Transcript Document
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