12.2 Features of Plate Tectonics

The Crust

The Mantle

Outer Core

Inner Core

VOCABULARY

Word to Know

epicentre

mantle convection

asthenosphere

lithosphere

plate boundary

ridge push and slab pull

subduction zone

volcanic belt

rift valley

What lies below Earth’s surface?

• Earth is made of four layers with distinct characteristics

What lies below Earth’s surface?

• Earth is made of four layers with distinct characteristics

CRUST

• Is Earth’s outermost layer made from solid, brittle rock and is 5–70

km thick.

CRUST

• CONTINENTAL CRUST – made from a lighter type of rock

called granite (can be 70 km thick)

CRUST

• OCEANIC CRUST – made from a dense, dark rock called basalt

(can be 10 km thick)

MANTLE

• Earth’s thickest layer (≈ 3000 km)

MANTLE

• Earth’s thickest layer (≈ 3000 km)

• Mostly solid, but a portion of the upper part is molten

MANTLE

• The upper mantle (660 km thick) is composed of partly molten rock

containing iron, magnesium, and radioactive elements

MANTLE

• Transition zone separates it from the lower mantle

MANTLE

• The lower mantle is made of solid, dense material that contains the

elements magnesium and iron

MANTLE

• The lower mantle is made of solid, dense material that contains the

elements magnesium and iron

OUTER CORE

• The layer below the mantle the outer core is liquid

• It is about 2300 km thick and is composed mainly of a mixture of

iron and nickel.

INNER CORE

• with a radius of 1200 km lies at Earth’s centre

• Composed of solid iron and nickel

• large, rigid, but mobile plates of rock

• Made up of the crust and upper portion of the upper mantle, they

form the lithosphere

• Lithosphere floats on the asthenosphere.

• The asthenosphere is a partly molten layer in the upper mantle.

• The asthenosphere is a partly molten layer in the upper mantle.

• Asthenosphere is kept molten by radioactive decay of radioactive

elements

• This molten mantle is moved by convection currents = mantle

convection

• The mantle convection is one of the driving forces behind plate

movement.

More about SPREADING RIDGES

• Magma (melted rock) rises to the Earth’s surface

• It cools down and hardens when it breaks through Earth’s surface

forming new sea floor = SPREADING RIDGE

More about SPREADING RIDGES

SPREADING RIDGE

(or OCEANIC RIDGE)

RIFT VALLEY

RIDGE PUSH

• The magma cools when it reaches the surface, solidifies, and is

pushed aside as new magma pushes from below

• As new material at a ridge or rift pushes older material aside, the

tectonic plates move away from the ridge

• This is called ridge push

SUBDUCTION

• when tectonic plates are pushed apart, eventually one or both will

bump into another plate

• If a dense oceanic plate collides with a continental plate, the heavy

oceanic plate will dive deep under the lighter continental plate

SUBDUCTION

• Subduction is the action of one plate pushing below another

• As the plate pushing below, the rest of the plate follows = slab pull

SUBDUCTION ZONES

• Theses areas of subduction, called subduction zones, typically

experience large earthquakes and volcanic eruptions

A PLATE BOUNDARY

• A region where two tectonic plates are in contact

• The way the plates interact is based on the type of plate and the

direction the plates are moving relative to each other

spreading apart colliding sliding by

More about SPREADING RIDGES

SPREADING RIDGE

(or OCEANIC RIDGE)

RIFT VALLEY

DIVERGENT PLATE BOUNDARIES

• mark the areas where tectonic plates are spreading apart (i.e.

RIDGE PUSH occurs)

DIVERGENT PLATE BOUNDARIES

• Examples: The Mid – Atlantic Ridge (in the ocean) and the East

African Rift (on the land)

DIVERGENT PLATE BOUNDARIES

• Divergent Plate Boundaries are areas where SPRADING RIDGES

and RIFT VALLEYS are formed

A PLATE BOUNDARY

• A region where two tectonic plates are in contact

• The way the plates interact is based on the type of plate and the

direction the plates are moving relative to each other

spreading apart colliding sliding by

CONVERGENT PLATE BOUNDARIES

• mark the areas where tectonic plates collide (i.e. SUBDUCTION

may occur)

• Because there are 2 types of tectonic plates (Oceanic and

Continental) 3 different interactions are possible

OCEANIC – CONTINENTAL PLATE CONVERGENCE

• EXAMPLE: the Juan de Fuca Plate (an oceanic plate) and the North

American Plate (a continental plate).

