PLATE TECTONICS - A POWERFUL UNIFYING THEORY

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

Earth’s lithosphere, which consists of the earth’s crust and upper mantle, is cut up into roughly 20 plates that move relative to one another atop of the asthenosphere.

PLATE TECTONICS INTRODUCED

Plates Interact: They converge, diverge or slide horizontally past one another.

Alfred Wegener in the early 1900’s proposed that the continents were once joined together in a single large land mass he called Pangea (meaning “all land” in Greek). He proposed that Pangea had split apart around 200-250 million years ago moving gradually to their present positions - a process that became known as continental drift.

CONTINENTAL DRIFT

The position of the continents today. The continents are still slowly moving, at about the speed your fingernails grow.

Continents fit together like a puzzle….e.g. the Atlantic coastlines of Africa and South America.

Wegner’s Evidence for Continental Drift

Wegner’s Evidence for Continental Drift

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

continents.

Wegner’s Evidence for Continental Drift

Mesosaurus 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.

Wegner’s Evidence for Continental Drift

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

Coal deposits are found abundant in Pennsylvania and Siberia. Why is this unusual?

Wegner’s Evidence for Continental Drift

•Glacial sediment deposits found in places where glaciers do not exist today.

•Glacial Scratches (scratches on rock caused by glacial movement) line up like a jigsaw puzzle when continents are reassembled.

•Both show that the land masses were all joined and partially covered by a single large ice cap over the ancient south pole!

Although today we know that Alfred Wegener was correct about continental drift, at the time his theory was not widely accepted. Wegener never supplied a viable mechanism to explain how continental movement could be accomplished.

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.

Sea floor spreading would eventually vindicate Wegener and lead to the most important unifying concept in geology: the theory of plate tectonics.

Wegner’s Evidence for Continental Drift

The Revival of Continental Drift 1940’s & 1950’s

Work in the 1940’s and 50’s set the stage for the revival of Wegner’s work.

During the 1950’s, intense oceanographic research and technological advancements provided maps of the sea floor showing mid-ocean ridges and deep sea trenches

Hess suggested that the continents may be moving along with the sea floor, not plowing through it as Wegener suggested.

Harry Hess and Sea Floor Spreading 1960’s

Harry Hess and Sea Floor Spreading 1960’s

Harry Hess concluded that new sea floor was being created at mid-ocean ridges (MOR) by volcanic activity. But the earth is not getting larger. Therefore he concluded that an equal amount of oceanic crust is probably being lost at trenches.

The driving force is convection. Locations of spreading ridges (upwelling) and trenches (downwelling) are determined by the convection cells.

Evidence of Sea Floor SpreadingPaleomagnetism

Throughout earth’s time the magnetic north and south have switched RANDOMLY and sporadically every 1000-10,000 years. When magnetism switches, its called a REVERSAL.

Evidence of Sea Floor SpreadingPaleomagnetism

Rocks record the direction of the earth’s magnetic field at the time the rocks form. Small magnetite (Fe) crystals in cooling magma act like compass needles that record the direction of the earth’s magnetic field when the magma solidifies.

Evidence of Sea Floor Spreading

Paleomagnetism

Frederick Vine and Drummond Matthews found that alternating normal and reversely polarized rock form a symmetrical stripe-like pattern parallel to the ridge crest.

Evidence of Sea Floor Spreading

Paleomagnetism

At MOR new sea floor is added and spreads laterally from the axis. As the magma cools and the iron bearing minerals crystallize they align themselves parallel to the lines of force of the earth’s magnetic field.

Therefore, the sea floor is a ticker tape recording of the earth’s magnetism through geologic time. (Only for about ~200million years….why?)

Sediments deposited on the sea floor and radiometric dating of basalt have ages no older than ~200 million years. Anything older has been recycled during subduction…so there is no sea floor older than the last Pangaea (~200-250 mya).

The rocks of the sea floor, are youngest close to the MOR and become progressively older the farther away they are from the ridges on either side. The age pattern is symmetrical across the ridge.

