Continental Drift Chapter 10

Wegener’s Hypothesis  Once a single supercontinent  Started breaking up about 200 mya  Continents drifted to current location  Evidence, but no mechanism

Pangaea

Pangaea

Wegener’s Evidence 1.Fossils –Similar ones found in areas thought to have been connected 2.Rock formations –Rock ages same on African & S. American coast –Continuation of mountain ranges across continents 3.Climatic evidence –Glaciers in Africa and S. America –Tropical fossils in colder climates

Rock Evidence

Fossil Evidence

Climate evidence Glacial evidence in south Africa, Australia, South America, and India. Occurred at same time.

Pangaea Breakup

Breakup of Pangaea (cont)

Mid – Ocean Ridges  Under sea mountain ranges  Sediment thicker farther away from ridges  Sediment closer to ridges is younger  Ocean floor is young compared to Earth

Mid - ocean Ridges

Mid – Ocean Ridges

Sea Floor Spreading  Suggested by Harry Hess  Ocean ridge is a crack (Rift) in Earth’s crust  Magma fills the crack  As ocean floor spreads, continents also move

Sea Floor Spreading

Paleomagnetism  Study of magnetic properties of rocks  Iron rich minerals in magma align with Earth’s magnetic field  As magma solidifies, magnetic orientation becomes permanent  Residual magnetism is paleomagnetism

Earth as a Magnet

Polar Wandering Makes more sense for the continents to move than for the North Pole to move

Magnetic Properties from Pangaea Paths coincide if continents started as Pangaea

Magnetic Reversals  Earth’s magnetic field does not always point north  Magnetic reversals – orientation is opposite of normal  “Normal” rocks and “Reversal” rocks line up by time period  Alternating normal and reversed polarity over time  Geomagnetic time scale

Magnetic Reversals

Plate Tectonics  Theory that explains how large pieces of lithosphere, called plates, move and change shape  Study of formation of features in Earth’s crust

Lithosphere  Solid outer layer of Earth  Consists of the crust and rigid upper part of the mantle  Broken into blocks called plates  Plates float on the asthenosphere

Asthenosphere  Solid plastic layer of mantle below the lithosphere  Made of mantle rock that flows very slowly  Allows tectonic plates to move

Lithosphere & Asthenosphere

Crust types 1.Oceanic crust –Dense –Rich in magnesium and iron 2.Continental crust –Low density –Rich in silica (SiO 2 )

Continental & Oceanic Crust

Tectonic plates  Can be either or both oceanic and continental crusts  15 major plates  Earthquakes – sudden shifts along plate boundaries  Volcanoes – plate motions generate magma that erupts

Tectonic Plates

Types of Plate Boundaries 1.Divergent boundaries –Plates move away from each other –Most on the ocean floor –Form ocean ridges (rifts) –Plates separate at rift valley

Divergent Boundary

Divergent Boundary (on land)

Types of Plate Boundaries 2.Convergent boundaries –Plates move toward each other –Plates eventually collide –3 types of collisions

Convergent Boundary  Continental – Oceanic Crust Collisions –Subduction zone – oceanic sinks under continental –Magma rises to surface and forms volcanic mountains

Continental – Oceanic Collision

Convergent Boundary  Oceanic – Oceanic Crust Collision –Forms ocean trench (subduction) –Magma forms island arc (Example: Japan)

Convergent Boundary  Continental – Continental Collision –Forms mountains –Edges crumple and thicken –Creates uplift

Continent Collisions

The Himalayas

Types of Boundaries 3.Transform boundaries –Plates slide past each other –Sudden motion produces Earthquakes –No magma involved

Transform boundary

San Andreas Fault

Earthquakes – 1 Year

North America – Earthquakes 10 Year Period

Causes of Plate Motion 1.Mantle convection –Convection currents drag plates 2.Ridge Push –Newly formed rock slides downhill at mid ocean ridge 3.Slab pull –Subducting plate pulls more crust with it

Plate Motion

Rifting  Breaking apart of Earth’s crust

Terrane  Piece of lithosphere with a unique geologic history incorporated into another piece of lithosphere

Western U.S. Terrane

Alaska Accretions

Continental Growth

Effects of Continental Change  Climate changes  Life –As populations separate, new species develop