plate tectonics (2) Flashcards
Plate Tectonics
A theory which proposes that Earth’s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself
Describe the view held by most geologists prior to the 1960s regarding the ocean basins and continents.
They believed that the continents didn’t move
Name the 20th century hypothesis that was at first rejected and then the theory that later replaced it.
Continental Drift Hypothesis
The Theory of Plate Tectonics
Continental Drift Hypothesis
A hypothesis, credited largely to Alfred Wegener, which suggests that all present continents once existed as a single supercontinent. Further, beginning about 200 million years ago, the supercontinent began breaking into smaller continents, which then “drifted” to their present positions
Supercontinent
A large landmass that contains all, or nearly all, of the existing continents
Pangaea
The proposed supercontinent that 200 million years ago began to break apart and form the present landmasses
What was the first line of evidence that led earthly investigators to suspect that the continents were once connected?
Shape
Explain why the discovery of the fossil remains of Mesosaurus in both South America and Africa, but nowhere else, supports the continental drift hypothesis.
Its a freshwater reptile, it lived in lakes not in oceans
In the early 20th century view of how land animals apparently migrated across vast expanses of open ocean?
They walked across skinny land bridges or floated on rafts
Describe two aspects of Wegener’s continental drift hypothesis that were objectionable to most Earth scientists
Mechanism of continental drift
Continents were moving independent of oceanic crust
Theory of Plate Tectonics
A theory which proposes that Earth’s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself
Lithosphere
The rigid outer layer of Earth, including the crust and upper mantle
Asthenosphere
A subdivision of the mantle situated below the lithosphere. This zone of weak material exists below a depth of about 100 km and in some regions extends as deep as 700 km. The rock within this zone is easily deformed
Lithospheric Plate
A coherent unit of Earth’s rigid outer layer that includes the crust and upper unit
Divergent Plate Boundaries
Where two plates move apart
Convergent Plate Boundaries
Where two plates move toward each other
Transform Plate Boundaries
Where two plates grind past each other
What new findings about the ocean floor did oceanographers discover after World War II?
The seafloor is quite young, the sediment that was deposited was relatively thin-> seafloor is being recycle
Divergent Plate Boundary
A boundary in which two plates move apart, resulting in upwelling of material from the mantle to create new seafloor
Ocean Ridge System
A continuous elevated zone on the floor of all the major ocean basins and varying in width from 500 to 5000 km. The rifts at the crests of ridges represent divergent plate boundaries.
Rift Valley
A long, narrow trough bounded by normal faults, It represents a region where divergence is taking place.
Seafloor Spreading
A hypothesis, first proposed in the 1960s by Harry Hess, which suggests that new oceanic crust is produced at the crests of mid-ocean ridges, which are the sites of divergence
Continental Rift
A linear zone along which continental lithosphere stretches and pulls apart. Its creation may mark the beginning of a new ocean basin
What is the average rate of seafloor spreading in modern oceans?
2 cm/yr in Atlantic
15 cm/yr in Pacific
Average: 5 cm/yr
List four features that characterize the oceanic ridge system.
Long, elevated, linear, magma rising
Convergent Plate Boundary
A boundary in which two plates move together, resulting in oceanic lithosphere being thrust beneath an overriding plate, eventually to be reabsorbed into the mantle. It can also involve the collision of two continental plates to create a mountain system
Subduction Zone
A long, narrow zone where one lithospheric plate descends beneath another
Deep-ocean Trench
A narrow, elongated depression of the seafloor
Partial Melting
The process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results
Continental Volcanic Arc
Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent. Examples include the Andes and the Cascades.
Volcanic Island Arc
A chain of volcanic islands generally located a few hundred kilometers from a trench where there is active subduction of one oceanic plate beneath another.
Why does oceanic lithosphere subduct, while continental lithosphere does not?
Density, age determines temperature
old is cold and dense-> subducts
What characteristic of a slab of oceanic lithosphere explains the formation of a deep oceanic trench as opposed to one that is less deep?
Density
The warmer it is the more buoyant it is
Deep trenches with deep subduction
Transform Plate Boundary
A boundary in which two plates slide past one another without creating or destroying lithosphere
Fracture Zones
A linear of irregular topography on the deep-ocean floor that follows transform faults and their inactive extensions
Mantle Plume
A mass of hotter-than-typical mantle material that ascends toward the surface, where it may lead to igneous activity. These plumes of solid yet mobile material may originate as deep as the core-mantle boundary
Hot Spot
A concentration of heat in the mantle, capable of producing magma that, in turn, extrudes onto Earth’s surface. The intraplate volcanism that produced the Hawaiian Islands is one example.
Hot-Spot Track
A chain of volcanic structures produced as a lithospheric plates moves over a mantle plume
Curie Point
The temperature above which a material loses its magnetization
Paleomagnetism
The natural remnant magnetism in rock bodies. The permanent magnetization acquired by rock that can be used to determine the location of the magnetic poles and the latitude of the rock at the time it became magnetized
Magnetic Reversal
A change in Earth’s magnetic field from normal to reverse or vice versa
Normal Polarity
A magnetic field the same as that which presently exists
Reverse Polarity
A magnetic field opposite that which presently exists
Magnetic Time Scale
A scale that shows the ages of magnetic reversals and is based on the polarity of lava flows of various ages
Magnetometer
A sensitive instrument used to measure the intensity of Earth’s magnetic field at various points
What does the orientation of transform faults indicate about plate motion?
Parallel
Which three plates appear to exhibit the highest rates of motion?
Pacific
Australia-India
Nazca
Convection
The transfer of heat by the mass movement or circulation of a substances
Slab Pull
A mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along
Ridge Push
A mechanism that may contribute to plate motion. It involves the oceanic lithosphere sliding down the oceanic ridge under the pull of gravity
Which of these forces-slab pull or ridge push-contributes more to plate motion?
Slab pull (subduction zones)
Describe the whole-mantle convection (plume) model.
Plumes rise from the core-mantle boundary
Subducted plates end up at the bottom of the mantle