Evidence of Sea Floor SpreadingOcean Floor Ages

Evidence of Sea Floor SpreadingHot Spot Volcanic Islands

Hot spot mantle plumes remain stationary while the lithosphere moves over it. This process forms a chain of volcanic islands.

The chain of islands formed indicates the direction of plate movement over the hot spot.

A hot spot is a persistent volcanic center located directly above a rising plume of hot mantle rock.

Evidence of Sea Floor SpreadingInteractions at Plate Boundaries

The majority of earthquakes and volcanic eruptions are concentrated in belts or linear chains at the boundaries of the lithospheric plates.

PLATE BOUNDARIES

Notice the three different types of plate boundaries. All plate boundaries are associated with either volcanism, earthquakes, or both.

PLATE BOUNDARIES

PLATE BOUNDARIES

Also called spreading centers and rifts; occurs where two plates move apart horizontally and new oceanic lithosphere is created.

Rates of sea floor spreading vary globally (1-17 cm/year).

Divergent Plate Boundaries

Continent-Continent Divergent BoundariesContinental rifting results from upwelling mantle beneath the continent. The continent thins out and is eventually torn apart producing earthquakes and volcanic eruption of basaltic magma. The upward rise of basaltic magma forms new oceanic crust between the two diverging continents.

Continent-Continent Divergent Boundaries

Examples include:The break-up of Pangea,The East African Rift Valley, Basin and Range of Nevada and Utah.

Ocean-Ocean Divergent BoundariesMid-ocean ridges where rising basaltic magma convects upward forming new ocean floor.

Examples include: Mid-Atlantic Ridge spreading ~1 cm/yr;East Pacific Rise spreading ~6 cm/yr.

Develop where two plates are moving horizontally toward each other and therefore are colliding. Can

result in orogenic events (mountain building) or volcanism and deep ocean trenches. Depends on

the plates involved.

Convergent Plate Boundaries

Convergent Plate Boundaries

Oceanic crust is denser (more Fe and Mg) than continental crust.When they collide, the denser oceanic plate will SUBDUCT

beneath that of the lower density continental plate.

Continent-Ocean Convergent Boundary

•Volcanism. Melting of the subducting plate generates a (mafic) magma. The magma, being less dense than the surrounding solid mantle, rises up through the continental (felsic) crust. The end result is a volcanic arc on the continent paralleling the oceanic trench

•Earthquakes.

Continent-Ocean Convergent Boundary

Examples include the Andes Mountains in South America, the Cascade Mountains in Western US

Continent-Continent Convergent Boundary

• Orogeny. When two plates carrying continental crust converge (after all the oceanic crust separating them is consumed by subduction) neither plate will subduct because of low densities. The result is collision and construction of large scale high pointy mountain chains.

• Earthquakes

Continent-Continent Convergent Boundary

Examples include the Appalachian Mountains formed during the formation of Pangaea, the Himalayas from the collision of India with Asia.

Collision of two oceanic slabs will result in the descent of one below the other initiating volcanic activity in a similar manner to ocean-continent collision. In the case of two oceanic plates colliding, the older (colder, denser) oceanic crust subducts.

Ocean-Ocean Convergent Boundary

Volcanism. Forms island arc.

Earthquakes.

Ocean-Ocean Convergent Boundary

Examples include Japan, Aleutian islands, Caribbean islands, Mariana Islands,

Transform Plate Boundaries

• Not associated with volcanism or mountain building.

• Lots of shallow earthquakes.

Occurs when two lithospheric plates slide past one another horizontally.

Ocean-Ocean Transform Boundaries or Ridge-ridge transform boundaries. Major offsets of mid-ocean ridge axis.

Continent-Continent Transform Boundaries.Example:San Andreas Fault

Driving force is not just convection?Ridge Push and Slab Pull

Plates tied to subducting limbs spread faster.

Ridge Push and Slab Pull

Examples include: Mid-Atlantic Ridge spreading ~1 cm/yr;East Pacific Rise spreading ~6 cm/yr.