PLATE TECTONICS Hypothesis: Earth’s Lithosphere moves on top of Aesthenosphere due to Mantle Convection Evidence of Lithospheric (Plate) motions (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 ofContinents at VariousTimes in the Past byAlfred 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

Topography of the Mid Ocean Ridge: N. Atlantic

New seafloor comes to the Surface at MORAt MIMOR’s

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 withEarth’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

Age of seafloor in m.y.

5 4 3 2 1 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 thePlates:

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.

IC

OC

MANTLEMOR

SUBDUCTION ZONE

CONVECTION CELLSin the EARTH

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 PlateS 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 theHawaiian Islands

Not all volcanic activity occurs at plate boundaries

SUMMARY TABLE:Process---Feature---Plate Boundary TypePlate Boundary Type

• Seafloor Spreading---Mid Ocean Ridges and Continental Rifts---Divergent Plate Divergent Plate BoundariesBoundaries

• Subduction---Subduction Zones---Convergent Plate BoundariesConvergent Plate Boundaries

• Shear---Strike-slip Faults---Transform Plate Transform Plate BoundariesBoundaries