OCEANIC – CONTINENTAL PLATE CONVERGENCE

• When the oceanic plate subducts under the continental plate, a deep

underwater valley called a trench forms

OCEANIC – CONTINENTAL PLATE CONVERGENCE

• When the oceanic plate subducts under the continental plate, a deep

underwater valley called a trench forms

OCEANIC – CONTINENTAL PLATE CONVERGENCE

• As subduction (and slab pull) occurs, magma can break through to

the surface, forming volcanoes

• A volcanic belt: a long chain of cone – shaped volcanoes may form

as a result

OCEANIC – CONTINENTAL PLATE CONVERGENCE

• Mountain ranges like the Coast Mountain range and Cascade

Mountain Range also form from the collision

OCEANIC – CONTINENTAL PLATE CONVERGENCE

• Many times, colliding plates resist the force of convection currents,

ridge push, and slab pull the plates get stuck in place and energy

in released in the form of an earthquake

• Most of the largest earthquakes are produced this way

OCEANIC – OCEANIC PLATE CONVERGENCE

• EXAMPLE: Aleutian Islands of Alaska and the islands of Japan and

Indonesia

• When the oceanic plate subducts under another oceanic plate

• Cooling will cause one plate to be denser than the other, and the

denser plate will slide deep into the mantle.

• A long chain of volcanic islands known as a volcanic island arc may

be produced

OCEANIC – OCEANIC PLATE CONVERGENCE

CONTINENTAL – CONTINENTAL PLATE CONVERGENCE

• EXAMPLE: the Himalayas

• When two continental plates collide

• NO SUBDUCTION - Since their densities are similar

A PLATE BOUNDARY

• A region where two tectonic plates are in contact

• The way the plates interact is based on the type of plate and the

direction the plates are moving relative to each other

spreading apart colliding sliding by

TRANSFORM PLATE BOUNDRIES

• EXAMPLE: The San Andreas Fault

• Convection currents in the mantle often cause tectonic plates to slide

past each other

• Usually are found near ocean ridges

TRANSFORM PLATE BOUNDRIES

• Since rock slides past rock, no mountains or volcanoes form

• However, earthquakes and faults (breaks in rock layers due to

movement on either side) may result

HOW ARE TECTONIC PLATES LINKED TO

EARTHQUAKES?

• a massive release of energy build up by stress of friction bewteen

tectonic plates that shakes the crust

• the location inside Earth where an earthquake starts is called the

focus

HOW ARE TECTONIC PLATES LINKED TO

EARTHQUAKES?

• The epicentre is the point on Earth’s surface directly above the focus

HOW ARE TECTONIC PLATES LINKED TO

EARTHQUAKES?

• Energy released by an earthquake produces vibrations known as

seismic waves

• There are 3 types of waves that are created during an earthquake and

each of them have different properties

MEASURING EARTHQUAKES

VOLCANOES

• The movement of tectonic plates produces three distinct types of

volcanoes: composite volcanoes, shield volcanoes, and rift eruptions

Very explosive or violent

NOT Very explosive or violent

NOT Very explosive or violent

COMPOSITE VOLCANOES

• found along plate boundaries

• Layers of ash and thick lava form a tall cone.

• As magma reaches the surface, it cools, hardens, and traps gases

below.

• Pressure builds; eventually, there is an eruption.

SHIELD VOLCANOES

• these are not found at plate boundaries but instead form over hot

spots.

• Thin magma/lava flows out from a hot spot and forms a low, wide

cone.

• The Hawaiian Islands are an example of a chain of shield volcanoes.

RIFT ERUPTIONS

• occur along long cracks in the lithosphere

• These are not explosive, but they release massive amounts of lava