Download 2. Plate Tectonics

PLATE TECTONICS
 Alfred Wegener – developed the Continental
Drift hypothesis
 The continents were once joined as one single
“supercontinent”
 Pangea was the name given by Wegener for
the connected large landmass (means “all
land”)
A. Continental Drift
 Similarities in the coastlines of South
America and Africa raised questions about
“one big land mass”
 Evidence to support the Continental Drift
 The coasts of different continents look similar
 Especially South America and Africa
 Similar rock formations and coal deposits found
on different continents
 Similar fossils on different continents
 Several mountain belts end at a coastline but reappear
on a landmass across the ocean
 Appalachian Mountains run NE through the Eastern US
 Similar mountains of the same age and same type of
rocks are found in British Isles
 When put together, they form a continuous chain
 Evidence of past tropical climates in the
Antarctic.
 Fossils of Glossopteris, a fern-like tropical plant
found in colder areas
 Arctic climates (glacier marks) in the tropics.
 Wegener’s Theory was heavily criticized
 He could not provide evidence or the cause of
the force that moved the continents to their
current positions
 A new theory was born – Plate Tectonics!
B. Plate Tectonics
 The geological theory that
states the pieces of Earth’s
lithosphere are in constant
slow motion
 This is driven by currents in the
asthenosphere
 The lithosphere is divided into
segments called plates which
effect each other
 Collide
 Pull apart
 Grind past each other
 These movements affect Earth’s surface
 Volcanoes & Earthquakes
 Mountains & Valleys
 Mid-Ocean Ridges & Deep Sea Trenches
 Plate Boundaries & Faults
 The edges of each piece of a plate is called a
boundary
 The cracks in the Earth’s crust along these
boundaries are called faults
Types of Plate Boundaries
 Transform Fault Boundaries
 2 plates grind past each other without creating
nor destroying the Earth’s crust
 San Andreas Fault in California
 These can create NEW PLATE boundaries
 Responsible for Earthquakes
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 Divergent Plate Boundaries
 When 2 plates move apart or divide
 Creates Mid-Ocean Ridges
 Mid-Atlantic Ridge formed by divergent
boundaries
 Longest chain of mountains/volcanoes in the world even
though its all under water
 Seafloor spreading – new oceanic lithosphere
produced
 Rift valleys also formed – plate separation on
land
 Rio Grand and Great Rift Valley
 Convergent Plate Boundaries
 When 2 plates come together
 Forms deep water trenches
 The canyon forms where the ocean crust plunges back
 The more dense and older rock sinks – called subduction
 Andes, Cascades, Sierra Nevadas
 Can form volcanoes
 The subducted plate melts
 The melted rock is less dense and rises to create the
volcano
C. Causes of Plate Motion
 Slab Pull
 Old ocean crust that is more dense sinks into the
asthenosphere and pulls some of the lithosphere with
it
 Ridge Push
 Oceanic lithosphere slides down the sides of
an oceanic ridge (gravity)
 Mantle Convection
 Mantle is the middle layer of the Earth
 It is constantly cooled and heated causing uneven
heating of the Earth
 It’s like the crust “floats” on the mantle
D. The Earth’s Major Plates