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PLATE TECTONICS

Continental DriftE.B. Taylor (1910) and Alfred Wegener(1915) published on Continental Drift.

Continental DriftWegener’s evidence

1. Fit of the Continents2. Fossil Evidence3. Rock Type and Structural Similarities4. Paleoclimatic Evidence

Continental Drift- Wegener’s EvidenceIn the 1960's, it was

recognized that the fit of the continents could be even further improved by fitting the continents at the edge of the continental slope — the actual extent of the continental crust.

Continental DriftWegener’s evidence

Fossils - Mesosaurs

Continental Drift – Wegener’s Evidence, fossils

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Continental Drift – Wegener’s EvidenceRocks and Geologic Structures

Same rock typesSame orderSame faults and foldsSame age300my old orogeny

Continental Drift- Wegener’s EvidenceClimates

Evidence of glaciationUnlikely placesBetter explained by fit

Continental Drift – Wegener’s EvidenceClimate

Coal DepositsShould be in warm temperate areas or equatorial areasBetter explained by fit

Continental DriftWegener lacked a mechanism for continental movement through oceanic crust.

Plate TectonicsEvidence from exploration of the ocean floor

Global oceanic ridge system and guyotsPaleomagnetic stripesPolar wanderingDeep ocean trenches associated with earthquakesAll seafloor less than 180my oldSediments thin where expected to be thick

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Plate TectonicsHarry Hess (1906-1969)

Surveyed ocean floorDiscovered guyotsHypothesized sea-floor spreading

The Ocean Floor

Plate Tectonics – the ocean floorThe mid-ocean ridge system is a nearly

continuous volcanic ridge found in all oceans.

There is nearly continuous volcanic activity somewhere along the mid-ocean ridge system.

Undersea volcano photographed from Japanese aircraft

Sea Floor Spreading

The Ocean Floor - guyots Sea Floor Spreading

A process known as seafloor spreading occurs where magma from the mantle wells up into the divergent boundary - forming new basaltic seafloor. Spreading rates average ~5 cm/year.

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Sea Floor SpreadingThe mid-ocean ridge has an elevated position on the seafloor because it is formed from relatively hot igneous rocks.

Plate Tectonics- Sea Floor Spreading

As the seafloor moves away from the ridge, it cools and contracts — thus the seafloor generally is at a greater depth as you move away from the mid-ocean ridge.

Sea Floor Spreading Plate Tectonics – Sea Floor SpreadingFred Vine and D.H. Mathews

Interpreted “magnetic stripes”Earth has experienced periods of reverse polarityAs magma solidifies it is magnetized according to the polarity at the timeStripes of magnetic polarity are formed at mid-oceanic ridges

Magnetic reversalsThe figure shows that the Earth’s magnetic field is currently oriented so that magnetic lines of force are entering the Earth near the north pole.

It has been recognized that the Earth’s magnetic field has reversed polarity many times in the past - known as magnetic reversals.

Magnetic Stripes

In the figure, regions of normal polarity are indicated in white where magnetic north is coincident with the geographic north.

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Magnetic Stripes

Stripes are parallel to mid-oceanic ridgeStripes are mirror images across ridgeDates of stripes indicate sea floor spreading

Evidence: Polar Wandering – Keith Runcorn PLATE TECTONICS

Sea-floor spreading provided the missing mechanism for Continental Drift

Plate TectonicsIf the sea floor is spreading, and the earth is not expanding, where does the excess crust go?Subduction zones

Subduction ZonesStudies of crust around deep sea trenches showed zones of earthquakes beneath the crust called “Wadati-Benioff Zones”

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Heat and heat flow studies around trenches revealed a “cold slab”

Plate Tectonics – Subduction Zones

The surface expression of a subduction zone is a deep-ocean trench - these trenches maybe thousands of km long, 50-100 km wide, and 8-12 km deep.

Earthquake epicenters revealed plate boundaries.

Plate tectonics is a theory about how the surface of the Earth evolves due to strong internal forces. The surface of the Earth is composed of rigid plates that are mobile and move relative to one another.

Plate tectonics is a unifying theory in Geology — different geologic phenomena such as mountain building, earthquakes, volcanoes, and the distribution of fossils and organisms can be explained through plate tectonics.

Earth’s Major Plates

The Earth’s surface is composed of a strong, rigid layer known as the lithosphere. The lithosphere is broken into pieces known as tectonic plates.

Lithospheric plates are thinnest in the oceans (<100 km thick) and may be more than 250 km thick on the continents. There are 7 major plates and over a dozen smaller plates.

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The lithospheric platesoverlie a weaker region of the mantle known as the asthenosphere.

The rocks in the asthenosphere are near their melting point and are relatively weak and ductile.

The asthenosphereallows the plates to move above it. Plates move slowly but continuously - generally on the order of a ~5 cm/year.

Note that the plates generally include a continent or a portion of a continent AND a portion of the ocean floor.

Each plate moves as a coherent unit relative to others

Plate Tectonics- Plate BoundariesPlate Boundaries are where earthquakes, volcanoes, and crustal deformation take place.There are three general types of boundaries:

Divergent –Plates move away from each otherConvergent- Plates move toward each otherTransform – Plates move past each other

Divergent Boundaries

Most divergent boundaries are located along mid-ocean ridges. Divergent plate boundaries are known as constructive margins because they are the site where new oceanic crust (lithosphere) is generated.

The process known as seafloor spreading occurs where magma from the mantle wells up into the divergent boundary -forming new basaltic seafloor.

Spreading rates average ~5 cm/year.

