Theory of Plate Tectonics Plate Tectonics Plate Boundaries Causes of Plate Tectonics

Theory of Plate Tectonics Plate tectonics is a relatively new theory that has revolutionized the way geologists think about the Earth. Plate: Large slab of solid rock Tectonics: from the Greek root “to build” Theory of Plate Tectonics – is a combination of two earlier ideas: continental drift and sea-floor spreading.

Plate Tectonics Earth's outermost layer, the lithosphere, is broken into 7 large, rigid pieces called plates. Several minor plates also exist. The plates are all moving in different directions and at different speeds, from 2 cm to 10 cm per year in relationship to each other.

PLATE TECTONICS the surface of the Earth is made of rigid plates  Size and position of plates can change over time  PLATE BOUNDARIES Edges of plates, where two plates “float” side-by-side, is where geologic activity takes place  Plates movement is from convection currents deep in the mantle  Size and position of plates can change over time  PLATE BOUNDARIES Edges of plates, where two plates “float” side-by-side, is where geologic activity takes place  Plates movement is from convection currents deep in the mantle

Plate Movement Convection currents beneath the plates move the crustal plates in different directions. The source of heat driving the convection currents is from radioactivity deep in the Earth’s mantle.

Plate Movement The lithospheric plates are thought to be moved around by circulating motions. This process is similar to what you see in a lava lamp. –The material in the lamp is heated by the bulb. The material then rises and is replaced by the cooler material that sinks to the bottom. –This causes circulating movement

Plate Movement “Plates” of lithosphere are moved around by the underlying hot mantle convection cells As magma is heated, it tends to rise, then cools and sinks again. This repeated heating and cooling results in a current which may be enough to cause continents to move.

What happens at tectonic plate boundaries?

Three Basic Types of Plate Boundaries Divergent Convergent Transform USGS Graphics

PLATE BOUNDARIES Divergent boundaries  plates move apart Convergent boundaries  plates move together Transform boundaries plates slide past 

DIVERGENT BOUNDARIES Make new crust When two plates move apart and magma surfaces forming NEW crust (usually oceanic) continental rift zones (landmass splits into two or more segments) mid-ocean ridges (sea-floor is elevated)

DIVERGENT BOUNDARIES Make new crust Rift valley continent-continent

DIVERGENT BOUNDARIES Make new crust: Sea Floor Spreading Youngest rocks form at ridge Older rocks are further from ridge Oldest rocks are located at subduction zones

DIVERGENT BOUNDARIES Make new crust: Sea Floor Spreading As magma rises and cools, iron and magnesium minerals align themselves to the Earth’s magnetic field “Magnetic Stripes” are formed The Earth’s magnetic field reverses polarity Magnetic stripes are mirrored on each side of the mid-ocean ridge Every ~20,000 years the polarity of the Earth changes. This is change is reflected in the rocks, whose polarity is fixed once the magma is cooled.

CONVERGENT BOUNDARIES plates colliding  Destroys old crust and forms new mountains  Three types of convergent boundaries  Destroys old crust and forms new mountains  Three types of convergent boundaries

CONVERGENT BOUNDARIES plates colliding Continent-continent convergence: Folded mountains Continent-continent convergence: Folded mountains Ex Himalayas Eurasian/Indian plates

CONVERGENT BOUNDARIES plates colliding Ocean-continent convergence: Trench & Coastal Volcanoes Ocean-continent convergence: Trench & Coastal Volcanoes Andes Mountains Nazca & S.American plates

CONVERGENT BOUNDARIES plates colliding Ocean-ocean convergence: Trench & Island arc Ex: The Caribbean Islands Ocean-ocean convergence: Trench & Island arc Ex: The Caribbean Islands

TRANSFORM BOUNDARIES When two plates slide past each other  no tearing or crushing - no damage occurs to the lithosphere  Faults –San Andreas Fault  Cause most earthquake damage  no tearing or crushing - no damage occurs to the lithosphere  Faults –San Andreas Fault  Cause most earthquake damage