• 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
continental crust.
• A deep oceanic trench
forms along a subduction
zone.
Continent-Continent
Convergence
• When two leading edges of continental crust
collide.
• Neither crust is conducted – they have the same
density.
• Colliding edges are crumpled and uplifted,
producing mountains.
Convergent 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 pressure
causing volcanoes.
Continental
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 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.
Divergent Boundaries
Transform 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!
Transform Boundaries
•Summary Illustration
Collision of ‘Drifting’ India with Eurasia
Side view of subduction, ‘drifting’ India, volcanoes, & mountain-building
Source: http://www.tectonics.caltech.edu/outreach/animations/index.html
Note on Source: First 8 slides are modified from a slide show prepared by Dr. Robert
Butler, University of Portland, and Jenda Johnson,
…the full source of which can be found at:
http://www.iris.edu/hq/programs/education_and_outreach/videos#B