Evidence for Continental DriftEvidence for Continental Drift

Jigsaw Puzzle fit of Jigsaw Puzzle fit of continentscontinents

Alfred Wegener during Greenland expedition

More evidenceMore evidence

Matching fossils on Matching fossils on continents now continents now located thousands located thousands of miles apart. of miles apart. Example = Example = Mesosaurus, a Mesosaurus, a freshwater reptilefreshwater reptile

Many othersMany others

Matching Matching mountain mountain rangesranges

More evidenceMore evidence

Matching geologic Matching geologic structures structures including:including:– Mountain chainsMountain chains– Ore depositsOre deposits– Same rocks of Same rocks of

same agesame age

More evidenceMore evidence

Climate change Climate change evidenceevidence– Glacial deposits at Glacial deposits at

current equatorcurrent equator– Fossilized palm Fossilized palm

trees in Greenlandtrees in Greenland Map shows why Map shows why

according to the according to the placements of placements of current continents current continents within Pangaeawithin Pangaea

Wegener not believedWegener not believed

Why? -Why? -– What could possibly force the What could possibly force the

continents to move across the continents to move across the ocean floor in this way. They ocean floor in this way. They would be crushed.would be crushed.

– He was a meteorologist, He was a meteorologist,

not a geologistnot a geologist

Developments 1950s and 1960sDevelopments 1950s and 1960s World War II submarines using sonar World War II submarines using sonar

found mountains under the oceans – found mountains under the oceans – the mid-ocean ridgesthe mid-ocean ridges

Sea floor drilling showed rocks Sea floor drilling showed rocks younger than expected and youngest younger than expected and youngest towards the center of the mid-ocean towards the center of the mid-ocean ridgeridge

Theory of seafloor spreading Theory of seafloor spreading suggested by Princeton professor Dr. suggested by Princeton professor Dr. Harry HessHarry Hess

Seafloor spreadingSeafloor spreading

First look at the earth’s layers as shown here.

Evidence for sea floor spreadingEvidence for sea floor spreading Alvin and other Alvin and other

submersibles found cracks submersibles found cracks and lava showing and lava showing spreading and volcanism spreading and volcanism at mid ocean ridges and at mid ocean ridges and odd life formsodd life forms

Matthews and Vine’s Matthews and Vine’s survey of the Indian survey of the Indian Ocean sea floor showed Ocean sea floor showed matching stripes of matching stripes of reversing polarities on reversing polarities on either side of ridge – what either side of ridge – what caused these?caused these?– Lava spreading during Lava spreading during

alternating magnetic periodsalternating magnetic periods

The Earth’s magnetic fieldThe Earth’s magnetic field It is produced by the It is produced by the

outer core of the earth outer core of the earth which is made of liquid which is made of liquid iron and nickeliron and nickel

This moving magnetic This moving magnetic material produces a material produces a moving magnetic field, moving magnetic field, which in turn produces which in turn produces a moving electric field. a moving electric field. It is a dynamo!It is a dynamo!

Earth’s magnetic field Earth’s magnetic field varies over time and it varies over time and it protects us from protects us from cosmic radiationcosmic radiation

Seafloor spreading and Seafloor spreading and paleomagnetismpaleomagnetism

Pangaea revisitedPangaea revisited

By piecing together By piecing together this information, this information, we can see how we can see how the continents the continents have moved over have moved over the past 200 the past 200 million years, due million years, due to seafloor to seafloor spreadingspreading

Plate tectonicsPlate tectonics Sea floor spreading provides the Sea floor spreading provides the

driving mechanism for movementdriving mechanism for movement However, it is not the continents that However, it is not the continents that

are moving, but the “plates” of are moving, but the “plates” of lithosphere “floating” in effect on the lithosphere “floating” in effect on the asthenosphereasthenosphere

The lithosphere is made up of about The lithosphere is made up of about 20 plates which move relative to 20 plates which move relative to each other in several wayseach other in several ways

Let’s look at a generalized sketchLet’s look at a generalized sketch

Evidence supporting the theory of plate tectonics:

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.

Confirmation of the Theory of Confirmation of the Theory of Plate TectonicsPlate Tectonics

Paleomagnetism: strips of alternating magnetic polarity at spreading regions.

Confirmation of Plate Tectonics TheoryConfirmation of Plate Tectonics Theory

The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed rock changes.

The PlatesThe Plates

Sea-Floor Spreading and Plate Sea-Floor Spreading and Plate BoundariesBoundaries

Age of Oceanic CrustAge of Oceanic Crust

Courtesy of www.ngdc.noaa.gov

Mid Atlantic Ridge

Mid Atlantic Ridge

Spreading ridgesSpreading ridges– As plates move apart new material is As plates move apart new material is

erupted to fill the gaperupted to fill the gap

Divergent BoundariesDivergent Boundaries

Converging Margins: India-Asia Converging Margins: India-Asia CollisionCollision

Transform Faults and Seafloor Transform Faults and Seafloor SpreadingSpreading

What causes plate tectonics?What causes plate tectonics? Convection in the mantle, as Convection in the mantle, as

the plastic asthenosphere the plastic asthenosphere flows, carrying the plates with flows, carrying the plates with it.it.

This is probably aided by slab This is probably aided by slab pull at subduction zones and pull at subduction zones and ridge push at mid ocean ridges ridge push at mid ocean ridges and rising plumes in the mantleand rising plumes in the mantle

This diagram shows several This diagram shows several different model hypothesesdifferent model hypotheses

““Plates” of lithosphere are moved around by the underlying hot mantle Plates” of lithosphere are moved around by the underlying hot mantle convection cellsconvection cells