Plate Tectonics

(Wrong kind of plate…and where’s my salad fork?)

The crust is broken!

The crust is broken!

We measure plate motion with GPS

Continental Drift Whiplash

Plate Boundary TypesPlate Boundary Types

Divergent plate boundaryDivergent plate boundary

Divergent plate boundaryDivergent plate boundary

• 2 plates are moving AWAY from each 2 plates are moving AWAY from each otherother

• Seen in mid-ocean ridgesSeen in mid-ocean ridges and rift valleysand rift valleys

Where in the world?Where in the world?

Divergent plate boundaryDivergent plate boundary(forming a rift valley)(forming a rift valley)

Mid ocean ridgeMid ocean ridge

Divergent plate boundaryDivergent plate boundary

Features:Features:

1)1) 2)2) 3)3)

Divergent plate boundaryDivergent plate boundary

Features:Features:

1)1) Mid ocean ridgesMid ocean ridges2)2) Undersea volcanoes—may make islandsUndersea volcanoes—may make islands3)3) Rift valleys, on landRift valleys, on land

Convergent plate boundariesConvergent plate boundaries

Convergent plate boundariesConvergent plate boundaries

• 2 plates move TOWARD each other2 plates move TOWARD each other

What is happening at the plate What is happening at the plate boundary?boundary?

Oceanic Plate

Oceanic Plate

SubductionSubduction• When 2 plates converge, one plate MAY move When 2 plates converge, one plate MAY move

downwards UNDER the other – this is called downwards UNDER the other – this is called SUBDUCTIONSUBDUCTION

SubductionSubduction

Earthquakes are Earthquakes are deeperdeeper the farther inland you go. the farther inland you go.

SubductionSubduction

Accretion—some of the lightest oceanic crust Accretion—some of the lightest oceanic crust joins the continentjoins the continent

• WhichWhich plate is subducting? plate is subducting?• What is What is happeninghappening to the subducted plate? to the subducted plate?

Convergent plate boundariesConvergent plate boundaries

Features:Features:

1)1) 2)2) 3)3)

Convergent plate boundariesConvergent plate boundaries

Convergent plate boundariesConvergent plate boundaries

Convergent plate boundariesConvergent plate boundaries

Features:Features:

1)1) … …in ocean/ocean convergencesin ocean/ocean convergences2)2) … …in ocean/continent convergencesin ocean/continent convergences3)3) … …in continent/continent convergencesin continent/continent convergences

Convergent plate boundariesConvergent plate boundaries

Features:Features:

1)1) Ocean trenches and volcanic island arcsOcean trenches and volcanic island arcs2)2) Benioff zone earthquakes, continental rim Benioff zone earthquakes, continental rim

volcanoes and accretionvolcanoes and accretion3)3) High plateaus, thrust faults and mountainsHigh plateaus, thrust faults and mountains

Transform Transform plate plate

boundaryboundary• 2 plates 2 plates slide pastslide past

each othereach other Pacific Plate

North American Plate

Transform Plate BoundariesTransform Plate Boundaries

• The area where these plates meet is called a The area where these plates meet is called a transform faulttransform fault

• The San Andreas Fault in California is The San Andreas Fault in California is possibly the most famouspossibly the most famous

Transform PlatesTransform Plates

Point Reyes, California Point Reyes, California

Transform plate boundariesTransform plate boundaries

Features:Features:

1)1) 2)2) 3)3)

Transform plate boundariesTransform plate boundaries

Features:Features:

1)1) Major earthquakesMajor earthquakes2)2) Offset linear featuresOffset linear features3)3) Fracture zonesFracture zones

What kind of boundary?What kind of boundary?11 22 33 44 55

What features do they make?What features do they make?11 22 33 44 55

Plate Boundary TypesPlate Boundary Types

Why do the plates move?

1) 2) 3)

Why do the plates move?

1) Heat2) Differences in density3) Gravity

Why do the plates move?

Inner core

Outer core

Mantle

Crust

Mantle and outer core are melted

Heat

• The Earth was originally molten• The heat within the Earth is left over from then• Radioactivity adds some heat

Density

• D=mass/volume• Less dense floats on more dense • If you heat an object or fluid, it expands, the

density decreases• A more dense fluid will force a less denseto rise

• The average density of the Earth provides evidence of the relative densities of the different layers of Earth

- crust, (lithosphere)-mantle (asthenosphere) and -core.

• The average density of the Earth provides evidence of the relative densities of the different layers of Earth

- crust, (lithosphere)-mantle (asthenosphere) and -core.

The whole Earth is more dense than the rocks we see

Density

• Mantle is more dense than crust

• Oceanic crust is more dense than continental crust

Gravity

g=Gm1m2/d2

If anybody cares. What we do care about is that

a) if you heat a fluid, its density decreases,

b) the cooler fluid is more dense, so it sinks.

All together--

Convection happens!

Why do the plates move?

Why do the plates move?

Convection happens!

• The accepted mechanism for plate motion is: convection in the asthenosphere.

Convection happens!

Convection happens!

This plate is pulled this way…

Convection happens!

…and this plate is pulled this way.

Wegener’s Evidence

• Wegener supported his hypothesis of continental drift with evidence from the puzzle fit of the continents, rock units, geologic structures, fossils and glacial evidence.

Evidence

The modern theory of plate tectonics is supported by evidence from:

Evidence

The modern theory of plate tectonics is supported by evidence from:

A. paleomagnetism, B. C. D.

a. b. c.

Seafloor crust shows the direction of the magnetic field

when the crust was formed

Evidence

The modern theory of plate tectonics is supported by evidence from:

A. paleomagnetism, B. age of ocean floor sedimentsC. D.

a. b. c.

Seafloor crust is older farther from the mid-ocean ridge

Evidence

The modern theory of plate tectonics is supported by evidence from:

A. paleomagnetism, B. age of ocean floor sedimentsC. ocean floor topography D.

a. b. c.

You can see the ridge and the suture zone around it

Evidence

The modern theory of plate tectonics is supported by evidence from:

A. paleomagnetism, B. age of ocean floor sedimentsC. ocean floor topography D. Wegener's evidence.

a. b. c.

Wegener had none of the above. He had to do it the hard way

Evidence

The modern theory of plate tectonics is supported by evidence from:

A. paleomagnetism, B. age of ocean floor sedimentsC. ocean floor topography D. Wegener's evidence.

a. fossils b. c.

Tropical fossils show up in polar regions

Evidence

The modern theory of plate tectonics is supported by evidence from:

A. paleomagnetism, B. age of ocean floor sedimentsC. ocean floor topography D. Wegener's evidence.

a. fossils, b. jigsaw continents, c.

They fit together pretty well

Evidence

The modern theory of plate tectonics is supported by evidence from:

A. paleomagnetism, B. age of ocean floor sedimentsC. ocean floor topography D. Wegener's evidence.

a. fossils, b. jigsaw continents, c. rock types Similar rocks show up on

different continents