mr. alfonso 12-2004. plate tectonics - a powerful unifying theory plate tectonics is a relatively...

Mr. Alfonso 12-2004

PLATE TECTONICS - A POWERFUL PLATE TECTONICS - A POWERFUL UNIFYING THEORYUNIFYING THEORY

Plate tectonics is a relatively new scientific Plate tectonics is a relatively new scientific concept, introduced some 30 years ago, but concept, introduced some 30 years ago, but it has revolutionized our understanding of it has revolutionized our understanding of the dynamic planet upon which we live. the dynamic planet upon which we live.

The theory has unified the study of the earth The theory has unified the study of the earth by drawing together many branches of the by drawing together many branches of the earth sciences.earth sciences.

It has provided explanations to questions It has provided explanations to questions that scientists had speculated upon for that scientists had speculated upon for centuries.centuries.

Alfred Wegener in the early 1900’s proposed that the continents about 200 million years ago were joined together in a single large land mass he called Pangaea (meaning “all land” in Greek). He proposed that Pangea had split apart and the continents had moved gradually to their present positions - a process that became known as Continental Drift.

CONTINENTAL DRIFTCONTINENTAL DRIFT

Pangea - Alfred Wegnercontended that around 200-250 million years ago the

supercontinent Pangea began splitting apart and since then the continents have moved to their

present positions.

Wegner’s Evidence for Continental DriftWegner’s Evidence for Continental Drift

Fossils of plants and animals of the same species found on different continents.

Wegner’s Evidence for Continental DriftWegner’s Evidence for Continental DriftMesosaurus a freshwater reptile fossil found in Africa and South America.

Glossopteris; a fern that requires warm climates was found on Antarctica, Southern South America, Australia, Southern Africa and India.


The distribution of climate sensitive sedimentary rocks on the different continents.

This suggests that the Artics were not always COLD!

Although today we know that Alfred Wegener was correct about continental drift, at the time his theory was not widely accepted. In spite of the evidence that the continents had once been joined, few scientists could understand how the massive continents, weighing billions of tons, could actually move. Wegener never satisfactorily explained this problem.

He suggested the centripetal force of the earth rotating about its axis slung the continents around, and that when the continents moved, they plowed through the ocean floor. Contemporary calculations did not support this. And without a mechanism Wegner did not have support from his colleagues!!!!

Confirmation of continental drift would have to wait until the 1960’s, when a better understanding of the ocean floor lead to the concept of sea floor spreading. Confirmation of sea floor spreading, and additional types of evidence, would eventually vindicate Wegener and lead to the most important unifying concept in geology: the theory of plate tectonics.


Why plates move…Why plates move…The earth is layered The earth is layered with respect to with respect to physical properties.physical properties.

The crust and the The crust and the upper part of the upper part of the mantle is called the mantle is called the LITHOSPHERE and LITHOSPHERE and it is RIGID, BRITTLE it is RIGID, BRITTLE and behaves and behaves ELASTICALLY.ELASTICALLY.

The part of the mantle The part of the mantle directly below the directly below the lithosphere is lithosphere is termed the termed the ASTHENOSPHERE ASTHENOSPHERE which behaves like a which behaves like a PLASTIC - a solid PLASTIC - a solid that flows.that flows.

CONVECTIONCONVECTIONDensity differences causes warmer part of the Density differences causes warmer part of the

plasti-like mantle to rise and the cooler to sink plasti-like mantle to rise and the cooler to sink caussing the “wax on–wax off” circular motioncaussing the “wax on–wax off” circular motion

PLATE TECTONICS INTRODUCEDPLATE TECTONICS INTRODUCEDEarth’s Earth’s lithospherelithosphere, which consists of the earth’s crust and , which consists of the earth’s crust and upper mantle, is cut up into roughly 20 plates that move upper mantle, is cut up into roughly 20 plates that move relative to one another atop of the relative to one another atop of the asthenosphereasthenosphere..

