Earth Science:
Plate Tectonics
How do we see inside the Earth?
• Waves
• Seismic waves
• Seismology
– Study & measurement of seismic waves
• What do we know that produces seismic
waves?
Two main types of waves
• “P” Waves
• “S” waves
P-waves
“P” or Primary waves
–
–
–
–
Longitudinal
Compress and expand
Fastest
Travel through all mediums ( solid, liquid
and air)
“S” Waves
• “S” or Secondary waves
–
–
–
–
Transverse
Vibrate up and down / side to side
Slower than “P” waves
Only travel through solids
Layers of the Earth
The Crust
•
•
•
•
•
•
Surface layer
Like an eggshell
Thin
Brittle
Can crack
Two regions:
– Oceanic (Basalt rock)
– Continental (granite rock)
•
•
Crust is less dense than the mantle
Like ice on water; the crust floats on the mantle
The Mantle
The Mantle
•
•
•
•
•
•
•
Rocky
Like the crust
Silicon
Oxygen
Iron, magnesium, calcium are heavier elements
Mantle is denser
How does the Earth’s crust add to the density
of the mantle?
• Weight bearing down on the mantle…it
compresses the minerals, increasing the
pressure and squeezing them into rocks.
The Mantle
•
•
•
•
Higher temperature
Why?
Increased pressure
Decay of radioactive elements
Layers of The Mantle
• Upper mantle
• Lithosphere
– Rigid rock upper portion
of upper mantle
• Asthenosphere
– Lies beneath lithosphere
and “flows” like plastic
– It is solid…silly putty
• Lower mantle
The Core
• Two layers: inner and outer
core
• 15% Earth’s volume
• 30% Earth’s mass
• 2x as dense as the Earth’s
mantle
• Why?
• Made of metallic iron
• Inner core is VERY hot!
• Some places as hot as the
surface of the sun!
The Inner Core
• Solid Iron
– How?
– Pressure from Earth keeps Iron packed
tightly; doesn’t allow it’s atoms to flow
The Outer Core
• Outer Core
– Liquid Iron
– Less pressure
– Flows and spins as Earth
rotates
– Creates convection currents
– Affects Earth’s surface
– Produces electrical charge
– Possibly responsible for
Earth’s magnetic field
Magnetosphere
Convection Currents
• What are
convection
currents?
• Current of heat
flows from core to
crust
• This sets up a
convection current
in the mantle
Effects of Convection
Currents
• When the current comes at a weaker part of
the crust, for example at a volcano, magma
comes above the earth's surface.
• This is called plate tectonics.
• The movement of these plates goes very
slowly.
• The bumping of two tectonic plates causes an
earthquake.
• The convection current along the bottom of
the crust causes the moving of the tectonic
plates.
Continental Drift & Tectonic
Plates
• Wegner’s Hypothesis on
Continental Drift
• The World’s continents are
in motion
• At one time, the continents
were joined together as
one…
• This was known as Pangea
• They fit together like a
jigsaw puzzle
What caused the continents
to separate?
Continental Drift
• Wegner’s theory that proposed the
landmass known as Pangea started
breaking up
• Separated into two parts: Laurasia and
Gondwanaland
• Wegner’s theory of the separation of
Pangea was supported by fossil,
biological, climatological and geological
evidence
Seafloor Spreading
• Seafloor is not permanent
• Constantly renewing itself
• Mid Atlantic Ridge (longest & tallest mountain
range in the world, center of the Atlantic
Ocean)
• Forms islands: Iceland and Azore
• There are rifts in the mountains
Plate tectonics
• Continents move because they are
embedded in plates
• The plates shuffle or move atop of the
Earth’s surface
• Plates move in different directions and
different speeds from each other
• There are nine large plates and several
smaller plates
Plate Boundaries
• Divergent Boundaries
• Plates move away from
each other
• Comes from rock in
athenosphere
• Partially melted
• The rock partially melts =
lava
• It was magma before it
left the surface of the
Earth
Convergent boundaries
• Plates move toward each other
Transform fault boundaries
• Plates slide past each other
Earthquakes
•
•
•
•
Devastating
Stress (force) applied to rock
Stress causes strain
Eventually the rock cannot bend
anymore
• It breaks and releases stored energy
• The released energy ( Seismic Waves)
travel out in all directions
Earthquakes cont
• The initiating site of the
seismic wave: focus
• The point at Earth’s surface
directly above the focus:
epicenter
• Measured on a Richter Scale
( the incraments are ten
fold in magnitude…
Tsunami
• Seismic sea wave
• Created by an
earthquake