Plate Tectonics

Sea-Floor Spreading

• Process by which new oceanic lithosphere is created as older materials are pulled away

• Creates Mid-Ocean Ridges– Underwater Mountain Chains

Theory of Plate Tectonics• Theory that both continental and oceanic

crust move• Theory states:– Earth’s lithosphere is divided into tectonic plates

that move around on top of the asthenosphere

Causes of Motion• Heat and Gravity• Convection– Hot material rises and cooler

material sinks. Drags tectonic plates sideways

• Slab Pull– Oceanic lithosphere is denser

than the asthenosphere.– Oceanic plate sinks and pulls the rest of the plate with it

Tectonic Plate Boundaries• Convergent – Two plates push into another – Result depends on what type of crust• Continental / continental– Push crust upward

• Continental / Oceanic–Oceanic plate slides under

• Oceanic / Oceanic–One of the plates slides under

Tectonic Plate Boundaries• Divergent – Two plates move away from each other– Examples:• Sea-floor spreading• Mid-ocean ridges are the most common• Can be found on continents

Sea-Floor Spreading Process• Plates move away from each other• Creates a gap in the sea floor• Magma rises to fill gap• Magma cools and forms underwater

mountains

• Newer Crust = At the Mid-ocean crust• Older Crust = Edges of continents

Tectonic Plate Boundaries• Transform– Two plates slide past each other

horizontally– Plates are smooth so they grind and jerk• Cause Earthquakes

– Example• San Andreas Fault in California

Tracking Plate Motion

• Slow and Gradual• Measured in centimeters per year• Measure using GPS– Global Positioning System