Download PLATE TECTONICS

PLATE TECTONICS
Hypothesis: Earth’s Lithosphere moves on top of
Aesthenosphere due to Mantle Convection
Evidence of Lithospheric (Plate) motions (Tectonics)
Driving Mechanism of Plate Tectonics
EVIDENCE OF LARGE-SCALE
HORIZONTAL MOTIONS OF THE
CRUST AND UPPER MANTLE:
Contributions of Land-based Geologists
(observed from 1600’s to present)
• Fit of coastlines, especially South America and
Africa
• Identical species of land-dwelling fossils on
different continents
• Alignments of rare minerals and fossils with other
continents
• Linear patterns of mountain ranges, volcanoes,
and earthquakes
Reconstructions of
Continents at Various
Times in the Past by
Alfred Wegener
Fossil Relationships of Rare or Limited Species
EVIDENCE OF A DYNAMIC EARTH
Evidence from Ocean-based Data
(1950-present)
• “Young” Age of Seafloor (<200 m.y.)
relative to Continents (up to 4.5 b.y.)
• Discovery of 40,000 km long Mid Ocean
Ridges (MOR)
• Age of seafloor increases symmetrically
away from MOR crest
Age of Seafloor
New seafloor comes to the
Surface at MOR
At MIMOR’s
Topography of the Mid Ocean Ridge:
N. Atlantic
MAGNETIC ANOMALIES
Earth’s magnetic field periodically reverses its polarity:
on average about every 400,000 years, but range is
10,000 yrs to millions of yrs
Fe-rich minerals crystallizing in molten rock align with
Earth’s magnetic field and point to N magnetic pole
Magnetic signature is “locked in” as rock solidifies
Subsequent pole reversals do not change the mineral alignment
in solid rock
Seafloor-Spreading at Mid-Ocean Ridges
a Divergent Plate Boundary
5
4 3
2
1
Age of seafloor in m.y.
0
0
1
2 3
4
5
Earthquake Evidence
• Quakes form linear zones
• Usually associated with volcanoes
• Deepest quakes reach 700km –implies
brittle failure occurring
• In some areas, there is an inclined plane of
seismicity, marking the contact between a
subducting plate and the overriding plate
Earthquakes define Plate
Boundaries
Andean-type Convergent Plate Boundary
Subduction Zone: where 2 plates collide, one sinks, or subducts.
Evidence for subduction comes from inclined plane of seismicity,
Called a Wadati-Benioff Zone
Earthquake Epicenters for central
South America
Color change from yellow
to dark blue represents
increasing depth
Driving Mechanism of the
Plates:
Convection in the Mantle
How Convection Works
Large convection currents in the aesthenosphere
transfer heat to the surface, where plumes of less dense
magma break apart the plates at the spreading centers,
creating divergent plate boundaries.
As the plates move away from the spreading centers,
they cool, and the higher density basalt rocks that make
up ocean crust get consumed at subduction zones. The
crust is recycled back into the aesthenosphere.
CONVECTION CELLS
in the EARTH
MOR
MANTLE
SUBDUCTION
ZONE
OC
IC
THREE TYPES OF PLATE BOUNDARIES
Transform Plate Boundary: San Andreas Fault
Japan-Sea Type Convergent Plate Boundary
Andean-type Convergent Plate Boundary
Himalayan-type Convergent Plate Boundary
Indian Plate Collides with Asian Plate
S
N
OBSERVATION:
Because ocean plates are denser than
continental plates, when these two types of
plates converge, the ocean plates are
subducted beneath the continental plates.
The collision of plates is accompanied by
earthquakes and volcanoes.
Hot Spot Beneath Pacific Plate Creates the
Hawaiian Islands
Not all volcanic activity occurs at plate boundaries
SUMMARY TABLE:
Process---Feature---Plate Boundary Type
• Seafloor Spreading---Mid Ocean Ridges
and Continental Rifts---Divergent Plate
Boundaries
• Subduction---Subduction Zones--Convergent Plate Boundaries
• Shear---Strike-slip Faults---Transform Plate
Boundaries