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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). - PowerPoint PPT Presentation
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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 PlateThe 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 TransformContinental 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 orOceanic/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”– Land Animal– 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
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