Plate Tectonics

Summary of Important Concepts Plate tectonics refers to the concept that the earth’s outer rigid

rock shell, called the lithosphere, is divided up into a number of separate pieces, called tectonic plates, that are shoved around by the slow convection of hot rock in the underlying asthenosphere.

The movement of the tectonic plates causes many of the major geologic features of the earth’s surface, including earthquakes, volcanoes, mountain ranges, and major features of the ocean floor like mid-ocean ridges and oceanic trenches. The movements of these plates causes the continents to move (drift) over time, opens up new ocean basins, and closes old ones.

Alfred Wegener, in the early 1900’s, was the first person to gather evidence that the continents had moved - a process called continental drift. His evidence suggested that the continents on both sides of the Atlantic Ocean had once been joined into a single large continent he called Pangaea, that gradually split apart over geologic time. In spite of the evidence, his theory was not widely accepted at the time.

Research in the 1960’s revived Wegener’s idea with new data from the ocean floor. Harry Hess proposed that new oceanic crust is created by sea floor spreading at mid-ocean ridges. This sea floor moves along horizontally until it sinks into the mantle at oceanic trenches and is destroyed: a process called subduction.

The concepts of continental drift, sea floor spreading, and subduction are all tied together into the theory of PLATE TECTONICS, in which the lithosphere is split up into a number of moving plates. These plates either:

move apart at divergent boundaries, creating new sea floor by volcanic eruptions at mid-ocean ridges (sea floor spreading);

come together at convergent boundaries, destroying old sea floor by subduction at oceanic trenches, creating volcanoes and mountain ranges in the process;

slide side-by-side at transform boundaries.

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.

PLATE TECTONICS INTRODUCEDEarth’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.Plates Interact: They converge, diverge or slide horizontally past one another.

CONTINENTAL DRIFTAlfred 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.

Wegner’s Evidence for Continental Drift Continents fit together like a puzzle….e.g. the Atlantic coastlines of

Africa and South America. Fossils of plants and animals of the same species found on different

continents. The distribution of climate sensitive sedimentary rocks on the

different continents. Glacial evidence that shows that the land masses were all joined

and partially covered by a single large ice cap over the ancient south pole!

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

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

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.

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

Harry Hess and Sea Floor Spreading 1960’s Hess suggested that the continents may be moving along with the

sea floor, not plowing through it as Wegener suggested. 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 SpreadingPaleomagnetismThroughout 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.

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.

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

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?)

Evidence of Sea Floor SpreadingOcean Floor Ages 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.

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).

Evidence of Sea Floor SpreadingHot Spot Volcanic Islands

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

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.

Evidence of Sea Floor SpreadingInteractions at Plate BoundariesThe majority of earthquakes and volcanic eruptions are concentrated in belts or linear chains at the boundaries of the lithospheric plates.

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

Continent-Continent Divergent Boundaries Continental 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.

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

Ocean-Ocean Divergent Boundaries Mid-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.

CONVERGENT PLATE BOUNDARIESDevelop 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.

Continent-Ocean Convergent 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.

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 Examples include the Appalachian Mountains formed during

the formation of Pangaea, the Himalayas from the collision of India with Asia.

Ocean-Ocean Convergent Boundary 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.

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

TRANSFORM PLATE BOUNDARIESOccurs when two lithospheric plates slide past one another horizontally. Not associated with volcanism or mountain building. Lots of shallow earthquakes.

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. Examples include:

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