The Theory of Continental Drift & Plate Tectonics Miss Loulousis Earth Science

The Theory of Continental Drift 10.1

Objectives Summarize Wegner’s hypothesis of continental

drift and the evidence for it Describe the process of sea-floor spreading and Explain how sea-floor spreading provides a mechanism for continental drift Identify how paleomagnetism provides support for the idea of sea-floor spreading and continental drift

Imagine Rice boiling, what path of

motion is the rice moving in?

It is moving in circles

The water/rice heats and rises

then cools at the surface and

sinks continuing the circle

*CHANGES IN DENSITY

-Hot Rises

-Cool Sinks

Why does the rice move in circles?

Boiling Water

Alfred Wegener (1912)

-Alfred Wegener developed the theory of continental drift

-He believed all the continents were at one time joined together

-These formed a supercontinent known as Pangaea

Continental Drift

• Continental Drift – hypothesis that continents were once a supercontinent and in different locations

• Evidence: – 1. Continents fit together like a puzzle

– 2. Fossil Correlation

– 3. Mountain Ranges and Rock layers line up

– 4. Climatic evidentce (Glacier Evidence left behind)

• If you look at a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle.

1. The continents coastlines fit together

According to Wegener

• Pangaea broke up into smaller continents about 200-250 mya during the Mesozoic Era

• Over millions of years drifted to current location

• Wegener also speculated that crumpling of Earth’s crust produced mountain ranges like the Andes on the western coast of S.America

2. Fossil Correlation -The same fossil can be found on different continents.

-Land animals can’t swim!

-How did the fossils get from continent to continent?

3. Mountain Ranges and Rock layers line up

• When the continents fit together like a puzzle, mountains also line up.

• Example: The Mountains in Norway align with the Appalachians

3. Mountain Ranges and Rock layers line up

-When the land matches up so does the layers of rock up and down

Think about how these layers can be found on separate continents!!!

Q: What does this suggest about there formation?

4. Climate/Glaciers

-Warm climates have evidence of glaciers which suggest the continent must have been moved from a different latitude.

• The climate of Antarctica was not always as harsh and cold as it is today. When the plant that became this fossil lived, the climate of Antarctica was warm and tropical.

4. Glaciers Cont.

• When Pangaea existed, Central Africa was over the South Pole

• Glaciers were formed on top of the land.

• Evidence of glaciers is still left behind today

Should coal and oil be found in Alaska?

-Recall that coal and oil form in

tropical environments

- Tell your neighbor why we find

oil in Alaska today.

New Evidence for Continental Drift • In 1940’s and 1950’s Wegner’s ideas were further supported by new evidence – 1. Paleomagnetism – 2. Convection currents – 3. Sea-floor spreading

– In the 1960's a gentleman by the name of Harry Hess

proposed that not only were the continents moving, but the sea floor was also moving.

– If the sea floor is spreading, then clearly the continents could diverge from one another. The rock of the sea floor also exhibited magnetic properties.

Sea-floor spreading

• Mid-ocean ridge long, undersea mountain chain that has a steep, narrow valley at its center, forms as magma rises from the asthenosphere, and that creates new oceanic lithosphere (sea floor) as tectonic plates move apart

Sea-Floor Spreading • Sea-floor Spreading formation of new oceanic lithosphere (sea floor)made as magma rises to Earth’s surface and solidifies

• Occurs at a mid-ocean ridge

Mid-Ocean Ridges Observations

1. Rocks closer to a mid-ocean ridge are younger than rocks farther from the ridge.

2. Rocks closer to the ridge are covered with less sediment than rocks farther from the ridge.

Sea-Floor Spreading, continued

As the ocean floor spreads apart, magma rises to fill the rift and then cools to form new rock.

Sea-Floor Spreading, continued

Hess Explains • What causes the sea-floor to spread or the continents to move? – mantle convection.

• At the ridge crest in the sea floor, hot magma rises because it is less dense than the surrounding magma.

• When it reaches the surface it cools and hardens becoming the sea floor.

• As more magma rises, the present sea floor is pushed aside.

• At plate boundaries, the cold and more dense rock sinks and melts, becoming magma once again.

• This mantle convection is the driving force for the motion of the sea floor and the continents themselves.

Plate Movement

• “Plates” of lithosphere are moved around by the underlying hot mantle convection cells

Convection is movement of heated material due to differences in density caused by difference in temperatures

Convection Currents • Convection Currents are the movement of

magma in the asthenosphere in circles. (like the rice)

• They move around the floating tectonic plates above them

Paleomagnetism

•Study of the alignment of magnetic minerals in rock •Rock acquires magnetic properties while forming from magma •Strips form in relation to the reversal of Earth’s magnetic poles •The magnetic strips line up in the rocks on different continents

Paleomagnetism Continued -Sometimes the North and South Pole switch changing magnetic alignment.

-When rocks form at the ocean floor the pattern changes back and forth

-This animation shows that as new crust is made there is a symmetrical pattern or rock formation with the same alignment

https://www.youtube.com/watch?v=WhiF6IqGACo

Exit Slip: Imagine you were Alfred Wegener in 1912

and you have to convince the National Science Foundation (NSA) of the Theory of

Continental Drift.

Summarize your speech at the end of your notes.