Chapter 14.3: The Theory of Plate Tectonics

The Theory of Plate Tectonics

Chapter 14 Lesson 3p510-519

Vocabulary• Plate tectonics (511) – Earth’s surface is made of rigid slabs of rock, or

plates, that move with respect to each other• Lithosphere (512) – the cold and rigid outermost rock layer• Divergent plate boundary (513) – forms where two plates separate• Transform plate boundary (513) – Forms where two plates slide past each

other• Convergent Plate Boundary (513) – form where two plates collide• Subduction (513) – A process in which the denser plate sinks below the

more buoyant plate • Convection (516) – the circulation of material caused by differences in

temperature• Ridge Push (517) – the forces that causes rising mantle material at mid-

ocean ridges that creates the potential for plates to move away from the ridges

• Slab Pull (517) – As a slab sinks, it pulls on the rest of the plate with this force

The Plate Tectonic Theory• Earth’s crust is constantly being created and

destroyed• The theory of plate tectonics states that Earth’s

surface is made of rigid slabs of rock, or plates, that move with respect to each other

• Each plate moves over Earth’s hot and semi-plastic mantle– The term tectonic describes the forces that shape

Earth’s surface and the resulting rock structures– Plate tectonics is used to explain earthquakes and

volcanic eruptions

Tectonic Plates

Tectonic Plates• The Pacific Plate is the largest plate• The Juan de Fuca is one of the smallest• The boundaries that run through oceans mark the positions

of the mid-ocean ridges• Earth’s outermost layers are cold and rigid compared to the

layers in Earth’s interior. It is called the lithosphere.– It is made up of the crust and the solid, uppermost mantle – It is thin below ocean ridges and thick below continents– The tectonic plates are just large pieces of the lithosphere

• Just below the lithosphere is called the asthenosphere. – This layer is so hot that it behaves like a plastic material– This enables to lithosphere to move

Plate BoundariesConvergent

Convergent

Divergent Plate Boundaries• Mid-ocean ridges are located

along divergent plate boundaries.• A divergent plate boundary forms

where two plates separate• When the seafloor spreads at a

mid-ocean ridge, lava erupts, cools, and forms new oceanic crust.

• Divergent plate boundaries can also exist in the middle of a continent– They pull continents apart and

form rift valleys– The East African Rift is an

example.

Transform Plate Boundaries• A transform plate boundary

forms where two plates slide past each other.– The San Andrews Fault in

California is a well known example.

– As the plates move past each other, they can get stuck and stop moving,

– Stress builds up where the plates are stuck.

– Eventually the stress is too great and the rocks break

– The resulting rapid energy release is an earthquake.

San Andreas Fault

Convergent Plant Boundaries

• Convergent plate boundaries form where two plates collide

• The denser plate sinks below the more buoyant plate in a process called subduction– The area where a denser

plate descends into Earth is called a subduction zone.

• When an oceanic plate and continental plate collide the denser oceanic plate subducts under the edge of the continent

• This creates a deep ocean trench

• A line of volcanoes forms above the subducting plate on the edge of the continent

• This process can also happen with two oceanic plates.– Typically the older, denser

plate will subduct beneath the younger plate.

– This creates a deep ocean trench and a line of volcanoes called an island arc

• When two continental plates collide, neither plate is subducted.

• Instead rock is uplifted and create mountains like the Himalayas

Evidence for Plate Tectonics

• Continents move apart or come together at speeds of a few centimeters per years

• Today scientists use a network of satellites called the Global Positioning System (GPS) to measure the movement of the plates

• The theory of plate tectonics explains why earthquakes and volcanoes occur in certain places

• All types of plate boundaries can result in earthquakes due to the rapid release of energy

• Diverging and converging plates result in volcanoes

• Mountains form when two continental plates converge

Plate Motion

• Convection Currents– Convection is the circulation of material caused by

differences in temperature and density• For example, the upstairs floors of most houses often

warmer than the lower floor• This is because warm air rises while denser, cold air

sinks.

• Tectonic plate activity is related to convection in the mantle.• Radio active elements heat Earth’s interior.• When materials such as solid rock are heated, they expand and

become less dense• Hot mantle material rises upward and comes in contact with Earth’s

crust• Thermal energy is transferred to the surface • As the mantle cools, it becomes denser and then sinks, forming a

convection current• These currents in the asthenosphere act like a conveyor belt moving

the lithosphere

FORCES CAUSING PLATE MOTIONScientists are still uncertain about which force has the greatest influence

Forces Causing Plate Motion• Basal Drag– This is when convection currents in the

asthenosphere move or drag the lithosphere much like how walking sidewalks at the airport move people.

Ridge Push• Recall that mid-ocean ridges have greater elevation than the

surrounding seafloor.• Because they are higher, gravity pulls the surrounding rock down

and away from the ridge• Rising mantle material at mid-ocean ridges creates the potential for

plates to move away from the ridge with a force called ridge push.– This moves the lithosphere away from the mid-ocean ridge.

Slab Pull• When plates converge, the denser plate will sink into

the mantel along a subduction zone• This plate is called a slab. It is usually old and cold

which makes it denser.• As a slab sinks, it pulls on the rest of the plate with a

force called slab pull.

A Theory in Progress

• Plate tectonics has become the unifying theory of geology

• It explains the connection between continental drift and the formation and destruction of crust along plate boundaries.

• It also helps to explain the occurrence of earthquakes, volcanoes, and mountains

Unanswered Questions

• Several questions remain unanswered:– Why is Earth the only planet in the solar system

that has plate tectonic activity?– Why do some earthquakes and volcanoes occur

far away from plate boundaries?• Part of the answer has to do with plate thickness. The

other part is the scientists are still learning how active the mantle really is.

– What forces dominate plate motion?