Some divergent boundaries occur under continental crust

The East African Riftrepresents a modern example of a continental rift. If this rift is successful, eastern Africa may split off from the rest of the continent and a new ocean basin may form between the two “Africas.”

This figure illustrates the different major types of plate boundaries.We just considered two types of divergent boundaries: mid-ocean ridges and continental rifts. Now we will consider the different types of convergent boundaries.

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Convergent margins are also known as destructive margins since oceanic crust is destroyed or consumed. Most convergent margins are associated with a subduction zone

The map shows the world’s oceanic trenches. Note that the Pacific isNearly encircled in deep-ocean trenches.

Convergent Boundaries: Oceanic-Continental Convergence

Oceanic-continentalconvergence occurs when leading edge of one plate is composed of continental rocks (granitic) and the other is oceanic (basaltic).

The denser oceanic plate dives beneath (subducts) the lower-density continental plate. Lower density granitic rocks tend to float in the asthenosphere.

Dewatering of the subducted slab causes melting in the wedge of the asthenosphere above it. The magma that is produced is buoyant and rises through the mantle toward the Earth’s surface.

The magma that is produced in the asthenosphere is basaltic in composition.

As the magma rises, it must penetrate through the thick continental (granitic) rocks.

As it assimilates the continental rocks, the composition of the magma changes from mafic to intermediate.

The magma results in volcanic activity along a line parallel to the subduction zone known as a continental volcanic arc.

Examples of continental volcanic arcs include the Cascade volcanoes such as Mt. Ranier and Mt. St. Helens and the volcanoes of the Andes mountains along the west coast of South America.

Mt. St. Helens

Convergent Boundaries: Oceanic-Oceanic ConvergenceOceanic-oceanic convergenceoccurs when the leading edge of both plates consists of oceanic crust. These plate boundaries have many of the same features as in oceanic-continental convergence. In oceanic-oceanic convergence, the line of volcanoes forms a string of islands parallel to the subduction zone known as a volcanic island arc.

Examples of island arc systems include the Aleutian Islands, Tonga, Indonesia, and Japan.

Ocean to Ocean ConvergenceNOTE: Oceanic crust subducts other oceanic crust

Volcanoes form islandsComposition of magma is mafic to intermediate

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Continent-continent convergence usually begins as oceanic-continental convergence (ex. Andes). As the oceanic crust is subducted, a continental block on the subducting plate may approach the continent.

The Himalayan mountains were formed by the collision of the Indian subcontinent into the Asian mainland.

These figures show the convergence of India into Asia over the last 71 million years.

Continent to Continent Collision

Uplifted continental crust

No volcanoes

Folding and faulting

NOTE:

Convergent Boundaries: Continental-Continental ConvergenceContinental-continental convergence defines a plate margin where the leading edge of both plates contains continental crust.

This type of plate boundary is associated with mountain-building.

Transform Fault Boundaries

Transform plate boundariesare where plates slide past one another.

Most transform boundaries are associated with mid-ocean ridges where they form linear breaks in the ridge system.

The active transform boundaryexists between the two offset ridge segments

Transform Faulting

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Evidence Supporting Plate TectonicsOverall pattern of volcanic and earthquake activityPolar Wandering

Evidence from Deep Sea Drilling

Evidence from Hot Spots

Evidence: Patterm of volcanoes, trenches, ridges, earthquakes.

Evidence: Ocean DrillingAn active program of sampling and drilling in the seafloor has provided considerable evidence in support of plate tectonics.The figure shows the age of the seafloor. The pattern is as is predicted by the theory of plate tectonics.

The seafloor is very young at the mid-ocean ridges and gets progressively older as a function of distance from the ridge.

Evidence: Ocean DrillingThe figure shows the thickness of sediment on the seafloor throughout the ocean basins.

The seafloor at the mid-ocean ridges is young and has essentially no sedimentary cover. Generally, the sedimentary cover increases with distance from the mid-ocean ridge.

Evidence: Hot SpotsMapping of the seafloor indicates that there are linear chains of volcanic islands structures known as seamounts.

The Hawaiian Islands are part of a chain of islands and seamounts extending to the Aleutian trench.

The Big Island of Hawaii is the only island in the chain with active volcanism.

Radiometric dating of the volcanic rocks of this chain indicate that they get progressively older as a function of distance from the Island of Hawaii.

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The origin of the volcanic islands and seamounts of this and other chains is from an anomalously hot portion of the mantle known as a hot spot that remains relatively stationary.

As the Pacific plate moves over the hot spot, volcanoes are formed from magmas generated by the hot spot. As the plate continues to move, the volcano moves off of the hot spotand becomes extinct but is replaced by a new one directly above the hot spot.

These observations are consistent with the theory of plate tectonics and support it.

The Breakup of PangaeaNow that we understand plate tectonics, we can use geologic data to reconstruct Pangaea and model the movement of the continents during the last 200 million years.

By about 150 m.y. ago, the N.Atlantic began to open.

~130 m.y. ago, the S. Atlantic was opening and India began its journey north toward Asia.

~44 m.y. ago, India began to collide with Asia forming the Himalayan Mtns.

Continental Drift animation

What Drives Plate Motions?

We have described plate motions but have not really defined the forces that drive the plates to move. This is an active area of research and there is a diversity of opinions. Most geologists agree on the following points about the driving forces for plate motion:

1. The Earth’s mantle is convecting - hotter rocks rise buoyantly and cooler denser rocks sink. This motion helps drive plate motion.

2. Mantle convection and plate tectonics are part of the same system.

3. Density differences due to the unequal distribution of heat within the Earth’s mantle ultimately drive the mantle convection cells and plate motion.