Chapter 12 Thermal Energy Transfer Drives Plate

tectonics

What is thermal energy • Thermal energy with respect to plate tectonics is energy of

molten rock that is constantly moving below your feet.

• Below the surface of the earth, there are plates that are

constantly moving by the movement of this churning melted

rock.

• This movement may cause plates to move in opposite

directions, and bring those plates together cause major

geological events such as volcanic eruptions and

earthquakes

Evidence for Continental Drift

12.1

Continental Drift • There is quite a bit of evidence that suggests that all

of the continents were once all joined together but

later drifted to their current locations

• Evidence for this includes o Coasts of the continents can be aligned

o Regions that are far apart have similar rocks, mountain ranges, fossils and

paleoglaciation

Continental Drift • The process of sea floor spreading provides a

mechanism for continental drift

• Each continent is attached to a huge slab of rock

known as tectonic plates

• When these tectonic plates move, they carry

continents with them

• The continents of the world have not always been in the same place....

Continental Drift Theory • Alfred Wegener...

• “The continents have “drifted” to their present locations over millions of years”.

• They were once joined as a “supercontinent” (Pangaea)

Pan - gaea

“all” “Earth”

Putting the Pieces Together...

• The continents do not fit “perfectly” like a jigsaw puzzle.

• A better fit is found by matching the continental shelves (original shorelines that are now underwater)

Matching Geological

Structures & Rock Evidence • Mountain ranges of

one continent, end at coastline begin again at another continent

(similar folds, ages)

Geological/ Rock Evidence

• Similarities between rocks of adjacent continents are evidence for continental drift.

• The rocks found in Newfoundland are the same type and age as rocks found in Greenland, Ireland, Scotland, Norway.

Matching Fossils • The fossils of some ancient

species suggests evidence for ‘pangaea’.

• Wegener found fossils of a fresh-water reptile ‘Mesosaurus’ in two places:

- SE South America and SW Africa

This small freshwater reptile could not have swam this

distance.

Fossil Evidence • Glossopteris (a fern)

Found In:

South America

Africa Australia

India Antarctica

Fossil Evidence • Problem: Ferns do not grow in cold climates (Antarctica)

Glacial Evidence • Evidence of glaciers

formed in ice ages provides support to CD theory

• During ice ages, glaciers covered large areas of land

• When the glaciers retreat/ advance, they leave behind proof o Deeply scratched rocks (striations),

U-shaped valleys, patterns of rock formations

• Paleoglaciation refers both to the extent of ancient glaciers and to the rock markings they have left behind.

“Paleoglaciation”

Paleoglaciation • Scientists were puzzled by evidence of glaciers in

places that are now tropical, like India and Africa!

• If they were at one time connected, this would

make sense.

More Evidence • Coal Deposits in Antarctica

- Coal forms from dead matter (usually tropical swamp material)

- Since the South Pole has never had a tropical climate....it must have been in a warmer location than it is now

How the Continents Moved & a

Possible Mechanism

• The current theory is that the continents moved due to the movement of tectonic plates.

Tectonic plates = movable slabs of rock that make up the Earth

Tectonic Plates • These large rigid plates slide over the surface of Earth

(over a layer of partially molten rock)

magma

partially molten surface

• Tectonic plates explain the location of volcanoes and earthquakes....

Volcanoes/ Earthquakes

Volcanoes are openings in the Earth’s surface that spew gases, chunks of molten rock

Earthquakes are sudden, ground-shaking releases of built up energy under Earth’s surface

Earthquake zones and volcanoes follow a pattern.... They occur along the boundaries (edges) of tectonic plates (form an outline)

Mid-Atlantic Ridge • Mapping of the ocean floor revealed

a long mountain range running down the middle of the Atlantic Ocean.

• It is called the “Mid-Atlantic Ridge”

How it was formed...

• The tectonic plates of the Earth slide apart (sliding is called “conduction”)

• Another way to think of this is “seafloor spreading”

• Something must replenish this lost crust!

Occurs at rate of ~ 1 inch /yr

Magma to the Rescue!!!! • Areas of oceanic crust are replenished by volcanic

activity.... o Magma (molten rock) rises up

o Cools and hardens when it reaches surface

o Forms the new sea floor

Magma and More Magma

• This process keeps going..... o Convection currents cause more magma to rise

o Magma forces apart hardened material

o Older rock is pushed aside as new rock forms!

“Sea Floor Spreading”

Animation....

Spreading Ridge

Proof of Sea Floor Spreading!

• In the 1940’s...oceanographers took samples of rock from the Mid-Atlantic Ridge

- Younger rocks were found closer to ridge - Older rocks were found farther from ridge (and in thicker layers...built up!)

More evidence in the rocks!!

• Scientists have also found that the rocks themselves have a “striped pattern”

• To understand this, we must understand the Earth’s magnetism....

Earth’s Magnetic Field

Every few thousand years....the direction of magnetic field REVERSES!

“MAGNETIC REVERSAL”

Consider a rock..... Rock contains metals like iron (Fe) Little pieces of iron “line up” with magnetic field and then they STAY that WAY! When MAGNETIC REVERSAL happens...they line up in opposite direction! Rocks “preserve” the direction of the Earth’s magnetic field that existed when the rock formed! Amazing!

Studying the magnetic properties of ancient rocks....

“Paleomagnetism”

Back to the Sea Floor....

• So the sea floor is spreading and new rocks (from magma) are formed...

• Looking closely at the sea floor, scientists found a striped pattern!

Normal magnetic polarity

Reverse magnetic polarity

“magnetic striping”

A HOT SPOT.... • A geological area

where molten rock rises to Earth’s surface!

• In mid-1960’s, Canadian geologist J. Tuzo Wilson suggested that chains of volcanic islands were formed when a tectonic plate passed over a stationary hot spot. Developed “plate tectonic theory”...unifying theory of geology.

HOT SPOTS....

Hawaiian Islands were formed by a tectonic plate passing over a hot spot. The plate slides in the direction of the arrow.... Forms a CHAIN of islands!