Plate Tectonics Notes - Hanover Area School District · 2012-10-05 · Plate Tectonics ! A theory that combines continental drift and seafloor spreading ! Earth’s plates are made

Plate Tectonics

Continental Drift

§  Wegener’s hypothesis that all continents were once connected and drifted slowly to their current locations

§  Controversial idea at the time

Pangaea

§  A large, ancient landmass (formed 200 million years ago) that was made of all of the continents joined together

§  Pangaea means “all land”

Evidence for Continental Drift a)  The continents look like puzzle pieces

that could fit together. b)  Fossil evidence of similar animals on

different continents §  Ex: Fossils of a freshwater reptile called

Mesosaurus found in S. America and Africa

c) Fossil evidence of warm weather plants in cold climates

§  Ex: Fossils of a fern-like plant called Glossopteris found in Antarctica

d)  Evidence of glacial activity in warm areas

e)  Similar rock structures found on different continents

Continental Drift Rejected

§  Wegener could not explain how, when, or why the continents could move.

Seafloor Spreading §  A possible explanation for how the

continents move §  Proposed by Harry Hess in the early

1960s

Seafloor Spreading

§  Caused when hot magma rises toward the surface, cools and hardens, then spreads seafloor sideways in either direction

§  Occurs at mid-ocean ridges §  Newest rocks are closest to the mid-

ocean ridge because that is where the new igneous rocks formed from magma cooling.

Evidence for Seafloor Spreading

1)  Using sound waves, researchers discovered underwater mountain ranges called mid-ocean ridges where volcanic eruptions and earthquakes occur.

2)  Rock samples taken from the seafloor by a ship called the Glomar Challenger were found to be youngest at the mid-ocean ridges and get increasingly older as they went toward the continents.

3) Using submersibles, scientists discovered that molten material erupts along the mid-ocean ridges.

4) Using magnetometers, scientists discovered that rocks on the seafloor show many periods of magnetic reversals. These reversals are recorded as alternating strips that run parallel to the mid-ocean ridges.

Plate Tectonics

§  A theory that combines continental drift and seafloor spreading

§  Earth’s plates are made up of parts of the seafloor and continents, so as the seafloor spreads it also makes the continents move.

§  Finally, there was an explanation for how the continents could move!

§  According to the theory of plate tectonics, Earth’s crust and part of the upper mantle are broken into sections called plates that move on the plasticlike layer of the mantle called the aesthenosphere.

Cause of Plate Tectonics

§  The transfer of heat in Earth’s interior provides the energy to move the plates.

§  This heat is transferred through convection currents.

§  Convection currents are caused by a cycle of heating and cooling.

Convection Currents in the Mantle

§  When the material in the mantle is close to the core, it becomes hot which makes it less dense and causes it to rise.

§  As it travels away from the core it cools which makes it become more dense and causes it to sink.

§  Eventually it will get close to the core and heat up again and the cycle will repeat itself.

§  See fig. 12 pg. 111.

Putting it all together…

§  Along the mid-ocean ridges, some of the less dense heated magma that is rising erupts forming new seafloor and causing it to spread outward.

§  Earth’s surface does not continue to get larger, though. Since new seafloor is being created at the mid-ocean ridges, there must be a place where the old seafloor is being destroyed.

§  Old seafloor is destroyed at subduction zones where plates are plunged back into the mantle.

§  The reason why the old seafloor subducts is that over time as it has cooled down and has become more dense. So, when it collides with another plate it slips under the less dense plate.

Plate Boundaries

§  As the Earth’s plates move around they can interact in three different ways. They can: § Collide – Convergent § Move apart - Divergent § Slip past one another sideways -

Transform

Convergent Boundaries

§  Where plates collide 1) Oceanic to Continental

§ Oceanic plate subducts § Deap-sea Trench § Volcanoes § Earthquakes § Ex: Andes Mountains along west

coast of South America

2) Oceanic to Oceanic § Older oceanic plate subducts § Deep-sea Trench § Volcanoes (can form island arc) § Earthquakes § Ex: Mariana Islands and Mariana

Trench (deepest spot in the world)

3) Continental to Continental § Neither plate subducts…instead

crumple up § Mountains § Earthquakes § Ex: Himalaya Mountains in Asia

(Mt. Everest)

Divergent Boundaries

§  Where plates move apart 1)  In the middle of an oceanic plate

§  Mid-ocean ridge with rift valley §  Earthquakes §  Ex: Mid-Atlantic Ridge

2) In the middle of a continental plate § Rift valley § Earthquakes § Ex: Great Rift Valley in eastern

Africa (if it keeps separating will eventually form an ocean)

Transform Boundaries

§  Where plates slip past one another sideways

§  Earthquakes §  Ex: San Andreas Fault in southern

California

Testing for Plate Tectonics

§  Today, scientists can use satellites to actually measure the movement of the Earth’s plates.

§  On average, plates move anywhere from 1 cm to 12 cm per year.