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