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Plate Tectonics
Divisions of the Earth’s Interior
Divisions of Earth’s Interior
1. CORE (about 3500 km thick)
• Metallic -- mostly iron, some nickel, sulfur
(Inner Core – Solid, Outer Core – Liquid)
2. MANTLE (about 2900 km thick)
• Rocky -- iron, magnesium, silicate (silicon &
oxygen) minerals
• Top layer of is called the Lithosphere - rigid
• Just below is Asthenosphere -- zone of weak
rock capable of plastic flow
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3. CRUST (only 10-65 km thick)
Rocky - varied minerals
Oceanic crust -- thin and dense; high in Fe & Mg
Continental crust -- thicker, less dense; contains
less Fe, Mg and more Al, Ca, Na, K
Alfred Wegener - Continental Drift
Hypothesis
Alfred Wegener, a German climatologist,
developed the Continental Drift
hypothesis in 1915
Some of Wegener’s Evidence at the Time:
Evidence for continental
drift
Problems with Continental Drift
Hypothesis
• Continents drift -- but what about the ocean
floor?
• What force could move continents?
• Studies of the ocean floor in the 2 decades
following WWII led to the development of the
plate tectonic theory
The Theory of Plate Tectonics
• The Earth’s is constantly changing
• The Earth’s crust is divided into 8 large plates
(and several small plates)
• Almost all major earthquake or volcano activity
occurs along the plate boundaries
• Because each plate moves as a unit, the interiors
of the plates are generally stable.
• Really not a theory due to overwhelming
evidence!!!!
Tectonic plates are made of
lithosphere + crust
How plates move - Convection Currents
Mantle convection
• Convection in the mantle brings hot material upward in
some places. Elsewhere, cooler rock sinks.
• Upwelling hot material can cause lithosphere to rift (split)
and plates drift apart.
TYPES OF PLATE BOUNDARIES
• Divergent boundaries -- plates
move away from each other
• Convergent boundaries -- plates
move toward each other
• Transform boundaries -- plates try
to slide past each other
DIVERGENT BOUNDARIES
• Plates rift (move apart) along a system
of fractures
• Blocks of rock are down-dropped along
fractures (faults) -- rift valleys
• Magma rises from asthenosphere along
rifts -- volcanic activity
DIVERGENT BOUNDARIES
• Movement along faults results in
earthquake activity
• Example: mid-ocean ridges volcanic
activity produces new seafloor as plates
drift apart -- seafloor spreading
• Examples: E. African Rift, mid-Atlantic
ridge
Diverging Plates
How
ocean basins
formed
CONVERGENT BOUNDARIES
• Oceanic crust + continental crust
gives subduction
• Dense oceanic crust is pushed under
(subducted) less dense continental
crust
Converging Plates - Subduction
Subduction zones
• High earthquake activity
• High volcanic activity -- partial melting
of asthenosphere above subducted slab
forms magma
• Produces volcanic mountains or volcanic
island arc
• Ocean trenches form where oceanic plate
drops below continental plate
Subduction zones -- examples
• Cascade Mountains (Pacific Northwest)
and Andes Mountain (western South
America) are continental volcanic
mountains over subduction zones
• Japan, Phillipines, Indonesia, Aleutian
Islands (Alaska) -- volcanic island arcs
CONTINENT-CONTINENT
CONVERGENCE
• Continental crust is not dense enough
to be subducted.
• Major deformation of crust, forms
high mountain belt
• Earthquake activity, metamorphism
• Example: Himalayan Mountains
Converging Continental Plates
TRANSFORM BOUNDARIES
• Major fault zone develops along
boundary; high earthquake activity
• High pressure metamorphism
along boundary
• Examples: San Andreas fault zone,
southern CA; New Zealand
Transform Boundaries
San Andreas Fault
Streams offset by San
Andreas Fault
GATHERING EVIDENCE
• Field work - geologists sampling rocks,
drilling, mapping formations
• Remote Sensing - observing from a
distance (satellite photos, sonar mapping of
ocean floors)
• Seismology - study of earthquakes and
seismic waves
• Volcanology - study of volcanoes