Unit 5 – The Dynamic Earth

Chapter 17 – Plate Tectonics

Major focus for the Regents Exam!

ESRT pages we will study in this unit:• Page 5 – Tectonic Plates• Page 10 – Inferred Properties of the Earth• Page 11 – Earthquake wave travel time & crust

composition

Plate Tectonics

Plate tectonics is a proven “theory” that describes the formation, movement, and interaction of lithospheric plates over a plastic asthenosphere.

• Lithosphere – is broken into plates that ride on the asthenosphere. (rigid mantle and crust)

• Asthenosphere – is the “plastic” mantle. (movement mechanism for lithosphere)

Drifting Continents

What caused people to consider that the earth’s surface might be moving?

 • Shape of South America and Africa• Fossil evidence• And much later…bands of reverse

magnetism in igneous rock on the ocean floor.

Principles of Plate Tectonics

• The lithospheric plates are made of both continental and oceanic crust, and cover the entire earth surface.

• As plates move…so do the continents.

• Plate tectonics explains how new crust forms and why earthquakes and volcanoes are located in concentrated belts.

Evidence of Plate Tectonics

Evidence that we use today to observe plate tectonics for minor crustal movement:

• Folds• Faults• Displaced fossils• Displaced strata

 

Evidence • Folds – occur when rocks are deformed slowly

due to pressure.

Evidence • Faults – happen when rocks are deformed

quickly due to pressure of tension.

Evidence

Displaced fossils –

• Marine fossils found high above sea level.

• Uplift of land (Grand Canyon, Alps, Rocky Mountains

• Fossils of land organisms (plants and animals) found below sea level.

• Subsidence of land (Gulf of Mexico)

Evidence

Displaced Strata – Sedimentary rocks found above sea level due to uplift.

 

 

Evidence

Evidence that we use today to observe plate tectonics for major crustal movement:

 

• Movement of plates (G.P.S.)

• Mountain Ranges

• Rift Zones

How does it work?

How can we assume that the processes that occur today…formed the “evidence” that we see from events of the past?

 Uniformitarianism – the laws of nature have

not changed (ex. Gravity, weathering, erosion and deposition) …”the present is the key to the past”

Start of Plate TectonicsWhere did the theory of plate tectonics start? • Observations 1.    Similarities in the shape of continents.2.    Mesosaurus (lived about 270 m.y.a) fossils are found

in both South America and Africa. • Theory of Continental Drift – proposed by Alfred

Wegner in 1912-        He used observations above to propose plate

movement-        Downfalls of his theory…was he could never

explain the mechanism for movement.

5 Pieces of Evidence to explain plate tectonics:

1.  Fit of Continents (modified from old theory of continental drift ~ Wegner)

 2.  Glaciation – evidence of glaciation in lower latitudes.

(ex. Scratches, glacial erratics, till) 3.  Correlation of rocks and mountain ranges on both sides

of the Atlantic Ocean. 4.  Age of ocean floor (Mid-Ocean Ridge) 5. Magnetic Reversals (Mid-Ocean Ridge)

Theory of Plate Tectonics

Plate Boundaries – tectonics plates touch at locations called plate boundaries. There are 3 main types of plate boundaries, based on how the plates are moving in relation to each other.

1. Convergent boundaries

2. Divergent boundaries

3. Transform boundaries

Convergent Boundaries• Locations where two plates are moving towards one another.• There are 3 specific types of convergent boundaries1. Continent – Continent boundary2. Ocean – Ocean boundary3. Continent – Ocean boundary

Continent – Continent• A location where two plates of continental

crust are coming together.

Ocean - Ocean• A location where two plates of oceanic

crust are coming together.

Continent - Ocean• A location where continental crust is

colliding with oceanic crust

Subduction

• Why does one plate usually sink below another plate?

Subduction

• The more dense plate sinks below the less dense plate.

Continental crust = 2.8 g/cm3

Oceanic crust = 3.0 g/cm3

Divergent Boundaries• A location where two plates are moving apart, due to

magma rising up and creating new crust. Sea-floor spreading.

Transform boundary• A location where two plates are sliding past

one another.

Animations of Plate Boundaries

http://www2.wwnorton.com/college/geo/egeo/flash/2_6.swf

http://www2.wwnorton.com/college/geo/egeo/flash/2_6.swf

Causes of Plate Motion

• Still much to be discovered in this area, but we have a better explanation of a mechanism for plate movement.

Mantle Convection

Convection explained…

Convection: the transfer of thermal energy by the movement of heated matter.

• Heated matter will expand.

• Matter that expands becomes less dense.

• Less dense matter rises in more dense matter.

Convection Current

Matter is heated = density is decreased = matter rises

Matter is cooled = density is increased = matter sinks

Mantle Convection

• The mantle is heated unevenly by deeper layers of the asthenosphere.

• As the lower parts of the mantle are heated they start to rise.

• As they rise, they enter colder regions of the mantle and begin to cool.

• This cooler mantle material will begin to sink…thus creating a convection cell.

Mantle Convection

• These convection cells in the mantle can move the lithosphere (plates) in several ways.

Types of Movement

Push – the weight of the uplifted rock at the ridge will push the crust away from the ridge.

Pull – the weight of a subducted plate pulls the rest of the plate down into the Earth.