PLATE TECTONICS HOW THE EARTH MOVES

STRUCTURE OF THE EARTH The Earth is made up of 3 main layers:  Core (inner and outer)  Mantle  Crust Inner core Outer core Mantle Crust

THE CRUST This is where we live! The Earth’s crust is made of: Continental Crust - thick (10-70km) - buoyant (less dense than oceanic crust) - mostly old Oceanic Crust - thin (~7 km) - dense (sinks under continental crust) - young

WHAT ARE THE TECTONIC PLATES? AKA: Lithospheric plate The ~100-km-thick surface of the Earth; Contains crust and part of the upper mantle; It is rigid and brittle; Fractures to produce earthquakes.

WHAT IS THE ASTHENOSPHERE? Asthenosphere: Is the hotter upper mantle below the lithospheric plate; Can flow like silly putty; and Is a viscoelastic solid, NOT liquid!! USGS Graphics

WHAT IS PLATE TECTONICS?

Alfred Wegener in the early 1900’s proposed the hypothesis that continents were once joined together in a single large land mass he called Pangea (meaning “all land” in Greek). He proposed that Pangea had split apart and the continents had moved gradually to their present positions - a process that became known as continental drift. CONTINENTAL DRIFT

Continents fit together like a puzzle….e.g. the Atlantic coastlines of Africa and South America. The Best fit includes the continental shelves (the continental edges under water.) WEGENER’S EVIDENCE FOR CONTINENTAL DRIFT Picture from

WEGENER’S EVIDENCE FOR CONTINENTAL DRIFT Fossils of plants and animals of the same species found on different continents. Picture from /vwdocs/vwlessons/pl ate_tectonics/part3.h tml /vwdocs/vwlessons/pl ate_tectonics/part3.h tml

WEGENER’S EVIDENCE FOR CONTINENTAL DRIFT Rock sequences (meaning he looked at the order of rock layers) in South America, Africa, India, Antarctica, and Australia show remarkable similarities. Wegener showed that the same three layers occur at each of these places. Picture from art4.html art4.html

SEAFLOOR SPREADING In the 1960’s, a scientist named Henry Hess made a discovery that would vindicate Wegner. Using new technology, radar, he discovered that the seafloor has both trenches and mid-ocean ridges. Henry Hess proposed the sea-floor spreading theory. Picture from USGS

SEAFLOOR SPREADING As the seafloor spreads apart at a mid-ocean ridge, new seafloor is created. The older seafloor moves away from the ridge in opposite directions. This helped explain how the crust could move— something that the continental drift hypothesis could not do. Picture from

PLATE TECTONIC THEORY Both Hess’s discovery and Wegner’s continental drift theory combined into what scientists now call the Plate Tectonic Theory. Theory of plate tectonics: The Earth’s crust and part of the upper mantle are broken into sections, called plates which move on a plastic-like layer of the mantle

WORLD PLATES

PLATE MOVEMENT “Plates” of lithosphere are moved around by the underlying hot mantle convection cells

CONVECTION CURRENTS/CELLS Plates move by the transfer of heat through heated material. Hot magma in the Earth moves toward the surface, cools, then sinks again. Creates convection currents beneath the plates that cause the plates to move.

CONVECTION CURRENTS

HOT SPOTS Stationary plumes of hot material that initiate at the core/mantle interface Hawaii: the plume is beneath oceanic crust

HOT SPOTS Yellowstone is associated with a hot spot under continental crust

HOT SPOTS

WHAT HAPPENS AT TECTONIC PLATE BOUNDARIES?

Divergent Convergent Transform Three types of plate boundary

Spreading ridges  As plates move apart new material is erupted to fill the gap Divergent Boundaries

Iceland has a divergent plate boundary running through its middle Iceland: An example of continental rifting

There are three styles of convergent plate boundaries  Continent-continent collision  Continent-oceanic crust collision  Ocean-ocean collision Convergent Boundaries

Forms mountains, e.g. European Alps, Himalayas Continent-Continent Collision

Himalayas

Called SUBDUCTION Continent-Oceanic Crust Collision

Oceanic lithosphere subducts underneath the continental lithosphere Oceanic lithosphere heats and dehydrates as it subsides The melt rises forming volcanism E.g. The Andes Subduction

When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. The worlds deepest parts of the ocean are found along trenches.  E.g. The Mariana Trench is 11 km deep! Ocean-Ocean Plate Collision

Where plates slide past each other E.g. Faults and earthquakes Transform Boundaries Above: View of the San Andreas transform fault

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