Theory of Plate Tectonics

Terri Henry

2nd lesson

Evidence of Continental Drift

• Continents fir together like puzzle pieces.

• Same fossils of animals and plants found on different continents.

• Similar rock layers on different continents.

• Striation left from glacier movement.

• Tropical plant fossils in Antarctica.• Satellite technology collects data

about plate movement.

Theory of Plate Tectonics• All of the evidence supports scientist belief that

the continents move. • Why?• Through a collection of data scientist have pieced

together the Theory of Plate Tectonics.• Scientist believe that the earth’s surface is broken

in to many different sized plates that move.

What is a Tectonic Plate?

• A massive, irregularly shaped slab of solid rock, usually composed of both continental and oceanic crust.

Why do Plates move?• Must investigate the interior of the earth.• 4.6 billion years ago, scientist believe that a huge

explosion of gas and matter occurred in space.• Over time the gas and matter cooled to formed planets.• A layer of solid matter or crust formed around the outside

of a a ball. • The lighter or less dense matter moved toward the

center. • Four main layers formed as the Earth cooled and its

matter separated.

Crust

• The outermost layer.• Includes soil, sand and rocks, deep rocks.• Very thin and brittle. • Contains elements Oxygen and Silicon.• Less dense than the mantle, this is why it floats.• Contains continental crust 20km – 70km and

oceanic crust 5km – 10km.• Also known as the lithosphere.

Mantle /Asthenosphere

• Mantle is a solid rocky layer below the crust.• It is made up of magnesium, silicon, and oxygen.• The mantle makes up 80% of the Earth’s volume.• The mantle makes up 68% of the Earth’s mass.• It begins 200km below the surface and extends down to

3,000km.• There is a big temperature difference that causes the

convection currents.• The convection currents are what move the crust.• Asthenosphere is the upper mantle to about 700km deep.• The Asthenosphere is a hot semi-solid material (plasticity).• The Asthenosphere glides over the lower mantle. This is what

causes the crust to move.

Outer Core• Made of iron and nickel.

• The temperature (3,800°C) is so great that these metals form a moving liquid layer. This liquid layer causes the magnetic field of earth.

• The outer core is 2,200km thick.

Inner Core

• A solid Sphere.

• Made of iron and nickel.

• Temperature is 5,000°C and pressure is so great (from the weight of all the other layers) the iron is squeezed into a solid form.

• About 1,300km thick. 

Why does the crust move?

• Inside the Earth, the source of heat is extreme.• Heat form the interior rises toward the surface of

the Earth. • This causes the liquid rock in the mantle to move

with it.• Hot or less dense liquid moves upwards.• Cool heavy rock sinks downwards.• The heated magma rises to the surface, spreads

and begins to cool, then sinks to the bottom of the mantle where it is reheated and rises again.

Convection currents continued

• The plates ride on the convection currents.• Plate motion results in volcanoes,

earthquakes, and mountain building.• Convection currents caused Pangaea to

break up.• Convection currents have pushed the

continents to where they are today.• Convection currents also causes the crust

to be destroyed or recycled

Why Does the Plate Float?

• Continental crust is composed of rocks that are made of relatively lightweight minerals such as quartz and feldspar.

• All of which are found in granite.

• Oceanic crust is composed of basaltic rock which is heavier.

Quartz Feldspar

Basaltic

Types of Plate Movement

• There are three main types of plate movement.

Divergent boundaries

Convergent boundaries

Transform boundaries

Divergent Boundaries• Occur along spreading centers

where plates are moving apart.

• New crust is created by magma pushing up from the mantle.

• Can be any combination of plates (oceanic and continental)

• Best known divergent boundary is the Mid-Atlantic Ridge or sea floor spreading.

Divergent Boundaries• Causes underwater volcanoes and islands.• Creates rift valleys.• The stress at this boundary is tension. A stress

that pulls on the crust, stretching rock so that it becomes thinner.

• Type of fault associated with divergent boundaries is a Normal fault.

Iceland

• Iceland lies across the Mid-Atlantic Ridge.

• It becomes wider every year as the North American and Eurasian Plates drift apart.

• Parts of Iceland are spreading 9cm per year.

Sea Floor Spreading

Transform Boundaries• The zone between two plates

sliding horizontally past one another.

• Do not easily slide past each other. There is a lot of pressure and stress that builds up between the plates as they move.

• The place where plates move and cause a break in the crust is called a fault. Transform boundaries create earthquakes when they slide past each other.

• Most transform boundaries exist on the ocean floor.

Transform Boundary• Type of stress: shearing a stress

that pushes a mass of rock in two opposite directions.

• Type of fault: Strike –Slip• Motion at the boundary: creates

faults• Crust action: there is no new

crust made at this boundary.• Land features: Faults are

created

San Andreas Fault

The San Andreas Fault slices through 2/3 or the length of California. The Pacific Plate is moving horizontally past the North American Plate at an average rate of 5 cm/yr.

Convergent Boundaries• Recycling of crust occurs along

these boundaries.• Plates are moving toward each

other or colliding.• Usually occurs where one

plate is sinking under another plate (subduction zone).

• Convergence may occur between an oceanic and continental plate, or between two oceanic plates, or between two continental plates.

Convergent BoundaryContinental to Continental

• Occurs when two continental plates collide with each other and push upwards.

• Tall folding Mountains form The land has no where to go but up.

• This is pushing up is known as uplifting.

• This type of convergent boundary does not cause subduction.

Convergent Continental to Continental

• Type of stress: Compression: a stress that squeezes rock until it folds or breaks.

• Type of Fault: Reverse• Motion at this boundary:

Folding and bending• Crust Action: no new crust

is created and crust is not recycled, it is reformed.

• Features: Mountain building

Creation of the Himalayan Mountain Range

The Himalayas continue to rise more than 1 cm a year—a growth rate of 10km in a million years.

Convergent BoundaryOceanic to Oceanic

• When two ocean plates collide, one plate moves under the other or subducts.

• Usually the plate with older rock moves under the younger rock because it is more dense.

• The old crust is forced into the Earth and melts to become liquid rock.

ConvergentOceanic to Oceanic• Type of stress: compression: stress that

squeezes rock until it folds or breaks.

• Motion at the boundary: subduction

• Type of fault: Reverse

• Crust is recycled into the earth.

• Land feature: trench is formed.

Marianas Trench

• Located between the Phillippine and Pacific plates.

• Deepest trench on Earth.• Reaches almost 11,000m

below sea level. • It is over 2,000m deeper

than the tallest mountain on Earth.

Convergent BoundaryContinental to Oceanic

• When oceanic and continental crust collide, the oceanic crust pushes under the continental crust.

• Oceanic crust slides under the continental crust because it is more dense.

• Eat and pressure below the earth’s surface get very intense. Earthquake and volcanoes occur near the subduction zones to release the pressure.

continental

ocean

ConvergentContinental to Oceanic

• Type of stress: compression: a stress that squeezes rock until it folds or breaks.

• Motion at the boundary: subduction

• Type of fault: Reverse• Crust Action: recycling of old

crust to form new crust• Land Form: Mountain ranges

and volcanoes

Cascade Mountain Range

• Occurs as the pacific plate moves under the Juan de Fuca plate.

• The heat and pressure cause the oceanic crust to melt.

• The crust is broken down when it forms liquid rock or magma.

• The magma rises to the surface of the earth and the result is a volcano.

Convergent Boundaries

India and Asia to create the Himalayas

Oceanic Nazca Plate is subducting under the South American Plate to create the Andes mountains and Peru Chili Trench

Two oceanic plates colliding, one must subduct under the other creating a trench (Marianas Trench)