some questions we will answer today: –how is the earth always changing? –what forces inside the...

• Some questions we will answer today:

– How is the earth always changing?

– What forces inside the earth create and change landforms on the surface?

– What is the theory of plate tectonics and how does it work?

– What two theories help make up the theory of plate tectonics?

– What is continental drift and sea floor spreading?

– What happens when the plates crash together, pull apart, and slide against each other?

Different theories about phenomena on earth’s surface

• Most of them contradict each other

• Not reliable

• Eg.– Roman Church– Copernicus– Aristotle– Galileo

Bases of studies of every discipline

Physics Chemistry Biology Earth Science

Gravity Atom Cell Plate tectonic

The Plate Tectonic Theory• One single theory• explains almost all phenomena prevalent in the

lithosphere – volcanism, – mid-oceanic ridges, – deep sea trenches, – earthquakes, – mineralization, – mountain-building and many more.

• “Paradigm Shift” Yet tip of the iceberg

To really understand how the earth became to look as it does today, and the theory of plate tectonics, you also

need to become familiar with two other ideas:

Continental Drift

and

Seafloor Spreading.

The idea of ‘moving’ continents

Continental Drift Theory• In the early 1900s a German explorer and scientist proposed the continental drift theory. He proposed that there was once a single “supercontinent” called Pangaea.

• Alfred WegenerAlfred Wegener

• Proposed hypothesis in Proposed hypothesis in 19151915

• Published Published The Origin of The Origin of

Continents and OceansContinents and Oceans

• Continental drift hypothesisContinental drift hypothesis

• Supercontinent Supercontinent PangaeaPangaea began breaking apart began breaking apart about 200 million years about 200 million years

agoago

Continental drift: An Continental drift: An idea idea

before its timebefore its time

CONTINENTAL DRIFT THEORY

• Alfred Wagener- 1912, revised in 1924.• He was a meteorologist studying global

climatic changes in the geologic past.• ‘jigsaw fit’ of continents

– Brazil- Gulf of Guinea

– North American coastline- Scandinavian coastline

– Ethiopia- Western India and Pakistan

– Australia- Bay of Bengal

THE THEORY

• Super continent- Pangaea• Covered by huge water body- Panthalassa• Pangaea- Laurasia & Gondwanaland• Seperated by a narrow sea- Tethys Sea• 250-300 mya, both started coming closer, Indian

peninsula started drifting north-eastwards, Tethys Sea became smaller

• Himalayas and Alps were formed• 180 mya, Americas started drifting westwards,

forming Rockies and Andes

Evidence used in support of Evidence used in support of continental drift hypothesis continental drift hypothesis

•Fit of the continents Fit of the continents

•Fossil evidenceFossil evidence

•Matching rock type and Matching rock type and mountain belts mountain belts

•Paleoclimatic evidencePaleoclimatic evidence

Evidence for Continental Drift

• Jigsaw Puzzle fit of continents

Alfred Wegener during Greenland expedition

Glacial Deposits

More evidence

• Matching geologic structures including:– Mountain chains

– Ore deposits

– Same rocks of same age

The Evidence for Continental Drift

• Fossil Evidence– Glossopteris– Cynognathus– Mesosaurus– Lystrosaurus

Wegener not believed

• Why? -– What could possibly force the continents to

move across the ocean floor in this way. They would be crushed.

– He was a meteorologist, not a geologist

Sea-Floor Spreading

• Sea-floor spreading: The process by which molten material adds new oceanic crust to the ocean floor

Seafloor Spreading

• Emerged from the study of the ocean floor.

• Series of mountains that extend around the world, stretching more than 64 thousand kilometers (40 thousand miles).

Developments 50s and 60s

• World war 2 submarines found mountains under the oceans – the mid-ocean ridges

• Theory of seafloor spreading suggested by Princeton professor Dr. Harry Hess

Seafloor spreading

First look at the earth’s layers as shown here.

Evidence for Sea-Floor Spreading

Trend of the Mid-Atlantic Ridge

• Similar to the coastlines of adjacent continents

• Europe and Africa to the east

• North and South America to the west

Oceanic Crust is thin

• Both sides of the ridge, oceanic crust is hardly 4 to 5 km thick

• Rest all oceans –between 10 to 20 km

Volcanoes

• Continuous chain of volcanoes all along the mid-Atlantic ridge

• What would be the effect of this at the ridge?

