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STRUCTURE OF THE EARTH

The Earth is made up of 3 main layers:

Core (inner and outer)

MantleCrust

Inner core

Outer core

Mantle

Crust

THE CRUST

This is where we live!

The Earth’s crust is made of:

Continental Crust

- thick (10-70km)- buoyant (less dense than oceanic crust) - mostly old

Oceanic Crust

- thin (~7 km)- dense (sinks under continental crust)- young

WHAT ARE THE TECTONIC PLATES?

AKA: Lithospheric plateThe ~100-km-thick surface of the Earth;

Contains crust and part of the upper mantle;It is rigid and brittle;Fractures to produce earthquakes.

WHAT IS THE ASTHENOSPHERE?

Asthenosphere:Is the hotter upper mantle below the

lithospheric plate;Can flow like silly putty; andIs a viscoelastic solid, NOT liquid!!

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WHAT IS PLATE TECTONICS?

•Alfred Wegener in the early 1900’s proposed the hypothesis that continents were once joined together in a single large land mass he called Pangea (meaning “all land” in Greek).

• He proposed that Pangea had split apart and the continents had moved gradually to their present positions - a process that became known as continental drift.

CONTINENTAL DRIFT

Continents fit together like a puzzle….e.g. the Atlantic coastlines of Africa and South America.

The Best fit includes the continental shelves (the continental edges under water.)

WEGENER’S EVIDENCE FOR CONTINENTAL DRIFT

Picture from http://www.sci.csuhayward.edu/~lstrayer/geol2101/2101_Ch19_03.pdf

WEGENER’S EVIDENCE FOR CONTINENTAL DRIFT

Fossils of plants and animals of the same species found on different

continents.

Picture from http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/part3.html

WEGENER’S EVIDENCE FOR CONTINENTAL DRIFT

Rock sequences (meaning he looked at the order of rock layers) in South America, Africa, India, Antarctica, and Australia show remarkable similarities.

Wegener showed that the same three layers occur at each of these places.

Picture from http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/part4.html

SEAFLOOR SPREADING

In the 1960’s, a scientist named Henry Hess made a discovery that would vindicate Wegner.

Using new technology, radar, he discovered that the seafloor has both trenches and mid-ocean ridges.

Henry Hess proposed the sea-floor spreading theory.

Picture from USGS http://pubs.usgs.gov/gip/dynamic/HHH.html

SEAFLOOR SPREADING

As the seafloor spreads apart at a mid-ocean ridge, new seafloor is created.

The older seafloor moves away from the ridge in opposite directions.

This helped explain how the crust could move—something that the continental drift hypothesis could not do.

Picture from http://www.pbs.org/wgbh/aso/tryit/tectonics/divergent.html

PLATE TECTONIC THEORY

Both Hess’s discovery and Wegner’s continental drift theory combined into what scientists now call the Plate Tectonic Theory.

Theory of plate tectonics: • The Earth’s crust and part of the upper mantle are broken into sections, called plates which move on a plastic-like layer of the mantle

WORLD PLATES

PLATE MOVEMENT

“Plates” of lithosphere are moved around by the underlying hot mantle convection cells

CONVECTION CURRENTS/CELLS

Plates move by the transfer of heat through heated material.

Hot magma in the Earth moves toward the surface, cools, then sinks again.

Creates convection currents beneath the plates that cause the plates to move.

HOT SPOTS

Stationary plumes of hot material that initiate at the core/mantle interface

Hawaii: the plume is beneath oceanic crust

HOT SPOTS

Yellowstone is associated with a hot spot under continental crust

HOT SPOTS

WHAT HAPPENS AT TECTONIC PLATE BOUNDARIES?

Divergent

Convergent

Transform

Three types of plate boundary

Spreading ridges As plates move apart new material is erupted to fill the gap

Divergent Boundaries

Iceland has a divergent plate boundary running through its middle

Iceland: An example of continental rifting

There are three styles of convergent plate boundaries

Continent-continent collisionContinent-oceanic crust collisionOcean-ocean collision

Convergent Boundaries

Forms mountains, e.g. European Alps, Himalayas

Continent-Continent Collision

Himalayas

Called SUBDUCTION

Continent-Oceanic Crust Collision

Oceanic lithosphere subducts underneath the continental lithosphere

Oceanic lithosphere heats and dehydrates as it subsides

The melt rises forming volcanism

E.g. The Andes

Subduction

When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone.

The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench.

The worlds deepest parts of the ocean are found along trenches.

E.g. The Mariana Trench is 11 km deep!

Ocean-Ocean Plate Collision

Where plates slide past each otherE.g. Faults and earthquakes

Transform Boundaries

Above: View of the San Andreas transform fault