Two Types of Plates1.Oceanic

• Under oceans

• Thin

• Dense and Heavy

• Always Sinks under

2. Continental

• Under continents

• Thick

• Lighter

• Never Sinks

So plates both exist and moveSo plates both exist and move

And the consequences are:And the consequences are:

– EarthquakesEarthquakes

– Volcanic activityVolcanic activity

– MountainsMountains

Types of plate BoundariesTypes of plate BoundariesTransform, Divergent, ConvergentTransform, Divergent, Convergent

Types of boundariesTypes of boundaries

Divergent Convergent Transform

(sea floor spreading) (with subduction) (San Andreas Fault)

Divergent Divergent (normal)(normal)

Convergent Convergent (reverse)(reverse)

Transform Transform (strike-slip)(strike-slip)

•plates are moving apart

•new crust is created

•Magma is coming to the surface

•plates are coming together

•plates are slipping past each other

•crust is not created or destroyed

• No volcanism

Boundaries Summarized

•crust is returning to the mantle

Sea Floor SpreadingSea Floor Spreading

Divergent:Divergent: Sea Floor SpreadingSea Floor Spreading• Mid Atlantic Ocean RidgeMid Atlantic Ocean Ridge

• longest topographic feature on Earth (70,000 km!) • 2-3 km above ocean basins

Divergent:Divergent: Sea Floor SpreadingSea Floor Spreading

Pillow Lava rocksPillow Lava rocks

Evidence that Evidence that the ocean floor is spreadingthe ocean floor is spreading

Magnetic Bands ReversalsMagnetic Bands Reversals– Lava comes up along ridge lines (mostly underwater) as Lava comes up along ridge lines (mostly underwater) as

plates separate.plates separate.– In 76 million years there’ve been 171 reversals of the In 76 million years there’ve been 171 reversals of the

earth’s magnetic field.earth’s magnetic field.– Lava contains iron.Lava contains iron.– Cooling lava locks in the prevailing magnetism.Cooling lava locks in the prevailing magnetism.– The ocean floor near the ridges has the prevailing field The ocean floor near the ridges has the prevailing field

and the floor further from the ridges shows field and the floor further from the ridges shows field reversals: reversals: evidence that the ocean floor is spreading.evidence that the ocean floor is spreading.

Rocks and fossils datingRocks and fossils dating– OlderOlder as one moves as one moves away from ridgesaway from ridges– YoungestYoungest rock is rock is next to the ridgenext to the ridge

Magnetic field reversalMagnetic field reversal• Proves the Earth magnetic field reverses itself Proves the Earth magnetic field reverses itself every 27000 yearsevery 27000 years

• Another proof that the sea floors are spreadingAnother proof that the sea floors are spreading

Age of sea floor as measured by fossilsAge of sea floor as measured by fossils- - OlderOlder as one moves as one moves away from ridgesaway from ridges

- - YoungestYoungest rock is rock is next to the ridgenext to the ridge

Transform Faults Transform Faults Plates move past Plates move past each othereach otherstrike slip faultsstrike slip faults

Example: Example: The San Andreas Fault The San Andreas Fault -California-California

3 Types of plate Collisions3 Types of plate CollisionsOceanicOceanic

VsVs

ContinentalContinental


VsVs


OceanicOceanic

VsVs

OceanicOceanicOceanic subducts Oceanic subducts or Sinksor Sinks

No Subduction No Subduction (mountain (mountain formed)formed)

One will subducts One will subducts or Sinks or Sinks (heaviest)(heaviest)

Fig. 2.11

Convergent Plate BoundariesConvergent Plate BoundariesOcean-Continent Ocean-Continent collision collision oceanic oceanic alwaysalways subducts or sinks subducts or sinks under continentalunder continental

Examples:Nazca plate

vs.South American plate(forming the Andes)

Convergent Plate BoundariesConvergent Plate Boundaries

Continent-Continent Continent-Continent collisioncollision

No SubductionNo Subduction

Example:Indian plate

v.s.Eurasian plate

(forming the Himalayas, Mt. Everest)

Convergent Plate BoundariesConvergent Plate BoundariesOcean-Ocean collisionOcean-Ocean collision

One of them always subducts or sinksOne of them always subducts or sinks

The Pacific Ring Of Fire• A collection of earthquakes and volcanoes that make a ring around the Pacific ocean• It shows the inter-relation of plate tectonics

This map, which shows 20th-century earthquakes (in red), illustrates how they cluster on the edges of the major tectonic plates (outlined in yellow).

Hot Spot Formation of HawaiiHot Spot Formation of Hawaii

Plates MapPlates Map

mr. alfonso 12-2004. plate tectonics - a powerful unifying theory plate tectonics is a relatively...

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