Age of rocks

• Youngest near the ridge

• As we move away from the ridge, rocks are older in age

• No rock in the Atlantic ocean is more than 200 million years old ( The crust is not more than 200 million years old)

How are these matching stripes evidence of sea-floor spreading?

• The pattern of stripes is the same on both sides of the ridge, indicating that the sea floor has spread from the mid-ocean ridge

What happens to the rock along the ridge when new molten material erupts?

• The spreading molten material pushes the older rock to both sides of the ridge

Where is the oldest part of the ocean floor?

• The oldest part of the ocean floor is farthest from the mid-ocean ridge

Pangaea revisited• By piecing together this

information, we can see how the continents have moved over the past 200 million years, due to seafloor spreading

If new crust develops from mid-oceanic ridges……….

• Where does the old crust go?

• Why the amount of lava coming out and forming crust does not reduce?

Look at the observations made by

• Hugo Benioff & Vine and Mathews- American GEOLOGISTS

• They observed certain phenomena in the eastern coast of Eurasia

Deep sea coast

• The sea near the east coast of China was very deep.

• Yellow Sea

• South China Sea

• East China Sea

Ring of Fire

• Chain of volcanic islands running parallel to the east coast of Eurasia– Aleutian– Japan– Phillipines

Volcanic activity

• Intense

• Explosive.

• Lava different than that of the Mid-Atlantic ridge

Frequent earthquakes

• Japan gets at least 2 earthquakes every day

Conclusion

• As Eurasia is pushed from Mid-Atlantic Ridge,– It pressed against and subsided under the crust

which make up the floor of the Pacific Ocean

– Constant rubbing of both the plates melts some rocks, magma forms and erupts out forming the islands.

Process known as??????????????????????????

Subduction

• Subduction: The process by which oceanic crust sinks through a deep-ocean trench and back into the mantle; a convergent plate boundary

Subduction at Deep-Ocean Trenches

Deep-Ocean Trenches

• Deep-Ocean Trenches: A deep valley along the ocean floor through which oceanic crust slowly sinks towards the mantle

What happens to the ocean floor at deep ocean trenches?

• At deep-ocean trenches, subduction allows part of the ocean floor to sink back into the mantle, over tens of millions of years

Plate Tectonics Unifying concept

• Sea floor spreading

• Continental Drift

• Earthquakes

• Volcanoes

• Mountains

• Sea islands

According to the theory of plate tectonics, the earth’s outer shell is not one solid piece of rock. Instead the earth’s crust is broken into a number of moving plates. The plates vary in size and thickness. They keep moving in different directions

2 Types of Plates

• Ocean plates - plates below the oceans

• Continental plates - plates below the continents

What is the Asthenoshere?

• The plastic layer below the lithosphere = asthenosphere

• The plates of the lithosphere float on the asthenosphere

Drifting of continents

• When the tectonic plates under the continents and oceans move, they carry the continents and oceans with them.

Sea-Floor Spreading

• Mid oceanic ridges

• Magma comes out

• Forms new crust

• Pushes existing crust on both sides

• Drifting continents

Subduction

• Heavier plate subsides under lighter plate.

• Magma erupts over the thinner plate

• Sometimes magma may pile over on the ocean to form islands

Plate Tectonic Theory• Plates of rigid lithosphere (oceanic and continental) move from the energy

of heat transfer below• Their interactions define divergent, convergent, and transform boundaries

and control many surface processes

Plate tectonics• Sea floor spreading provides the driving

mechanism for movement• However, it is not the continents that are

moving, but the “plates” of lithosphere “floating” in effect on the asthenosphere

• The lithosphere is made up of about 20 plates which move relative to each other in several ways

• Let’s look at a generalized sketch

The Plates

Types of Plate Margins

• Plate boundariesPlate boundaries• Interactions among individual plates Interactions among individual plates

occur along their boundariesoccur along their boundaries • Types of plate boundariesTypes of plate boundaries

– Divergent plate boundariesDivergent plate boundaries

– Convergent plate boundariesConvergent plate boundaries

– Transform fault boundariesTransform fault boundaries

DIVERGENT PLATE BOUNDARY

• At a divergent plate boundary lithospheric plates move away from each other.

