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Physical Geology: Chapter 2- Plate Tectonics: The Unifying Theory

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Evidence for Continental Drift
- Puzzle fit of the continents
- Similar rock assemblages across oceans
- Similar rock ages across oceans
- Distribution of fossils across oceans
Eduard Suess
An Austrian geologist who postulated that the present-day southern continents had once formed a single giant continent called Gondwana (or Gondwanaland)
Alfred Wegener
wrote a book on the breakup and drift of continents, in which he laid out the remarkable similarity of geologic features on opposite sides of the Atlantic (Figure 2.1). In the years that followed, Wegener postulated a supercontinent, which he called Pangaea (Greek for "all lands"), that broke up into the continents as we know them today.
Glomar Challenger Expedition
• 20,000 nautical miles
• 44 holes drilled
• 1,000 cores
• Deep Sea Drilling
Project retrieved 145,728 feet of sediment and ocean floor
-"unequivocally established" the geological youthfulness of oceanic crust in comparison to most continental rocks
• helped confirm that sea-floor spreading and widespread crustal motions occurred
mid-ocean ridges
New crust is formed, look like zippers
Sea Floor Spreading
The mechanism by which new oceanic crust is formed at a spreading center on the crest of a mid-ocean ridge. As two plates move apart, magma wells up into the rift between them to form new crust, which spreads laterally away from the rift and is replaced continually by newer crust.
Plate Tectonic Theory
The theory that describes and explains the creation and destruction of Earth's lithospheric plates and their movement over Earth's surface. (From the Greek tekton, meaning "builder.")
The Lithosphere
The lithosphere is not a continuous shell, but is broken into a mosaic of rigid plates that move over Earth's surface. Each plate travels as a distinct unit, riding on the asthenosphere, which is also in motion.
How many major plates are there?
13
Divergent boundary
A boundary between lithospheric plates where two plates move apart and new lithosphere is created.
Convergent boundary
A boundary between lithospheric plates where the plates move toward each other and one plate is recycled into the mantle.
Transform fault
A plate boundary at which the plates slide horizontally past each other and lithosphere is neither created nor destroyed.
Continental crust
1. Because continental crust is lighter, it is not as easily recycled back into the mantle as oceanic crust.
2. Because continental crust is weaker, plate boundaries that involve continental crust tend to be more spread out and more complicated than those that involve oceanic crust.
Ocean spreading centers
On the seafloor, the boundary between separating plates is marked by a mid-ocean ridge, an undersea mountain chain that exhibits earthquakes, volcanism, and rifting, all caused by the tensional (stretching) forces of mantle convection that are pulling the two plates apart.
Plates
Plates = lithosphere (crust + upper mantle)
• Float on asthenosphere (rest of mantle)
Oceanic plate separation
rifting, volcanoes, and earthquakes (mid-alantic ridge)
Continental plate separation
rift valleys, volcanoes, and earthquakes (East African rift valley)
Ocean-ocean convergence
oceanic trench, volcanic island arc, and deep earthquakes (marianas trench)
Ocean-continent convergence
volcanic mountain chain, folded mountains, and deep earthquakes (Andes mountains)
Continent-continent convergence
crustal thickening, folded mountains, and earthquakes (Tibetan plateau, Himalaya mountains)
Mid-ocean ridge transform fault
lateral (transform) faults and earthquakes
Continental transform fault
lateral (transform) fault and earthquakes (san andreas)
Geomagnetic reversal
change in the Earth's magnetic field such that the positions of magnetic N and S change.
• Alternate between periods (chrons) of normal polarity and reverse polarity.
• Last reversal occurred 780,000 yrs ago.
• Brief disruptions are called geomagnetic excursions.
Geomagnetic reversal
due to dynamo action in which convection of molten iron core generates electric currents that give rise to magnetic fields
• Magnetic field is weaker during reversals • Field protects us from the solar wind • Can use the records of reversals to create
a time scale • Oldest ocean floor ~180 m.y.
Two major supercontinents
Rodinia
• Existed 1.1 billion and 750 million years ago • Russian for Motherland
- Pangea
• Rodinia breaks up to form Pangea • Pangea forms 237 million years ago • Greek for Entire Earth
Subduction
The sinking of oceanic lithosphere beneath overriding oceanic or continental lithosphere at a convergent plate boundary.
Theory 1
Whole mantle convection: Plate recycling extends to the core-mantle boundary.
Theory 2
Stratified convection: The lower mantle convects more sluggishly than the upper mantle. Boundary near 700 km separates the two different convection systems.
The Engine of Plate Tectonics
spreading centers and hot spots
isochron
A contour that connects rocks of equal age
What is the theory of plate tectonics?
According to the theory of plate tectonics, the lithosphere is broken into about a dozen rigid plates that move over Earth's surface. Three types of plate boundaries are defined by the direction of the movements of plates in relation to each other: divergent, convergent, and transform-fault boundaries. Earth's surface area does not change over time; therefore, the area of new lithosphere created at divergent boundaries equals the area of lithosphere recycled at convergent boundaries by subduction into the mantle.
What are some of the geologic characteristics of plate boundaries?
Many geologic features develop at plate boundaries. Divergent boundaries are typically marked by volcanism and earthquakes at the crest of a mid-ocean ridge. Convergent boundaries are marked by deep-sea trenches, earthquakes, mountain building, and volcanism. Transform faults, along which plates slide horizontally past each other, can be recognized by earthquake activity and offsets in geologic features.
How can the age of the seafloor be determined?
We can measure the age of the seafloor by using thermoremanent magnetization. Magnetic anomaly patterns mapped on the seafloor can be compared with a magnetic time scale that was established using the magnetic anomalies of lavas of known ages on land. Seafloor ages have been verified through dating of rock samples obtained by deep-sea drilling. Geologists can now draw isochron maps for most of the world's oceans, which allow them to reconstruct the history of seafloor spreading over the past 200 million years. Using this method and other geologic data, geologists have developed a detailed model of how Pangaea broke apart and the continents drifted into their present configuration.
What is the engine that drives plate tectonics?
The plate tectonic system is driven by mantle convection, the energy for which comes from Earth's internal heat. Gravitational forces act on the cooling lithosphere as it slides downhill from spreading centers and sinks into the mantle at subduction zones. Subducted lithosphere extends as deep as the core-mantle boundary, indicating that the whole mantle is involved in the convection system that recycles the plates. Rising convection currents may include mantle plumes, intense jets of material from the deep mantle that cause localized volcanism at hot spots in the middle of plates.
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