Plate Tectonic Model

Did you read chapter 30 before coming to class?

A. YesB. No

So how do you move a continent?

What drives plate motion?

Forces driving plate motion• Ridge Push• Slab Pull• Basal Drive?

Forces resisting plate motion• Basal Drag?• Transform Fault Friction• Collision• Mantle Resistance

Plate Boundaries The action in plate tectonics is almost always at the plate

boundaries. Two plates moving towards each other form a convergent boundary

1. Ocean crust meets ocean crust2. Ocean crust meets continental crust3. Continental Crust meets continental crust

Two plates moving away from each other form a divergent boundary

Divergent Plate Boundaries in the oceans Extensional Stresses stretching ocean crust Creates New Seafloor Volcanoes

• Eruptions of basalt• Low in Si, high in Fe

Earthquakes• Shallow foci• Low to modest magnitudes

Partial melting of peridotite in the mantle produces basalt. Why doesn’t this process just produce more peridotite?

Different mineral components in the peridotite melt at different T’s

What will happen if only a small fraction of the rock melts?

Will the melted part have the same composition as the original rock?

The mid ocean ridges

Iceland

Mid Atlantic Ridge above the water

The mid ocean ridges have a rich variety of life and geologic structure

Tube Worms Spider Crabs

Formations on the ridges

Black SmokersPillow Basalt

Divergent Plate Boundaries under continents

Extensional Stresses stretching

Eventually creates New Seafloor

Volcanoes• Basalt and granite

(Bimodal composition) Earthquakes

• Shallow foci• Modest magnitudes --

larger than oceanic rifts.

The rift valley in Africa is one of the most clear examples of continental rifting

Oldoinyo Lengai

Erta Ale

Killimanjaro

The rift occurring under the Red Sea is more advanced

Why do we get both high & low Si rocks at continental rifts?

Basalt is dense and ponds at base of crust

Heat from basalt melts some of the Si-rich crust producing granite/rhyolite magma

Both types of magma may eventually erupt at the surface

Convergent Plate Boundaries

Compressional Stresses Old Seafloor Destroyed (Subduction) Volcanoes

• Explosive and Very Dangerous (lots of gas) Earthquakes

• Shallow to Deep foci• Large magnitudes

Mountain Belts/High Deformation Accretion – Building of the Continents

Ocean-Ocean Collision

Island arcs: Indonesia, Japan, Taiwan, Philippines, Kamchatka Peninsula, Aleutian Islands.

Ocean-Continent Collision

The Andes mountains are the result of a ocean-continent collision

Deep, severe earthquakes and explosive volcanoes occur as oceanic crust subducts

Continent-Continent Collision

Why are there no volcanoes when continents collide?

No plate is sinking into the mantle and melting!

The Indian subcontinent slammed into Asia several million years ago

India is now a part of Asia. The Asian continent has grown.

Appalachian mountains were similar to the Himalaya mountains 200-300 MYA (collision between North America and Africa)

Build-up of North America through time

Pieces added to western North America in the last 200 million years

Transform Plate Boundaries

Shear Stresses No destruction or creation of crust Volcanoes are rare Earthquakes

• Shallow foci• Large magnitudes

Occur both in ocean basins and on continents

Transform faults are evident in the ocean ridges

The San Andreas fault is a transform boundary

Island Chains

Created by plates drifting over hot spots.

Other “Hot Spots” around the world

Major plate boundaries today

Lets play “Name that boundary”!

Island Arc

Transform Boundary

Subduction(Trench) Divergent plate

boundary

Continental Rift Zone

ConvergingContinental

The interior structure of Earth has been determined mostly from

a) Drillingb) Explorationc) X-Raysd) Earthquake

wavese) Chemical

analysis