April 17 Plate Tectonics Allen

Plate Tectonics Earth’s crust is cracked and fragmented Fragments called plates The Earth’s mantle is very hot and fluid, so the plates move atop the mantle

Earth’s Crust Two kinds of crust exist Granitic (felsic) and basaltic (mafic)

Earth’s Crust Think of the Earth as completely covered with a layer of thin, dense basalt rock with chunks of thicker, less dense granite here and there The granite chunks are the continents riding on the basalt like wood floats on water

Plate Boundaries--where tectonic plates meet and interact Where can you find this image?

Plate Boundaries Three types Convergent Divergent Transform

Divergent Boundaries Where plates are moving apart Cracks in the crust are filled by upwelling molten rock Give one example of a divergent boundary:

Divergent Boundaries Divergent boundaries on continents are called rifts Example: the African Rift Valley Splitting Africa in two Divergent boundaries under oceans form mid-ocean ridges

Divergent Boundaries Several plates are pulling apart beneath the Atlantic ocean forming the Mid-Atlantic Ridge Diverging ocean plates cause sea floor spreading Where new sea floor is constantly being added

EAST AFRICAN RIFT VALLEY – CONTINENTAL RIFT

In Afar, Ethiopia, a 40-mile magmatic rift that opened up 1 year ago

April 17 Spreading Center Allen

Mid-ocean ridges (rifts) normally form where tectonic plates are (1) converging (3) stationary (2) diverging (4) sliding past each other The motion of the convection currents in the mantle beneath the Atlantic Ocean appears to be mainly making this ocean basin (1) deeper (3) wider (2) shallower (4) narrower

Convergent Boundaries Where plates are coming together Here for example What is the name of this boundary?

Convergent Boundaries Where mountains form Where earthquakes occur (and sometimes volcanoes) Several plates are colliding with the Pacific Plate creating the Pacific Ring of Fire An area that experiences lots of earthquake and volcanic activity

Pacific Ring of Fire

Convergent Boundary Types Ocean-Ocean Continental-Continental Ocean-Continental

Convergent Boundaries Oceanic crust - Oceanic Crust older, denser crust subducts forming ocean trench Oceanic Crust – Continental Crust denser oceanic crust subducts forming ocean trench Continental Crust – Continental Crust resists subduction - folding, faulting, and mountain building

Ocean-Ocean Convergence Forms ocean trenches Deepest places on Earth Name one ocean-ocean boundary

Oceanic Crust – Oceanic Crust Older, denser oceanic crust is SUBDUCTED VOLCANIC ISLAND ARC FORMED – ALEUTIAN ISLANDS

Ocean-Continental Convergence Site of subduction zones Denser ocean plate slides beneath the less dense continental plate Example: the Pacific Northwestern U.S. Creates many of the Earth’s volcanic mountain chains as the ocean plate dives down, melts and is forced back to the surface Name one ocean-continental boundary

Oceanic-Continental Convergent Boundary - SUBDUCTION April 17 DENSER OCEANIC CRUST GOES DOWN INTO MANTLE Allen

Continental-Continental Convergence How non-volcanic mountains are formed Example: the Appalachian chain and the Himalayas Rising of land due to plate collisions is called uplift Name one continental-continental boundary

Continental Crust – Continental Crust Convergent Boundary NO SUBDUCTION - COLLISION ZONE

Collision of India and Asia April 17 Collision of India and Asia Allen

TRANSFORM PLATE BOUNDARIES Transform Fault Boundaries TRANSFORM PLATE BOUNDARIES At a transform fault boundary, plates grind past each other without creating or destroying the lithosphere. SHALLOW EARTHQUAKES  Transform faults • Most join two segments of a mid-ocean ridge. • At the time of formation, they roughly parallel the direction of plate movement.

Transform Fault Boundary AT MID-OCEAN RIDGES April 17 Transform Fault Boundary AT MID-OCEAN RIDGES Allen

So, what causes plates to move? Convection currents: drag and move the lithospheric plates above the asthenosphere (three sources of heat produce the convection currents): Leftover heat from earth’s formation Decay of Radioactive elements Plate friction

Convection Cells

Convection Currents in the mantle drag and pull the lithospheric plates above them

Mid-Atlantic Ridge As the plates pull apart new ocean floor is added What 2 continents are moving away from each other as a result of the mid-Atlantic Ridge?

Convection Currents It is theorized convection currents within the Mantle cause plates to move around There are other theories about what causes plates to move!

Evidence for Plate Tectonics  Ocean Drilling The data on the ages of seafloor sediment confirmed what the seafloor spreading hypothesis predicted. The youngest oceanic crust is at the ridge crest, and the oldest oceanic crust is at the continental margins.

Which graph best represents the geologic age of the surface bedrock on the ocean bottom?

More Evidence for Plate Tectonics  Earthquake Patterns Scientists found a close link between deep-focus earthquakes and ocean trenches. The absence of deep-focus earthquakes along the oceanic ridge system was shown to be consistent with the new theory.

Convergent plate (subduction) boundaries have deeper earthquakes Divergent plate boundaries and collision zones have shallow earthquakes

April 17 f07_58_pg196 f07_58_pg196.jpg f07_58_pg196 Allen

SUMMARY 3 PLATE BOUNDARIES DIVERGING CONVERGING (SUBDUCTION ZONE) Oceanic Crust – Continental Crust Oceanic Crust - Oceanic Crust CONVERGING (COLLISION ZONE) Continental Crust – Continental Crust TRANSFORM

Boundaries: Diverging

Boundaries: Converging SUBDUCTION

CONVERGING –- COLLISION ZONE No subduction – continental crust less dense - buoyant

Boundaries: Transform

Due to plate tectonics the world is constantly changing