Ch 10 Plate Tectonics

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Continental DriftAlfred Wegener (1912)

• German Meteorologist

• Continents moved slowly to their current locations.

• Pangaea- the single landmass of Earth that broke apart

about 200mya . “All Land”

Evidence Puzzle-like Fit

• Coastlines of continents fit together like puzzle pieces.

• Examples: S.America &

Africa

Fossil Clues• Certain fossils of ancient animals &

plants are found on distant landmasses.

Freshwater/land-dwelling reptile Mesosaurus fossils have been found on South America & Africa.

Fossils of the ancient plant Glossopteris were found in Africa, South America, Australia, India, & Antarctica.

Climate Clues-Fossils of warm climate plants were found in

present-day cold climates. (Coal)

-Rocks deposited by glaciers from the ice ages have been found in present-day warm climates.

Rock Clues• Similar rock structures are found on different continents separated by oceans.

-Appalachian mountains of the USA are similar to mountains found in Greenland & western Europe

-Rocks of South America match those in Africa

Main objection to Wegener’s hypothesis was its inability to provide a mechanism for the movement of the continents.

Seafloor SpreadingSeafloor Spreading Hypothesis

Radio waves were used to map the ocean floor.

Mid-ocean Ridges- underwater mtn ranges

• Less dense magma flows sideways dragging the seafloor along.

• As the seafloor spread apart, magma flows up through cracks in at the ridge forming new rock.

Rift Valley Midocean Ridge

Evidence• Drilling

– Ocean floor rock samples show that the age of the rock gets older the further the rock is from the mid-ocean ridge

Magnetism

– Changes in the magnetic alignment of the Earth are found in alternating bands in the seafloor rock.

Theory of Plate Tectonics Theory

Combined theory of Continental Drift & Seafloor Spreading.

Earth’s crust & upper mantle (lithosphere) are broken into plates and move around on a plastic-like layer of the mantle (asthenosphere.)

Seven major plates:Pacific (largest) N. American

S. American African

Eurasian Australian

Antarctic

Plate Boundaries• Divergent Boundaries

– Two plates spread apart.

– Where seafloor spreading occurs.

– New crust is formed.

Ex: Mid-Atlantic Ridge

• Convergent Boundaries

–Two plates come together (collide)

–Oceanic plate will sink under a continental plate (subduction) producing a deep trench. Ex: Andes & Cascades

– Two oceanic plates will converge resulting in a volcanic island arc.

Ex: Mariana Islands

– Two continental plates will converge & crumple forming folded mountains.

Ex: Himalayas, Appalachians, & Alps

• Transform Fault Boundary

– Two plates slide past each other.

– Move at different rates.

– Also known as strike-slip faults.

Ex: San Andreas Fault

Causes (Hypothesis)

• Convection Currents

– Hot magma rises in the mantle.

– Magma hits the crust & cools.

– Cooling magma sinks dragging the lithospheric plate with it (centimeters.)

– Magma sinks into the mantle & reheats.

Features Formed by Tectonics

• Faults

– Cracks in the tectonic plates caused by stress.

– Plate boundaries are also faults.

– Movement along faults cause earthquakes.

• Volcanoes

• Magma produced by friction during subduction oozes up through the crust, erupts, & forms volcanoes.

Mountains

• Convergent & Divergent boundaries form mountains.

• Process of mountain building is called orogenesis.

-Volcanic Mountains

• Folded Mountains

oHorizontal rock layers are squeezed from opposite sides, causing them to buckle & fold. EX: Himalayans & Appalachians

Fault-block Mountains: Huge, tilted Fault-block Mountains: Huge, tilted blocks of rock that are separated by blocks of rock that are separated by faults.faults. EX: Sierra NevadaEX: Sierra Nevada

• Upwarped Mountains- blocks of Earth’s crust are pushed up by forces inside the Earth. (Domes) EX: Rocky Mountains