STRUCTURE OF THE EARTH
The Earth is made up of 3
main layers:
 Core (inner and outer)
 Mantle
 Crust
Crust
Mantle
Outer core
Inner core
THE CRUST
This is where we live!
The Earth’s crust is made of:
Continental Crust
Oceanic Crust
- thick (10-70km)
- buoyant (less dense
than oceanic crust)
- mostly old
- thin (~7 km)
- dense (sinks under
continental crust)
- young
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.
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!!
WHAT IS PLATE TECTONICS?
CONTINENTAL DRIFT
•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.
WEGENER’S EVIDENCE FOR
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.)
Picture from
http://www.sci.csuhayward.edu/~lstrayer/geol2101/2101_Ch19_03.pdf
WEGENER’S EVIDENCE FOR
CONTINENTAL DRIFT
Picture from
http://volcano.und.edu
/vwdocs/vwlessons/pl
ate_tectonics/part3.h
tml
Fossils of plants and animals of the
same species found on different
continents.
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/p
art4.html
SEAFLOOR SPREADING
Picture from USGS
http://pubs.usgs.gov/gip/dynamic/HHH.html
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.
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.
CONVECTION CURRENTS
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?
Three types of plate boundary
Divergent
Convergent
Transform
Divergent Boundaries
Spreading ridges
 As plates move apart new material is erupted to fill the
gap
Iceland: An example of continental rifting
Iceland has a divergent plate
boundary running through its
middle
Convergent Boundaries
There are three styles of convergent plate
boundaries
Continent-continent collision
Continent-oceanic crust collision
Ocean-ocean collision
Continent-Continent Collision
Forms mountains, e.g. European Alps, Himalayas
Himalayas
Continent-Oceanic Crust Collision
Called SUBDUCTION
Subduction
Oceanic lithosphere subducts
underneath the continental
lithosphere
Oceanic lithosphere heats and
dehydrates as it subsides
The melt rises forming volcanism
E.g. The Andes
Ocean-Ocean Plate Collision
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!
Transform Boundaries
Where plates slide past each other
E.g. Faults and earthquakes
Above: View of the San Andreas
transform fault
REVIEW