Plate Tectonics
Tools of the Trade
• How do we learn about
the nature of the ocean floor and structure of the oceanic crust? • a. Ocean Floor Mapping using sound: multibeam (SEA BEAM), side-scan (SEA MARK) • B. Seismic studies: reflection and refraction of low frequency sound (earthquake) waves
• • • •
c. Magnetic Studies d. Dredging/Camera Tows e. Deep Submersibles f. Satellite geodesy: measuring gravity and sea-surface height • g. Deep Sea Drilling
Evidence for Plate Tectonics
• • • • •
A. Evidence from the continents 1. Shape of the continental boundaries 2. Paleomagnetic pole positions 3. Matching geologic features 4. Large displacement on transcurrent faults
• 5. Most seismicity and volcanism
restricted to narrow zones • 6. Seismicity of Benioff zones (deep earthquakes only in such zones; near trenches)
B. Evidence from the Oceans
• 1. Improved fit of the continents with • • • • •
continental margins rather coast lines 2. Mid-ocean ridges 3. Trenches and Island arcs 4. Magnetic anomaly patterns 5. Seismicity of transform faults 6. Island chains (hot spot traces) aligned parallel
• 7. Age of ocean
floor and thickness of sediment increases away from mid-ocean ridges (MOR's) • 8. Heat flow & Depth of the oceans: ocean floor gets deeper and colder away from MOR's
• Divergent Plate Boundaries: Spreading
Centers & Creation of Oceanic Crust • New oceanic crust is created at spreading centers because the decompression of rising mantle causes the mantle to melt.
II. Structure of the Earth
• The Earth can be divided into the
following layers: • Core (Fe-Ni alloy): 31 % of mass of Earth; inner: solid; outer: liquid • Mantle (Mg-Fe silicates): 2/3 the mass of the Earth (68%) • Crust: 0.4% of mass of Earth
• Continental Crust: average 30 km thick: ‘
granitic’, low density, permanent • Oceanic Crust: typically 6 km thick; ‘basaltic’, temporary (maximum life: <200 My) • The outer part of the Earth can be divided into: • Lithosphere: consists of the crust and uppermost mantle (0-200 km, thickness varies)
• The lithosphere deforms brittlely (it
breaks rather than flows) • Asthenosphere: the mantle immediately below the lithosphere • The asthenosphere deforms plastically (it flows rather than breaks).
Isostatic Equilibrium
• Buoyancy – ability of an object to float in
water (p.56, f.3.4) • Isostatic Equilibrium – support provided to any region of a continent that projects above sea level.
Transform Boundaries and Convergent Boundaries
• Transform Boundaries: Plates sliding
by one and other result in zones of high earthquake frequency. • Convergent Boundaries: One plate slides beneath the other in a process called subduction. • Oceanic lithosphere cools and contracts as it rides away from mid-ocean ridges.
• Eventually, its density may exceed that
of underlying asthenosphere. • As a result, it will sink, or subduct back into the mantle.
Processes at Destructive Plate Margins
• Downward motion of
lithosphere creates trench • Compaction of sediments expels water • Sediments, and rarely crust, may be scraped off and accreted • Release of water from subducting oceanic crust causes mantle above it to melt and volcanism.
Oceanic Island Volcanoes, Hotspots, and Mantle Plumes
• Volcanic island chains such as Hawaii
form as the lithosphere passes over a hotspot. • These hotspots are fixed relative to one another. • Hot spots are thought to be caused by mantle plumes.
• A mantle plume is a narrow (~100 km)
column of mantle rock that rises (at about 10 cm/year or so) from the deep mantle, probably the core-mantle boundary. • When it reaches the upper 100 or 200 km of the mantle, it partially melts (1020% molten).
• After the lithosphere
has passed over the plume, the volcanoes cool and erode, eventually sinking beneath the sea, forming chains of seamounts.
Growth of Ocean Basins
• Plate tectonics has been continually
creates and destroys ocean basins. • Over the last 200 million years, one ocean, the Tethys, has been destroyed, and another, the Atlantic, has been created.
• New oceans are
created as continents rift apart. • The African Rift valley is an example of a rift in a very early stage. • Extensive volcanism is associated with rifting.
• The Red Sea is an
example of a rift in a slightly more advanced stage.