• The mid-Atlantic Ridge, a topographically high area near the middle of the Atlantic Ocean, is an example of a divergent plate boundary.

• New crustal material

Features of Divergent Boundaries

• Mid-ocean ridges

• rift valleys

• fissure volcanoes

They’re Pulling Apart!

• When plates pull away from one another they form a diverging plate boundary, or spreading zone.

Thingvellir, the spreading zone in Iceland between the North American (left side) and Eurasian (right side) tectonic plates. January 2003.

Divergent: Atlantic RidgeLAVA FOUNTAINSKRAFLA VOLCANOICELAND

Divergent boundaries in ContinentsDivergent boundaries in Continents

• Continental riftsContinental rifts• Splits landmasses into two or more Splits landmasses into two or more

smaller segmentssmaller segments

The East African RiftThe East African Rift

CONVERGENT PLATE BOUNDARY

• At a convergent plate boundary, lithospheric plates move toward each other.

• The west margin of the South American continent, where the oceanic Nazca Plate is pushed toward and beneath the continental portion of the South American Plate, is an example of a convergent plate boundary

• Types of convergent boundariesTypes of convergent boundaries• Oceanic-oceanic convergenceOceanic-oceanic convergence

– When two oceanic slabs converge, one descends When two oceanic slabs converge, one descends beneath the other beneath the other

– Often forms volcanoes on the ocean floorOften forms volcanoes on the ocean floor

– If the volcanoes emerge as islands, a volcanic If the volcanoes emerge as islands, a volcanic island arcisland arc is formed (Japan, Aleutian islands, is formed (Japan, Aleutian islands, Tonga islands) Tonga islands)

– Subducting plate bends downward forming an oceanic trench

Convergent plate boundariesConvergent plate boundaries

Oceanic-oceanic Collision

Figure 12.16

Oceanic-Continental Collision

ANDES• The Andes Mountain

Range spans the entire length of South America, along the western coast. During this subduction some Nazca crust is scraped off along base of the Andes, adding height to the entire range.

RING OF FIRE

OCEANIC-CONTINENTAL COLLISONS SUSTAIN MOSTVOLCANIC ACTIVITY AROUND THE PACIFIC OCEAN

• Types of convergent boundariesTypes of convergent boundaries• Continental-continental convergenceContinental-continental convergence

– Continued subduction brings continents togetherContinued subduction brings continents together

– Less dense, buoyant continental lithosphere does Less dense, buoyant continental lithosphere does not subductnot subduct

– Result is a collision between two continental Result is a collision between two continental blocksblocks

– Process produces mountainsProcess produces mountains (Himalayas, Alps, (Himalayas, Alps, Appalachians)Appalachians)

Convergent plate boundariesConvergent plate boundaries

The collision of India and Asia The collision of India and Asia produced the Himalayasproduced the Himalayas

Transform fault boundariesTransform fault boundaries

• Third type of plate boundaryThird type of plate boundary • Plates slide past one another and no new Plates slide past one another and no new

lithosphere is created or destroyedlithosphere is created or destroyed

Transform Boundaries

• Transform faults mark fractures in the crust where plates slide laterally past each other

• The San Andreas fault separates the Pacific plate from the North American plate

• These areas are likely to have a rift valley, earthquake, and volcanic action.

San Andreas Fault, CA

• Caused by rising plumes of mantle Caused by rising plumes of mantle materialmaterial

• Volcanoes form over themVolcanoes form over them (Hawaiian (Hawaiian Island chain)Island chain)

• Mantle plumes are long-lived structures Mantle plumes are long-lived structures and originate at great depth, perhaps at and originate at great depth, perhaps at core-mantle boundarycore-mantle boundary

Hot spotsHot spots

The The Hawaiian Islands form over Hawaiian Islands form over stationary hot spotstationary hot spot

OTHER HOTSPOTS

Plate motions also can be looked at into the future, and we can have a stab at what the geography of the planet will be like. Perhaps in 250 million years

time there will be a new supercontinent.