plate tectonics chapter 19. alfred wegener proposed hypothesis in 1915 published the origin of...
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Plate TectonicsChapter 19
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Alfred Wegener• Proposed hypothesis in 1915 • Published The Origin of
Continents and Oceans
Continental drift hypothesis • Supercontinent Pangaea began breaking
apart about 200 million years ago
Continental drift: An idea before its time
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Pangaea approximately 200 million years ago
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The great debate
Objections to drift hypothesis • Inability to provide a mechanism capable
of moving continents across globe • Wegner suggested that continents broke
through the ocean crust, much like ice breakers cut through ice
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Matching of mtn ranges
on continents
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Paleoclimatic evidence for Continental Drift
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The great debate
Continental drift and the scientific method
• Wegner’s hypothesis was correct in principle, but contained incorrect details
• For any scientific viewpoint to gain wide acceptance, supporting evidence required
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Continental drift and paleomagnetism
Renewed interest in continental drift came from rock magnetism
Magnetized minerals in rocks • Show direction to Earth’s magnetic poles • Provide a means of determining their
original latitude
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Continental drift and paleomagnetism
Polar wandering• Apparent movement of magnetic poles in
volcanic rocks indicates continents move • Shows Europe was closer to equator when
coal-producing swamps existed
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Apparent polar-wandering paths for Eurasia and North America
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The scientific revolution begins
During the 1950s and 1960s technological strides permitted extensive mapping of the ocean floor
Seafloor spreading hypothesis was proposed by Harry Hess in the early 1960s
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The scientific revolution begins
Geomagnetic reversals • Earth's magnetic field periodically
reverses polarity – north magnetic pole becomes south magnetic pole, vice versa
• Dates when polarity of Earth’s magnetism changed were determined from lava flows
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Paleomagnetic reversals recorded by basalt at mid-ocean ridges
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Age of Oceanic Crust
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Geomagnetic reversal• Paleomagnetism was the most convincing
evidence to support concepts of continental drift and seafloor spreading
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Plate tectonics: The new paradigm
More encompassing theory than continental drift
Mix of ideas that explained motion of Earth’s lithosphere by subduction and seafloor spreading
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Plate tectonics: The new paradigm
Earth’s major plates • Associated with Earth's strong, rigid
outer layer– Known as the lithosphere
– Consists of uppermost mantle and overlying crust
– Overlies a weaker region in the mantle called the asthenosphere
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Basal tractions drive plate motions
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Earth’s major plates• Seven major lithospheric plates• Plates are in motion and change in shape
and size• Largest plate is the Pacific plate• Several plates include an entire continent
plus a large area of seafloor
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Earth’s major plates• Plates move relative to each other at a
very slow but continuous rate
– Average about 5 centimeters (2 inches) per year
– Cooler, denser slabs of oceanic lithosphere descend into the mantle
– Motion defined by rotation around a pole
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Plate boundaries• Interactions among individual plates
occur along their boundaries • Types of plate boundaries
– Divergent plate boundaries
– Convergent plate boundaries
– Transform fault boundaries
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Types of Plate Margins
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Divergent plate boundaries
Most are located along the crests of oceanic ridges
Oceanic ridges and seafloor spreading • seafloor is elevated forming oceanic ridges
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Oceanic ridges and seafloor spreading• Seafloor spreading occurs along the oceanic ridge
system
Spreading rates and ridge topography • Ridge systems exhibit topographic differences
• Topographic differences are controlled by spreading rates (see map of age of oceanic crust for width of ridges relative to their age)
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Divergent boundaries are located mainly along oceanic ridges
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Spreading rates and ridge topography• Topographic differences are controlled by
spreading rates– Slow rates (1-5 cm/year), rift valley develops
on ridge crest (30 to 50 km wide, 1500-3000 m deep)
– Intermediate spreading rates (5-9 cm/year), rift valleys are shallow with subdued topography
– At rates > 9 cm/year no rift valley develops or are narrow and extensively faulted
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Divergent boundaries in Continents
Continental rifts• Splits landmasses into two or more
smaller segments
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Divergent boundaries
Continental rifts• Example includes East African rifts• Produced by extensional forces acting on
the lithospheric plates • Not all rift valleys develop into spreading
centers• Otherwise Nevada would be an ocean!
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The East African Rift
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Development of Continental Rift into Ocean Basin
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Convergent plate boundaries
Old portions of oceanic plates are returned to the mantle
• Surface expression of descending plate is an ocean trench
• Called subduction zones • Average angle at which oceanic
lithosphere descends into the mantle is about 45
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All have same basic characteristics, but can have highly variable features Types of convergent boundaries
• Oceanic-continental convergence – Denser oceanic slab sinks into the asthenosphere – Bathymetry marked by trench– As plate descends, partial melting of mantle rock
makes basaltic or andesitic magmas– Volcanic mountains associated with subduction
of oceanic lithosphere are called continental volcanic arcs (Andes and Cascades)
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Types of Arcs
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Types of convergent boundaries• Oceanic-oceanic convergence
– When two oceanic slabs converge, one descends beneath the other
– Often forms volcanoes on the ocean floor
– If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands)
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QuickTime™ and a QuickDraw decompressor are needed to see this picture.
Swim through the Marianas Trench
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Types of Arcs
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Types of convergent boundaries• Continental-continental convergence
– Continued subduction brings continents together
– Less dense, buoyant continental lithosphere does not subduct
– Result is a collision between two continental blocks
– Process produces mountains (Himalayas, Alps, Appalachians)
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The collision of India and Asia produced the Himalayas
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Transform fault boundaries
Third type of plate boundary Plates slide past one another and no new lithosphere is created or destroyed Transform faults
• Most join two segments of a mid-ocean ridge as parts of linear breaks in the oceanic crust known as fracture zones
• Accommodate simultaneous movement of offset ridges
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Transform faults accommodate movement on offset ridge segments
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Testing the plate tectonics model
Plate tectonics and earthquakes • Plate tectonics model accounts for the
global distribution of earthquakes – Absence of deep-focus earthquakes along the
oceanic ridge is consistent with tectonic theory
– Deep-focus earthquakes associated with subduction zones
– The pattern of earthquakes along a trench provides method to track plate's descent
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Deep-focus earthquakes occur along convergent boundaries
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Earthquakes near Japan trench
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Evidence from ocean drilling• Most convincing evidence confirming
seafloor spreading comes from drilling directly into ocean-floor sediment
– Age of deepest sediments – Thickness of ocean-floor sediments verifies
seafloor spreading
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Hot spots • Caused by rising plumes of mantle
material • Volcanoes form over them (Hawaiian
Island chain)• Mantle plumes are long-lived structures
and originate at great depth, perhaps at core-mantle boundary
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The Hawaiian Islands form over stationary hot spot
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No one driving mechanism accounts for all major facets of plate tectonics Researchers agree that convective flow in 2,900 km-thick mantle is main driving force of plate tectonics (by basal traction) Other mechanisms generate forces that contribute to plate motion
• Slab-pull• Ridge-push
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Importance of plate tectonics
Provides a unified explanation of Earth’s major surface processes, especially oceans
Within framework of plate tectonics, we find explanations for the distribution of earthquakes, volcanoes, and mountains
Plate tectonics provides explanations for distribution/evolution of plants and animals and climate record