Theory of Plate Tectonics

Plate Tectonics

Plate Boundaries

Causes of Plate Tectonics

What is Plate Tectonics?

• The Earth’s crust and upper mantle are broken into sections called plates
• Plates move around on top of the mantle like rafts
• Convection currents drive the motion

Why do the plates move?

• Heat energy from the core of the Earth spreads outward
• Tremendous heat in asthenosphere causes convection currents
• Convection currents cause hot material to rise and expand (plates diverge) and cooler material to sink and contract (plates converge)
• Plates “ride” on hot, flowing rock at about 2 cm a year

VOCABULARY CHECK-IN

• Lithosphere
• The crust and the "rocky"upper mantle
• 100 km thick
• Less dense than the material below it
• Asthenosphere
• The plastic layer below the lithosphere
• The plates of the lithosphere float on the asthenosphere

2 Types of Crust

• Ocean crust - plates below the oceans, thinner, denser, composed of basalt, higher iron content
• Continental crust - plates below the continents, thicker, less dense composed of granite, higher silicate content.

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MOST PLATES HAVE BOTH TYPES OF CRUST...

The North American Plate has continental crust to the west and oceanic crust to the east.

Plate Boundaries

Three types of plate boundary

A plate boundary is where two or more plates meet.

Divergent Boundaries: where two plates move away from each other

• Spreading ridges

As plates move apart new material comes up to fill the gap

Oceanic-Oceanic Divergent

Where tectonic boundaries are oceanic crust

Features:

• mid-ocean ridge with a rift (split) down the center
• undersea mountain range or ridge system

Effects:

• shallow earthquakes
• volcanoes

Example: Mid-Atlantic ridge

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Iceland has a divergent plate boundary running through its middle. You can walk "between" or "over" the two major plates in some areas.

Iceland: An example of divergent plates

Continental-Continental Divergent

Where tectonic boundaries are continental crust

Features:

• rift zone
• volcanic mountain range split by a rift valley
• basalt flows (new oceanic crust)

Effects:

• shallow focus earthquakes
• volcanic activity

Example: East African Rift

African Rift Valley: An example of divergent plates

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The are fault block "mountains."

How oceans are formed from continents...

Convergent Boundaries: where two plates move toward each other

There are three styles of convergent plate boundaries

• Continent-continent collision

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• Continent-oceanic crust collision

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• Ocean-ocean collision

Where tectonic boundaries are continent crust

Features:

• large mountains

Effects:

• extensive earthquake
• no volcanoes, no subduction
• highly deformed and folded metamorphic rocks

Example: Himalayas

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Type 1: Continent-Continent Collision

The Himalayas: an example of c-c convergence

The Himalayas

Where tectonic boundaries are continent crust and ocean crust

Features:

• deep ocean trench parallel to shoreline
• volcanic mountain range inland

Effects:

• deep earthquake activity
• subduction of ocean plate
• explosive volcanism

Example: Andes Mountains and Cascade Mountains

Type 2: Continent-Oceanic Crust Collision

Andes Mts., South America:

an example of o-c convergence

Cascade Mts., North America:

an example of o-c convergence

Mt. St. Helens the day before the 1982 eruption.

Mt. St. Helens two years after the 1982 eruption.

C-O Subduction

Where tectonic boundaries are ocean crust

Features:

• deep ocean trench
• volcanic island arc

Effects:

• deep earthquake activity
• subduction of ocean plate
• explosive volcanism

Examples: Philippines, Mariana Trench, Aleutians (Alaska), Japan

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Type 3: Ocean-Ocean Plate Collision

Aleutian Islands, Alaska:

an example of o-o convergence

Mariana Trench:

an example of o-o convergence

The Mariana Trench is 11 km deep, the deepest spot in the ocean.

Transform Boundaries: where two plates slide past each other

Plates can slide in opposite directions

Plates can slide in the same direction but at different speeds.

Above: View of the San Andreas transform fault

Transform plate boundaries

Where tectonic plates moved past one another

Features:

• possible earthquake scar
• often no features visible

Effects:

• extensive earthquake activity

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Examples: San Andreas Fault, Alpine Fault in New Zealand

San Andreas Fault:

an example of a transform boundary

Examples of Evidence for Transform Boundaries

Questions...

• What are the three types of boundaries?
• In what direction do plates go for each?
• Which boundary has a subduction zone…what occurs at a subduction zone?

Causes of Plate �Tectonics

Convection Currents

• 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.

Questions...

• What causes plates to move?
• How is a convection current formed?
• Describe the lab you completed and compare the results with convection current in the asthenosphere.