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