Plate Tectonics TheoryChapter 9 Notes

Continental Drift

• German Climatologist and Geophysicist named Alfred Wegener.

• Proposed the idea of continental drift.

• Wrote a book in 1912 called the Origin of Continents and Oceans.

Matching a Puzzle

• Others before him noticed the patterns of the continental plates.

• Abraham Ortelius (1596)• Frances Bacon (1625)• Benjamin Franklin (?)• Antonio Snider-Pelligrini

(1858) picture above

Pangaea

• Wegener proposed that at one time in the distant past, all of the continental plates converged into one supercontinent.

• He called this supercontinent Pangaea.

Evidence for Continental Drift

• Matching fossils found on adjacent continents across the ocean.

• Matching rock types found on adjacent continents across the ocean.

• Erosion and depositional features found on adjacent continents across the ocean.

• Magnetic patterns found on the ocean floor (not discovered until the 1960s).

• Hawaiian Islands produced by Hot Spots.

Matching Fossils

• Mesosaurus – Extinct crocodile-like reptile.

• Glossopteris – Extinct seed-fern type plant.

• Lystrosaurus – Extinct dinosaur.

• Cynognathus – Extinct dinosaur.

Matching Rock Types

• Matching Rock Types found on adjacent continents across the ocean.

• Appalachian and Caledonian Mountains

Erosion and Deposition

• Deposits that resemble glacial till found on separate continents.

• Large striations found in solid bedrock.

• This could also be explained by erosion and deposition caused by submarine landslides.

Magnetic Reversals

• In the 1960’s, scientists discovered that basalt on the ocean bottom shows that the magnetic field has reversed polarity in the past.

• It is not understood how this happens and what affect it might have on living organisms.

• It does appear to be very strong evidence that the bottom of the ocean has spread.

Hawaiian Hot Spots

Hawaiian Hot Spots

Hawaiian Hot Spots

Geologic Time

• Uniformitarianism vs. Catastrophism• Current seafloor spreading is moving at about a few

centimeters per year. That is about the speed at which your fingernails grow.

• So where is all that lithosphere going? • Subduction and Mountain building.• The current rates of erosion and uplift vary depending on

the mountain range.• Some mountains are growing and some are slowly

shrinking due to erosion.• Appalachians, Alps, Rockies, Andes, Himalayas

Modern Plate Tectonics• In spite of the numerous evidences, most

scientists rejected Wegener’s idea of seafloor spreading until the 1960’s.

• With the aid of sonar technology and the ability to map the ocean floor, substantial evidence was able to be given.

Plate Boundaries

• The earth’s lithosphere (crust) is made up of several plates which rest upon a hot molten magma called the asthenosphere.

• Wherever these plates come in contact with another plate is called a boundary.

• There are three types of plate boundaries.

1. Divergent Boundaries: Plates move apart2. Convergent Boundaries: Plates come together3. Transform Boundaries: Plates slide past each other

Plate Boundaries• At Divergent boundaries, new lithosphere

is rising up and being produced.

• At Convergent boundaries, old lithosphere is either subducted or compressed to build thicker regions of lithosphere (mountains)

• At transform boundaries, two plates slide past one another. (San Andreas Fault)

Divergent Plate Boundaries

• Divergent plate boundaries are found on both oceanic crust and continental crust.

• Sometimes a rift valley that begins on land can develop into an ocean ridge on the bottom of the ocean.

• New lithosphere expands and forms a ridge. Once it cools down it shrinks.

Convergent Plate Boundaries

• At Convergent Plate Boundaries, one plate slides and descends downward toward the mantle and eventually begins to melt.

• Some of the lithosphere gets compacted into the crust, causing the plates to thicken.

• This process is called accretion.• This thickening of the crust creates a

difference in topography at the surface.• Lithosphere that descends gets recycled.

Convergent Plate Boundaries

• Three types of convergent plate boundaries.

1. Ocean– Continental2. Ocean–Ocean3. Continental-Continental

•Differences in density is a major factor contributing to subduction of lithosphere.•As dense lithosphere comes in contact with buoyant lithosphere, it descends below it. •Where oceanic crust descends below continental crust you find an ocean trench.

Ocean–Continental Convergence

• Oceanic lithosphere is made of dense basalt.• Dense basalt descends below buoyant continental

crust rich in silicates (quartz, feldspar, granite etc.)• Continental Volcanic Arcs form adjacent to

Ocean-Continental Convergent Plate Boundaries.

Oceanic-Oceanic Convergence

• This plate boundary forms volcanic chains on the bottom of the ocean floor located adjacent to the plate boundary.

• When the submarine volcanoes reach the ocean surface, they form island chains called volcanic island arcs.

Continental-Continental Convergence

• Sometimes two continental plates collide.• Continental plates resist subduction due to their

low density, which makes them very buoyant.• The crust becomes very thick.• Isostasy and isostatic adjustment and

equilibrium takes place.• This creates uplift and builds the mountains.• The largest mountains of the world were formed

in this way. (Himalayas, Everest)

Isostatic rebound (readjustment)

Continental-Continental Convergence

• Example: The collision of India and Asia to form the Himalayan mountains.

Transform Plate Boundaries

• Plates slide past one another horizontally.• Most of these are found along divergent plate

boundaries in the ocean (mid-ocean ridge).• Some are found on the continents, like the San

Andreas Fault in California.

Mechanism for Plate Tectonics

• One of the main reasons why scientists remained skeptical of plate tectonics until the 1960s was because of a lack of a mechanism that could explain it.

Convection Currents

• The earth’s mantle is solid, but behaves like a ductile fluid.

• Uneven heat throughout the earth’s mantle causes convection currents.

• It is believed that these convection currents cause the earth’s plates to move.

• Two forces that drive plate movement are the ridge-push and slab-pull