Chapter 4 - ESC1000 Flashcards
What was the view held by most geologists prior to the 1960s regarding the geographic positions of the ocean basins and continents?
That ocean basins and continents had a fixed geographic position and were of great antiquity.
Who is Alfred Wegener?
German meteorologist and geophysicist who wrote “The Origin of Continents and Oceans” and proposed the continental drift theory in 1915.
What is continental drift?
A theory which originally proposed that the continents are rafted about. It has essentially been replaced by the plate tectonics theory.
What is a supercontinent?
A large landmass that contains all, or nearly all, of the existing continents.
What is Pangea?
The proposed supercontinent that 200 million years ago began to break apart and form the present landmasses.
What did Wegener hypothesized?
That a single supercontinent he named Pangea began to fragment 200 million years ago during the Mesozoic era and then over a span of millions of years the continents drifted to their present locations.
What evidence did Sir Edward Bullard find to support the continental drift theory?
The continental shelf of South America and Africa which lies submerged below sea level remarkably fit.
What fossil evidence supported the continental drift theory?
Identical fossil organisms had been discovered in rocks from both South America and Africa. Most paleontologists agreed there must have been a type of land connection between both continents.
Which 2 fossils were used as evidence to support the continental drift theory?
- The Mesosaurus, a small aquatic freshwater reptile.
- Glossopteris, a fern which its seeds were to large to be carried by wind.
What geologic features were used to support the continental drift theory?
Wegener found the same highly deformed igneous rocks in Brazil and Africa. Also the mountain belt that includes the Appalachians trends northeastward and disappears at the coast of Newfoundland matches with the structures found at the British Isles and Scandinavia.
What evidence of ancient climates support the continental drift theory?
Wegener found evidence of glaciation in the late Paleozoic era in southern Africa, South America, Australia, and India. Wegener proposed that all these areas where part of Pangea and at that time they were located in the South Pole.
What was the main objection to Wegener’s theory?
His inability to identify a clear mechanism for continental drift.
Which details about continental drift were incorrect?
Continents do not break through the ocean floor and the tidal energy is much too weak to move continents.
What is the theory of plate tectonics?
A well tested 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.
What is the litosphere?
The rigid outer layer of Earth, including the crust and upper mantle.
Compare and contrast the oceanic lithosphere vs the continental lithospere.
The oceanic lithosphere is about 100 km thick while the continental lithosphere is about 150 km thick.
The oceanic crust is composed of basalt whereas the continental crus mainly of granitic rocks; therefore, the oceanic crust is more dense than the continental crust which makes the oceanic lithosphere more dense than the continental lithosphere.
What is the asthenosphere?
A subdivision of the mantle situated below the lithosphere. This zone of weak material exists below a depth of about 100 km (60 miles) and in some regions extends as deep as 700 km. The rock within this zone is easily deformed.
How does rock in the asthenosphere responds to forces?
Because the rock is nearly at melting point it responds to forces by flowing.
How does the rock in the lithosphere respond to forces?
The lithosphere is cool and rigid and tends to respond to forces by bending or breaking but not flowing.
What is the result between the differences in how the asthenosphere and lithosphere respond to forces?
Earth’s rigid outer shell is effectively detached from the asthenosphere, which allows these layers to move independently.
What is a lithospheric plate?
A coherent unit of Earth’s rigid outer layer that includes the crust and upper unit. Also called simply a plate.
What are the 7 major plates?
North American, South American, Pacific, Eurasian, Australian-Indian and Antarctic plates.
What are the 7 intermediate plates?
Caribbean, Cocos, Nazca, Philippine, Arabian, Scotia, and Juan de Fuca plates.
What are the 3 types of plate boundaries?
- Divergent plate boundaries: two plates move apart resulting in upwelling and partial melting of hot material from the mantle to create new seafloor.
- Convergent plate boundaries: two plates move toward each other resulting in oceanic lithosphere descending beneath an overriding plate and eventually absorbed into the mantle or the collision of two continental blocks to create a mountain belt.
- Transform plate boundaries: where to plates grind past each other.
What is a divergent plate boundary?
A region where the rigid plates are moving apart, typified by the mid-ocean ridges. Also called a spreading center.
What is a spreading center?
Another term for a divergent plate boundary.
Where are most divergent plate boundaries situated?
Most are along the crests of oceanic ridges.
What are oceanic ridges?
Elevated areas of the seafloor characterized by high heat flow and volcanism.
What is the oceanic ridge system?
A continuous elevated zone on the floor of all the major ocean basins and varying in width from 500 to 5000 kilometers. The rifts and crests of ridges represent divergent plate boundaries.
What is a rift valley?
A long, narrow trough bounded by normal faults. It represents a region where divergence is taking place.
What is seafloor spreading?
The process of producing new seafloor between two diverging plates.
