Plate Tectonics: Overview Earth’s crust: broken into about 12 major plates Plates move several cm each year Asthenosphere drives the plate Slow moving layer Convection currents Plates can be either Oceanic: thinner but MORE dense (sink) Continental: thicker but LESS dense (rise)

Table of Contents: Types of Plate Boundaries Convergent boundaries : Compression forces push plates together Divergent boundaries: Tension forces move plates apart Transform Plate boundaries: Shearing forces cause plates to slide against each other (opposite directions or same direction @ different rates)

Convergent Boundary Type 1 Continental—oceanic: denser oceanic crust subducts under continental crust. Oceanic plate melts as it is driven down, less dense material punches through continental plate to create volcanoes and/or mountains Forms a subduction zone and oceanic trench Earthquakes occur as plate is subducted. Ex: Andes Mtns and Peru-Chile trench animation of the processes at a destructive plate boundary

Convergent Boundary Type 2 Oceanic—oceanic: one subducts creating a VERY deep trench and an island arc Ex. Kuril Islands and Marianas Trench: plunges deeper into the Earth's interior (~11,000 m) than Mt Everest, the world's tallest mountain, rises above sea level (~8,854 m).

Island arcs (archipelagos) are created along plate boundaries! Hot spots (like Hawaii) are located within the margins of plates, @ center, at plate boundaries (everywhere) magma pushes to surface Do NOT confuse the creation of islands at convergent boundaries with islands created at hot spots! Hotspot animation

Convergent Boundary Type 3 Continental—Continental: Both resist subduction Land buckles/folds upward to create fold mountains No volcanic activity but major earthquakes often occur due to collision. Ex: Himalayas: Indo-Australian and Eurasian Plate The Making of: the Himalayas

Divergent Plate boundaries Processes: Hot magma rises to fill the 'gap' creating new crust and mountain ranges to form the mid-ocean ridge. Both earthquakes and volcanoes occur at this type of boundary. Rising magma can build up above ocean surface to create a volcanic island (for example Surtsey, Iceland). (ex. North American and Eurasian Plate form the Mid-Atlantic Ridge) animation

Transform Plate boundaries Movement: two plates move past each other crust is neither created or destroyed Processes: Shearing forces build btw. the rock Pressure builds b/c of friction between the plates, slipping releases energy = earthquake. Example: San Andreas Fault (N. American + Pacific Plates)

Divergent Convergent Transform Continental crust  rift valley Oceanic crust  mid-ocean ridge 2 continental plates  mountain range Plates move against each other Stress builds up Stress is released earthquake 2 oceanic plates or Oceanic/continental subduction

Lets put it all together…

Review Name the type of plate boundaries Name the plates: continental or oceanic Name resulting features or earthquakes

Wegener:Early Observations German Scientist Alfred Wegener (1912) proposed the idea of moving continents scientific hypothesis based on fossil evidence, rock & mineral formations

Evidence from Rock Formations Similar rock types on opposite sides of the Atlantic Ocean. Appalachian Mountains in the US identical to rock in Greenland and Europe.

Fossil Evidence: Reptiles Lystrosaurus: Early Triassic Period (250 mya) Land Animal Antarctica, India, S Africa Cynognathus: Late Triassic Period “Mammal-like reptile” S Africa, South America, China, Antarctica

Evidence from Fossils

Climate Evidence: Ferns & Coal Certain climates support diff fern types Land connections were proved by same fossils on different continents Coal in Northern US originated in an equatorial climate but drifted North over millions of years

Why wasn’t it accepted? Problems with his theory: Couldn’t explain how large land masses were pushed such great distances Didn’t explain how continents moved through the solid crust

Accepted Today because: Know that asthenosphere has convection currents Currents push magma which then pushes plates Plates move at different rates Not all pieces of plate move at same rate or same direction as the plate in general!

Seafloor Spreading

Age of Rock Rock NEAR ocean ridge= YOUNGER Rock FAR from ocean ridge= OLDER Oceanic rocks are relatively young compared to continental rocks—>3.8 billion years old but oceanic is recycled @ subduction zones Thickness increases with distance from ridge

Magnetism & Seafloor Earth’s magnetic field generated by flow of molten iron in outer core (Fe)compass Flow changes magnetic reversal compass point S Oceanic crust basaltic rock-contains large amts of Fe-bearing minerals  MAGNETIC! Ocean floor shows record of past reversals using magnetometers under water “strips” along seafloor “strips” are mirror images on either side of ridge symmetrical!! WOW!

When iron cools it is magnetized by the magnetic field of the earth. So the magnetic field of the sea floor at any time is aligned with the Earth's magnetic field. But the Earth's magnetic field reverses every several thousand years and so when that happens there is new material on the sea floor which is magnetically orientated differently than the older material. This discovery led to the understanding that we can study rock samples far from ridge and understand what core was like/magnetic polarity millions of years ago.

Magnetic Seafloor Symmetry

Isochron Isochron imaginary line on map showing points that have same age (formed @ same time)

Seafloor Spreading New oceanic crust formed @ ocean ridges destroyed @ deep-sea trenches Magma forced to crust along ridge b/c hotter and less dense than mantle Fills gap of spreading ridge Magma solidifies ocean floor Continents like passengers ride along while ocean crust slowly moves