Plate Tectonics

The Earth’s Crust in Motion…

Earth’s Interior• Scientists have not been able to directly

study much of the earth’s interior due to high temperatures and pressures. They do get much of the information they possess from the study of seismic waves (shock waves produced by earthquakes). These are measured by an instrument known as a seismograph.

• There are 3 types of seismic waves – Primary waves (P waves). These are

the fastest seismic wave. They travel through any material but are slowed by less dense materials such as liquids & gases. They are push/pull waves.

– Secondary waves (S waves). These are slower waves that will not travel through liquids. They are upward waves that cause the land they travel through to move at right angles.

– Surface waves (L waves). These bend and twist Earth’s surface & cause the most damage at the surface.

Structure of the Earth’s Interior

The Core• The center of the earth. • The core is split into 2 layers the

solid inner core and the liquid outer core. Both layers are made of iron and nickel.

• The composition of the core and rotation of the Earth causes a magnetic field called the magnetosphere.

• The inner core is very dense due to pressure & has a temperature of around 50000C.

• The diameter of the inner core is approximately 2600 km (1616 miles). It is found 5150 km (2781 miles) below the surface.

• The outer core (liquid) is 2900 km (1802 miles) below the surface and 2250 km (1398 miles) thick.

The Mantle• The middle layer of the Earth.• The largest layer of the Earth.

(80% of volume 68% of mass).• The mantle is made mostly of

silicon, oxygen, magnesium and iron.

• There is more iron as you get closer to the core.

• The temperatures range from 870C near the crust to 2200C near the core.

• The mantle is a solid but it can flow like a liquid so it has a property known as plasticity(This is similar to mercury or Jell-O or silly putty).

• Sample of the mantle have been taken beneath the ocean.

The Crust• The thin outermost layer of the Earth. • The crust is made mostly of silicon,

oxygen and 6 trace elements. • There are 2 types of crust.

– Continental crust – crust beneath the continents. Average thickness is 32 km (19.84 miles). It is thicker beneath mountains. Mostly made of igneous rocks granite and basalt.

– Oceanic crust –crust beneath the ocean. Averages 8 km (5 miles) thick. Mostly made of basalt.

• Lithosphere- The topmost solid part of the earth where life exists. The lithosphere is made of 7 major plates that are split into minor plates.

The Moho

• Short for Mohorovičić discontinuity.

• A thin layer of AIR between the crust and mantle.

• It explained jumps on the seismograph.

• Identified in 1909 by Andrija Mohorovičić, a Croatian seismologist

The letter A is the Moho

Continental Drift• The theory made by German

meteorologist Alfred Wegner in 1912. It said that the continents used to all be together in a supercontinent called Pangea which drifted apart and are still moving.

• 3 pieces of evidence to support this theory– 1.continent boundaries fit together

like a puzzle – 2.fossils of same organisms found

on different continents• a. Mesosuras- reptile• b. Glosopteris- fern plant

– 3.Glaciar evidence in different climates

• *Wegner was not believed for a long time because he could not give a way for the continents to move. The discovery of lithospheric plates and sea floor spreading helped to give his theory more credibility.

Supercontinent

Pangea – means all lands

Tens of Millions of years!

Plate Tectonics and Sea Floor Spreading

• Plate tectonics – theory that earth’s crust is made of plates that move driven by convection currents in the asthenosphere and sea floor spreading.

• (Based on continental drift ideas but stronger).

• Sea floor spreading – a hypothesis that there is a crack between plates under the sea that allows magma to reach the surface and form new seafloor

How does it work?

Plates – pieces of the lithosphere

Plates fit closely together along cracks called Plate Boundaries

Convection Currents in asthenosphere cause movement

Earth’s Puzzle

• What plate do we live on?

• What plates do we border?

• USGS Earth’s Plates

• How far do we move every year?

Types of Boundaries

• Divergent

• Convergent

• Transform

A

Divergent

B

Convergent

C

Transform

•plates are moving apart

•new crust is created

•Magma is coming to the surface

•plates are coming together

•crust is returning to the mantle

•Subduction (pushing down) is occurring

•plates are slipping past each other

•crust is not created or destroyed

A

Divergent

B

Convergent

C

TransformContinental crust

rift valley

Oceanic crust mid-ocean ridge

2 continental plates mountain range

Plates move against each other

Stress builds up

Stress is released

earthquake

2 oceanic plates oroceanic + continentalsubduction or underwater mountains

The Big Picture

The Big Picture 2