What is a 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 a 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. Also called a subduction zone.
What are deep-ocean trenches?
An elongated depression in the seafloor produced by the bending of oceanic crust during subduction. Also called simply a trench.
What are the 3 types of convergence zones?
- Oceanic - Continental Convergence
- Oceanic - Oceanic Convergence
- Continental - Continental Convergence
What happens at a oceanic-continental convergence?
The less dense continental plate floats atop the more dense oceanic plate which descends beneath the continental plate into the asthenosphere where it melts at a depth of about 100 kilometers.
What is 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.
What is a continental volcanic arc?
Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent.
What happens at an oceanic-oceanic convergence?
Very similar to the oceanic-continental convergence where one plate subducts beneath the other; however, the oceanic convergences produce volcanic island arcs.
What is a volcanic island arc?
A chain of volcanic islands generally located a few hundred kilometers from a trench where active subduction of one oceanic slab beneath another is occurring.
What happens at a continental-continental convergence?
The two continental plates collide and no major subduction occurs as both plates are moderately buoyant. The collision creates folds and deforms the sedimentary rocks along the continental margins creating a new mountain belt.
What is a transform plate boundary?
A boundary in which two plates slide past one another without creating or destroying lithosphere. Equivalent to a transform fault.
What is a fracture zone?
Any break or rupture in rock along which no appreciable movement has taken place.
What plate is shrinking in size?
The Pacific Plate.
What plates are growing in size?
The African and Antarctic Plates.
What new ocean basin was created by the breakup of Pangea?
The Atlantic ocean.
What evidence is used to support the tectonic plate theory?
Ocean drilling, mantle plumes and hotspots, and paleomagnetism.
Explain the evidence of ocean drilling.
Scientists drilled the ocean floor and tested sedimentary rock along the ocean basin by fossil dating and found that older fossils lie farther away from the ridge crests than more recent fossils thus supporting the seafloor-spreading hypothesis.
Explain the mantle plume and hot spot evidence.
Hot spots are thought to have a relatively fixed position within the mantle. As the Pacific plate moves over this hot spot a chain of volcanic structures known as a hot-spot track was built. The age of each volcano indicates how much time has elapsed since it was situated over the mantle plume. Hawaii is the perfect example of this as Kauai the older modern island presents no activity and the younger island such as the Big Island presents high volcanic activity.
What is a mantle plume?
A mass of hotter than normal 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.
What is a hot spot?
A concentration of heat in the mantle capable of producing magma, which in turn extrudes onto Earth’s surface. The intraplate volcanism that produced the Hawaiian islands is one example.
What is a hot spot track?
A chain of volcanic structures produced as lithospheric plate moves over a mantle plume.
What is the curie point?
The temperature above which a material loses its magnetization.
What is 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. Also called fossil magnetism.
How does paleomagnetism proves the tectonic plate theory?
Scientists plotted the location of the magnetic north pole and it appears to be wandering. The explanation is that the continents drift over the fixed position of the pole and that is why it appears as it is wandering about the Earth. Another evidence is the pattern of paleomagnetism on the ocean seafloor that can only be explained by seafloor spreading. As basaltic rocks form at the oceanic ridges they become magnetized according to Earth’s current magnetic field.
What is a magnetic reversal?
A change in Earth’s magnetic field from normal to reverse or vice versa.
What is normal polarity?
A magnetic field in which the north and south magnetic poles are at the same ends of Earth (relative to the rotational axis) as at present.
What is reverse polarity?
A magnetic field opposite to that which exists at present.
What is the magnetic time scale?
A scale that shows the ages of magnetic reversals and is based on the polarity of lava flows at various ages.
What is a magnetometer?
A sensitive instrument used to measure the intensity of Earth’s magnetic field at various points.
What do transform faults that connect spreading centers indicate about plate motion?
They reveal the direction of plate movement.
What is convection?
The transfer of heat by the movement of a mass or substance. It can take place only in fluids.
What is 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.
What is ridge push?
A mechanism that may contribute to plate motion, which involves the oceanic lithosphere sliding down the oceanic ridge under the pull of gravity.
How does convection flow drive plate movement?
Warm buoyant mantle rocks rise while cool, dense lithospheric plates sink.
How is mantle convection and plate tectonics part of the same system?
Subducting oceanic plates drive the cold downward moving portion of the convection flow while shallow upwelling of hot rock along the oceanic ridge and buoyant mantle plumes are the upward flowing arms of the convective mechanism.
What is the major mechanism for transporting heat away from Earth’s interior to the surface?
Convective flow in the mantle.
What is the whole-mantle convection model?
Cold oceanic lithosphere sinks to great depths and stirs the entire mantle. Suggests the burial ground for the lithospheric slabs is the core-mantle boundary.
What type of plumes have been proposed for the whole-mantle convection model?
Narrow tube like plumes and giant upwellings.
