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Layers of the Earth
CRUST/LITHOSPHERE
• thinnest layer
- is cool and solid
- between 3 - 40 km thick
• floating on mantle
•
Oceanic Crust: 3 - 8 km thick
•
Continental Crust ~ 40 km thick
Mantle/Athenosphere
• average thickness is about 2880 km
• solid but very hot
• behaves like plastic, flowing and circulating
slowly without breaking
Core
•
•
•
•
•
radius ~ 3470 km
half of the Earth’s diameter
metallic - made of iron and nickel
outer core - liquid
inner core -solid
Supercontinent
Pangea – all lands
•Similar Reptiles
and insects
•Tropical forests
•coal deposits
Tens of Millions of
years ago!
Pangaea
Wegener revived the early idea of continental drift, contending
that all of the present-day continents were connected,
side-by-side, as long ago as the Carboniferous (~300 Myr).
He called the supercontinental mass Pangaea,
Greek for ‘all lands’.
Today’s Continents
As soon as maps of the globe became available, people
wondered about the arrangement of the continents and oceans.
Hundreds of years later, valid explanations were constructed.
The crust and lithosphere are broken up into 25 Plates
USGS
Movement of the Plates:
Continental Drift
•Evidence
•How it works
Continental Drift: Fossil Evidence
Mesosaurus: purely freshwater reptile
Glossopteris: seeds too large to be effectively wind-transported
Continental Drift: Rock Ages
Rock ages showed strong correlation across the Atlantic, as did
mountain ranges of similar age
Continental Drift:
Geometry evidence
• shape of the continents
• eg. the shape of the west coast of Africa and
the east coast of South America are
remarkably similar and were perhaps once
joined
More Evidence:Mantle Convection
Materials that can flow
tend to lose thermal energy
by the convection process.
This explains circulation in
a pot of water that is being
heated from below in the
same way it describes the
convection currents in the
Earth’s mantle.
Mantle Convection
Giant convection cells within the upper mantle
drag the plates along laterally.
Where convection rises sea floor spreading takes place.
Where the convection cells descend they drag crust
down, causing subduction
How does it work?
Plates – pieces of the
lithosphere
Plates fit closely together
along cracks called
Plate Boundaries
Convection Currents 
movement
Here is another version of the Rock Cycle
http://www.volcanoworld.org/vwdocs/vwlessons/lessons/Metrocks/Metr
Mechanisms of Plate Tectonics:
Mechanisms of Plate Tectonics:
1
2
RidgeRIDGE
Push
TRENCH
3 MANTLE
drag
convective flow of mantle
Sea Floor Spreading
Hot material rose at the oceanic ridges, thus explaining the high
heat flow and volcanic activity, and why the ocean floor is bulged
up at the ridges.
The logical next step is that where continent and ocean meet, at
the trenches, ocean crust is being returned to the mantle at the
same rate it is being generated at the ridges.
Types of Boundaries
• Divergent
• Convergent
• Sliding
To summarize……
A
Divergent
•plates are
moving apart
•new crust is
created
•Magma is
coming to
the surface
B
Convergent
C
Sliding
•plates are
coming
together
•plates are
slipping past
each other
•crust is
returning to
the mantle
•crust is not
created or
destroyed
A
Divergent
Continental crust
 rift valley
B
Convergent
2 continental plates 
mountain range
C
Sliding
Plates move against
each other
Stress builds up
Oceanic crust  midocean ridge
2 oceanic plates or
oceanic + continental
subduction
Stress is released
earthquake
DIVERGENCE: Sea Floor Spreading
Sea floor spreading, leads to continental drift
*This hypothesis
makes a number of
testable predictions.*
Magnetic Reversals
Interestingly, the polarity of the magnetic field shifts every
0.5 - 1.0 Myr. That means rocks formed over time will record
either ‘normal’ magnetic orientation (like today), or
‘reversed’. This leads to alternating bands of normal and
reversed magnetism.
We are apparently
headed into a
polarity reversal, to
be complete in
~3000 yr.
* Taking magnetic stratigraphy
back in time is paleomagnetism. *
Paleomagnetism
and
Sea Floor Spreading
The magnetic
stripes as
products of
steady creation of
new ocean crust
over geologic
time.
Oceanic Ridges
The ridge is a Divergent Plate Margin and divergence
takes place by Sea Floor Spreading.
New crust is added from upwelling magma (molten
rock) from the upper mantle.
Older crust is
pushed laterally
away from the
ridge axis – so that
the sea floor
spreads away from
the ridge axis.
From http://www.uwsp.edu/geo/faculty/ritter/glossary/s_u/sea_flr_spread.html
CONVERGENCE: Oceanic Crust – Continental Crust
The denser oceanic crust descends beneath lighter
continental crust.
Coastal mountain chains develop due to compressive
forces and volcanics (e.g., the Andes of South
America).
Magma material rises from descending slab and builds
volcanoes in
the rising
mountains.
CONVERGENCE: Oceanic Crust-Oceanic Crust
The oldest, densest crust normal descends beneath the
younger crust.
Volcanic islands develop at the surface of the overriding crust (forming Island Arcs) - e.g. Hawaii
CONVERGENCE:
Continental Crust-Continental Crust
Neither plate subducts (both too light).
Compressive forces driving plates fold and thrust the
continental margins forming an extensive mountain
belt (e.g., the Himalayan Mountains).
SLIDING: Slip-Strike Faults
Plate margins along which the plates slip by each
other. Termed: Slip-Strike Faults/Transform Faults
On either side of a fault plate motions are in opposite
directions.
The Big Picture