Plate Tectonics

Sir Francis Bacon, 1620, noted that the continental coasts on

opposites sides of the Atlantic fit together like puzzle pieces.

Could North and South America once have been joined to Europe

and Africa?

Fossil evidence from Western Africa and Eastern South America

supports this assertion (Mesosaurus, Glossopteris)

Alfred Wegener• 1915 - German geophysicist

• Tried to explain climate patterns

indicated by fossils

• Suggested all continents might have

been a single super continent – Pangaea

– Said that Pangaea began to break apart

180 million years ago

Although rejected, Wegener’s idea did not die…

1940 - Hugo Benioff plotted locations of deep earthquakes at edge

of Pacific (Pacific Ring of Fire)

Seismographs revealed that earthquakes and volcanoes do not occur

at random - but in zones

Other support:

Radiometric dating - oceanic crust much younger than Earth

Echo sounders used to map sea floor (ocean ridges match shorelines)

1960 - Harry Hess suggests that new seafloor develops at ocean

ridges and spreads outward with trenches along edges of ocean

basins, powered by convection currents in the mantle

Hypothesis: Seafloor Spreading!

94% of energy comes from radioactive atoms breaking down releasing energy,

must be areas of cooling—near trenches!

Seafloor spreading was an idea

proposed in 1960 to explain the

features of the ocean floor. It

explained the development of the

seafloor at the Mid-Atlantic Ridge.

Convection currents in the mantle

were proposed as the force that

caused the ocean to grow and the

continents to move.

(right) The Mid-Atlantic Ridge

conforms to the shape of the

adjacent continents. The inset

shows the central rift.

Seafloor Spreading - A Key Idea

Evidence for Seafloor Spreading…

• 1963 - Frederick Vine & Drummond

Matthews

– Magnetic patterns in ocean floor were

mapped

– Noticed that there were magnetic patterns

on either side of an ocean ridge

– Crust is moving like a large conveyer belt

away from ocean ridges!– http://www.wwnorton.com/college/geo/egeo/f

lash/2_3.swf

• 1965 - Sir Edward Bullard used a computer to

compare coastlines at the continental shelf edge

Theory which describes the movement of

crustal plates on the mantle – Plate Tectonics!

Work of John Tuzo Wilson - lithospheric plates

floating on the asthenosphere

Types of Plate Boundaries

Some plates move fast ~ (16 cm/year) and some

slowly ~ (2-3 cm/year)

All plate motions can cause earthquakes at plate edges

(mostly ridges & trenches)!

Plate Boundaries…

1. Transform - locations where crustal plates move past

one another (ex. San Andreas Fault)

Translation at transform boundaries causes shear.

2. Divergent - boundaries where plates move apart; new ocean floor is

created, spreading rates can differ at ridges (slower, steep slopes) and

rises (faster, gentler slopes)

•Divergent Oceanic Crust - the Mid-Atlantic Ridge, East Pacific Rise

•Divergent Continental Crust - the Rift Valley of East Africa

Extension of divergent boundaries

causes splitting and rifting.

http://www.wwnorton.com/college/geo/egeo/flash/2_7.swf

3. Convergent Plate Boundaries - regions where plates are

pushing together, trenches have the largest earthquake

magnitudes, can be further classified as:– Island Arc Trench Systems - subduction of oceanic crusts, lithosphere is

heated, rises back to surface as volcanoes (islands), ex. Japanese Islands

– Continental Arc Trench Systems - oceanic plate will subduct under continental

plate, volcanoes will form on continents (ex. Peru-Chile Trench & Andes)

– Continental Mountain Systems - two continents collide & sedimentary rock is

folded to form mountains (ex. Himalayas)

http://www.wwnorton.com/college/geo/egeo/flash/2_6.swf

Evidence for Plate Tectonics:

• Ancient Life & Climates (fossils)

– Coral reefs in eastern North America, western Europe, Alps, &

Himalayas rocks that are 350 myo

• Ocean Sediments - sediments are younger & much thinner than

should be (must be recycled over time)

• Continent Fit

Paleomagnetism: most igneous rocks contain magnetite -

becomes permanently fixed as rock cools (mini-compass)

1.Strips of alternating magnetic polarity at spreading regions

2. Apparent Polar

Wandering: plate movement

causes the apparent position of

the magnetic poles to have

shifted

The paleomagnetic fields in

the rocks would indicate a

single pole until the

continents drift apart.

Hot Spots: surface expression of plumes of magma (ex. Hawaii, Iceland,

Yellowstone)

6% of energy for plate tectonics comes from hot material (magma) rising

up from the core

A volcanic island chain can form when a plate passes

over a hot spot and a stationary mantle plume.

show Exploring the Earth

video clip

Website for plate tectonics

worksheet

• http://geoinformatics.sdsc.edu/doe/student

/tectonics/continental_drift.html