Plate TectonicsPlate Tectonics

BIGBIG Idea: Idea: Most geologic activity

occurs at the boundaries between plates.

I. Drifting ContinentsI. Drifting Continents

• MAIN IDEA: The shape and geology of the continents suggests that they were once joined together.

Continental DriftContinental Drift

theory proposed by Alfred Wegener (VAY guh nur) in 1912

stated that the continents were once a single landmass

the continents have moved...

--Pangaea (“all the earth”): Pangaea (“all the earth”): supercontinent; single landmass single landmass --Panthalassa (“all seas”):Panthalassa (“all seas”): huge ancient huge ancient oceanocean

A. Evidence of Continental A. Evidence of Continental DriftDrift

1. Puzzle-Piece Fit

2. Rock Formations2. Rock Formations

Ages, types and layers of coastal rocks of widely separated regions matched…

Western Africa and Eastern Brazil

Appalachians (along eastern U.S.) fit Greenland and Europe

3. Fossils3. Fossils

Mesosaurus: freshwater reptile that lived 270 million years ago

• remains (fossils) found in South America and Africa

• impossible for these reptiles to have crossed the Atlantic

4. Climate4. Climate

(a) Glossopteris: fern that grew in temperate climates

places where fossils had been found were once closer to the Equator

rocks containing these fern fossils had once been joined

(b) Coal Deposits: Coal forms from ancient

swamp plants Coal beds found in

Antarctica indicated that this frozen land once had a tropical climate

Antarctica must have been closer to the Equator

(c) Glacier Deposits: found in Africa and South

America climates are much warmer

there today

A Rejected Notion…A Rejected Notion…

• Wegener’s Continental Drift Hypothesis was never accepted by the scientific community

• He could not explain what forces could cause such massive movement

• Wegener died in 1930, on expedition in Greenland, while collecting evidence to further support his theory

II. Seafloor SpreadingII. Seafloor Spreading

• MAIN IDEA: Oceanic crust forms at ocean ridges and becomes part of the seafloor.

1.Mid-Atlantic Ridge: undersea mountain range with a steep narrow valley down the center

• the middle of the ocean floor is very young

2. Renewal of the Ocean Floor:seafloor spreading - floor moving

away from the centerHarry Hess and Robert Dietz

3. Paleomagnetism: as magma solidifies the iron-rich minerals align with magnetic north like a compass

reverse polarityreverse polarity: magnetic : magnetic orientation pointing southorientation pointing south

III. Plate BoundariesIII. Plate Boundaries • MAIN IDEA: Volcanoes,

mountains, and deep-sea trenches form at the boundaries between the plates

Theory of Plate TectonicsTheory of Plate Tectonics• Plate Tectonics:

theory that the lithosphere is made of rigid plates that ‘float’ on the asthenosphere

• Tectonics: the study of the formation of Earth’s features

What are the What are the inferredinferred properties of Earth’s properties of Earth’s

interior?interior?

Zones of EarthZones of Earth1. Crust: thin,

solid outermost layer; surrounds Earth like a shell surrounds an egg.

Two Types of Crust:Two Types of Crust:

Continental Crust: makes up the continents (land); thicker, less dense; granitic rock

Oceanic Crust: under the oceans; thinner, MORE dense; basaltic rock

2. Moho: boundary between the crust and mantle.

3. Mantle: thickest layer sits below the crust makes up 2/3 of the Earth’s

mass

•Lithosphere (‘rocky sphere’): upper part of mantle, cool and rigid

•Asthenosphere (‘weak sphere’): lower (‘plastic’) mantle; flows like hot tar

•Plasticity: the ability of a solid to flow

4. Core: center of the Earth; two parts:

• InnerInner Core: ball of hot, solidsolid metals (iron and nickel)

OuterOuter Core: only true liquidliquid zone

If we’ve never been If we’ve never been there, how have we there, how have we learned about the learned about the interior of Earth?interior of Earth?

By studying seismic By studying seismic waves…waves…

Seismic Wave:Seismic Wave: vibration that travels through the Earth; results from earthquakes or huge explosions

Types of Plate Types of Plate BoundariesBoundaries

1. 1. Divergent BoundariesDivergent Boundaries: • two plates that are moving

AWAY from each other• as plates move apart, the

asthenosphere flows up to fill the empty space

What forms when plates What forms when plates DIVERGEDIVERGE??

a. Mid-Ocean Ridges Mid-Ocean RidgesMid-Atlantic RidgeMid-Atlantic Ridge

b. Rift Valleys:b. Rift Valleys:(1) narrow valleys in the (1) narrow valleys in the middle middle of ocean ridgesof ocean ridges

(2) where continents (2) where continents separateseparate

East African Rift East African Rift ValleyValley

2. 2. Convergent Convergent BoundariesBoundaries

• direct collision of one plate with another

Three types of converging Three types of converging boundaries:boundaries:

• Types of Convergent Boundaries

OceanicOceanic vs. vs. ContinentalContinental

Subduction Zone:Subduction Zone: – oceanic crust moves (dives) under continental crust

forms an oceanic trench and volcanic mountain range

oceanic crust is more dense than continental, thus it dives down or subducts

Andes Mountain RangeAndes Mountain Range

Mount Saint Helens - Mount Saint Helens - Cascade Cascade

Mountain RangeMountain Range

OceanicOceanic vs. vs. OceanicOceanic

older, more dense oceanic crust subducts

forms trenches and arcs of volcanic islands

Aleutian IslandsAleutian Islands

Mariana TrenchMariana Trench

ContinentalContinental vs. vs. ContinentalContinental

no subduction two continental plates collide

pushing up and creating folded mountains

3. 3. Transform BoundariesTransform Boundaries • Plates slide horizontally

past each other

IV. Causes of Plate IV. Causes of Plate MotionsMotions•MAIN IDEA: MAIN IDEA: Convection Convection

currents in the mantle currents in the mantle cause plate cause plate motions.motions.

coolcool material sinkssinks as hothot material risesrises, due to

DENSITY DIFFERENCESDENSITY DIFFERENCES.