Download Plate Tectonics Continental Drift

Plate Tectonics
• Stated that the continents had once
been joined to form a single
supercontinet
• First proposed by Alfred
Weagner
• Supercontinet called
Pangaea
Evidences for Continental Drift
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The Continental puzzle
Matching Fossils
Rock Types and Structures
Ancient Climates
Matching Puzzle
• Coastlines on opposite coasts line up
Matching Fossils
• Fossil evidence for continental drift includes
several fossil organisms found on different
landmasses (ex: Mesosaurus)
Rock Types and Structures
• Mountain belts that end at
one coastline, reappear on
a landmass across the
ocean
Ancient Climates
• Glacial evidence found in areas that are now
warm.
• Glaciers do not move from sea to land only
land to sea so continents had to be in different
place.
Rejecting Continental Drift
• Wegner had no way to explain a mechanism
that moved the continents
• Most people rejected his idea
• A few scientists agreed and continued to look
for evidence to support his idea
Plate Tectonics
• Emerged around 1968
• New technology allowed the ocean floor to be
mapped
• New earthquake data and information on
Earth’s magnetic field were available
• Similar to Wegner’s idea
Earth’s Major Plates
• The uppermost mantle along with the crust
behave as a strong, rigid layer known as the
lithosphere
• The lithosphere is divided into segments called
plates
• There are 7 major plates
• All the plates are moving and changing shape
• They move about 5cm a year
• Move due to convection in the mantle
• Movement causes earthquakes, volcanoes,
mountains
Earth’s Plates
Types of Plate Boundaries
• Divergent
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When two plates move apart
Material up wells from the mantle
Creates new sea floor
Ex : East African Rift Valley
• Convergent
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Where two plates move together
Ocean lithosphere goes under continental crust
Continent runs into continent creating mountains
Ex: Himalaya Mountains
• Transform Fault
– Two plates grind past each other neither destroying nor
creating lithosphere
– Ex: San Andreas Fault
Divergent Boundaries
• Constructive plate margins (lithosphere being made)
• Plates move apart molten rock comes up fills
the space, hardens making new rock
– Seafloor Spreading – produces new ocean floor
due to this the ocean floor is young 180 myo
• Oceanic Ridges are made (underwater mountain ranges)
• Rift Valleys (Ex: East Africa)
– Associated with volcanoes
Divergent Boundary
Convergent Boundaries
• Destructive Plate Boundaries (lithosphere
being destroyed)
• Subduction Zones –occurs when one oceanic
plate is forced down into the mantle beneath
a second
– Produces trenches
Subduction Zone
Convergent: Oceanic-Continental
• More dense ocean sinks beneath the less
dense continent
• Descending plate melts
• New magma rises back up creating volcanoes
• Creates Continental Volcanic Arcs
– Ex: Andes Mountains (caused by subduction of Nazca
plate beneath the South American Plate)
Convergent: Oceanic-Continental
Convergent: Oceanic-Oceanic
• One oceanic plate will
subduct under
another (similar to
oceanic-continental)
• Volcanoes form on
the ocean floor
– Creates chain of
volcanic islands
• Called volcanic island
arc
• Ex: Aleutian Islands,
Alaska
Convergent: Continental-Continental
• Creates non-volcanic mountains
• Ex: Himalaya Mountains (India and Asia)
Transform Fault Boundaries
• Plates grind past each other without creating
or destroying lithosphere
• Common along mid-ocean ridges
• On land example is the San Andreas Fault
• Trigger Earthquakes
All Boundaries
Evidence For Plate Tectonics:
Paleomagnitsm
• Paleomagnetism
• Mineral grains in rock cool parallel to the existing
magnetic field
• Earth’s magnetic field reverses polarity at
different times
– Normal polarity (what we currently have)
– Reverse polarity (opposite than now)
• On the ocean floor rocks show alternating bands
of polarity on both sides or a mid-ocean ridge
Paleomagnitism
Evidence for Plate Tectonics:
Earthquake Patterns
• There is a close link between deep-focus
earthquakes and ocean trenches
– Occur where crust is subducting
Earthquake Map
Evidence for Plate Tectonics: Ocean
Drilling
• Youngest rock is at the ridge crest and oldest
rock is at the continental margin
Evidence For Plate Tectonics: Hot Spots
• Weak spot in the lithosphere where magma is
rising
• Hot spot does not move plate moves over hot
spot
– Creates a chain of volcanic islands with oldest
farthest from hot spot and youngest closest to hot
spot
Hot Spot
Causes of Plate Motion
• Convection occurring in the mantle is the
basic driving force for plate movement
– Slab-pull is thought to be the primary downward
arm of convective flow
– Ridge-push causes oceanic lithosphere to slide
down the sides of the oceanic ridge
• Unequal distribution of heat within Earth
causes the thermal convection in the mantle
that ultimately drive plate motion
Mantle Convection