CH. 2: Plate Tectonics Flashcards
Which of the following issues initially prevented the widespread acceptance of Alfred Wegener’s continental drift hypothesis?
Wegener’s lack of a good mechanism to explain why the continents moved
Seafloor spreading is driven by volcanic activity that occurs:
along mid ocean ridges
In the Atlantic Ocean, where does the youngest oceanic crust occur?
adjacent to the mid ocean ridge
Based on the map image below, where do you think the spreading rate is greatest?
in the Pacific Ocean
As the map above shows the oldest seafloor is generally less than 200 million years old. Why?
Most oceanic lithosphere eventually gets subducted beneath the continents by the time it’s 200 million years old because the older it gets, the colder and denser it gets
Where would you expect to find volcanoes in an oceanic–continental convergence zone?
always on the continental plate
Why are rocks that formed in an oceanic environment found in the highest parts of the Himalayan Mountains today?
ocean sediments were trapped and deformed during the continent-continent collisions that formed the Himalayan Mountains
Many people say that California is going to fall off the face of the Earth, using what we know about plate tectonics, which of the following is most likely?
the western most edge of California and the Baja Peninsula will slowly move north relative to the rest of the North American continent
Are the vertical or horizontal lines the transform boundary?
horizontal
Magnetic anomalies result from the seafloor spreading in conjunction with:
magnetic polarity reversals
What is the significance of the ages of the Hawaiian Islands in terms of plate tectonics?
the ages decrease toward the big island of Hawaii, showing that the Pacific plate is moving NW over a stationary hot spot
Convection works because _____
cold material is denser than the same material when it is hot
Which one of these processes does NOT contribute to plate motions?
tidal forces
What is a mantle plume?
an upwelling of hot mantle from the Earth’s interior that is cylindrical in shape
Why can’t scientists retrieve information about the Atlantic Ocean basin that is older than roughly 200 million years?
there was no Atlantic Ocean 200 million years ago
Which of the following is NOT a piece of evidence Alfred Wegener used to describe his hypothesis of Continental Drift?
paleomagnetism
Use the map of GPS velocities below and indicate which of the following plates is moving slowest
Caribbean
A typical rate of plate motion is:
3 centimeters/year
Where in the ocean does oceanic lithosphere sink into the mantle?
at ocean trenches
The major difference between oceanic-oceanic convergence and oceanic-continental convergence is that:
oceanic-oceanic convergence produces A VOLCANIC ISLAND ARC and oceanic continental convergence produces A CONTINENTAL VOLCANIC ARC
Match the following geologic features on Earth with their tectonic boundaries:
- Appalachian Mountains
- Andes Mountains
- The Arabian Peninsula and Madagascar
- San Andreas Fault
- continent-continent
- continent ocean
- continental rift
- transform
Refer to the following hypothetical plate map below to answer the following questions
How many tectonic plates are shown on this map?
Which plates A,B, and/or C likely have active volcanoes?
In which direction is continent A moving relative to the adjacent subduction zone?
- 5
- A & B
- Southeast
Imagine that you are studying seafloor spreading along two different mid oceanic ridges. Using data from a magnetometer, you produced the two following maps. From these maps, what can you determine about the relative rates of seafloor spreading along these two ridges?
Spreading Center A produces new oceanic crust faster than B
When considering evidence of glaciation on the southern continents, why did Wegener reject the explanation that the entire planet had experienced a period of extreme cooling and was covered in glaciers 250 million years ago?
Because geologic evidence supported the existence of tropical swamps in the Northern Hemisphere at the same time as the glaciers existing in the Southern continents
The former late Paleozoic (250 million years) age supercontinent is known as:
Pangaea
Rigid lithosphere plates move ____
over the more ductile asthenosphere
Match the following description of relative motion at plate boundaries with their boundary type:
- lithospheric plates move toward each other
- lithospheric plates slide past each other
- lithospheric plates move away from each other
- convergent
- transform
- divergent
When Earth’s magnetic field reverses polarity, it’s recorded in ____ formed at that time. Paleomagnetic researchers of the find fairly ____ patterns of alternating magnetism that parallel ___. Such a pattern forms when an older crust is carried away in an opposite directions by ____.
- oceanic crust
- symmetric
- mid-ocean ridges
- sea-floor spreading
Which of the following statements regarding transform plate boundaries is not correct?
New oceanic crust is formed at transform plate boundaries.
- Crust is neither created nor consumed at transform plate boundaries.
Continued movement of the continental crust on the west side of the San Andreas fault could result in which of the following features?
an island
- As the continental crust on the west side of the San Andreas continues to migrate in a northwest direction, it could separate from North America and form an island.
Which of the following geologic processes and/or phenomena is not found at transform plate boundaries?
volcanoes
- Volcanic activity characterizes both divergent and convergent boundaries, but is not found at transform boundaries because transform boundaries do not cause crustal melting to produce magma.
Which tectonic boundary results when two tectonic plates pull apart from each other?
divergent boundaries
- Divergent boundaries form because of tensional stress.
Divergent boundaries are not one, smooth ridge of separation. Instead, they are broken into a series of segments, with different segments of fault offset from others. What features allow for the offset of divergent boundaries?
transform faults
- Transform faults account for the lateral offset that is seen along divergent boundaries.
Crustal rifting is currently occurring in East Africa. Which of the following features would you expect to find there?
All of the above are correct
normal faults + long/wide valley + lava flows + only normal faults and long, wide valleys
- All of these physical features are evidence that the landscape is experiencing tensional stress from rifting.
Which type of tectonic boundary is the result of two tectonic plates colliding with each other?
convergent boundary
- Convergent boundaries form as a result of compressional forces pushing two plates together.
At a subduction zone where oceanic lithosphere meets continental lithosphere, which would subduct and why?
The oceanic lithosphere would subduct because it has a higher density at 3.0 grams per cubic centimeter.
- Oceanic lithosphere, which is basaltic, has a higher density than continental lithosphere.
Why are volcanic chains associated with convergent boundaries?
partial melting occurs due to subduction
- Partial melting is triggered by water loss in the plate. This melts mantle material, which rises to the surface.
Plate A has oceanic lithosphere on its leading edge that is 10 million years old. Plate B has oceanic lithosphere on its leading edge that is 200 million years old. Which plate will be subducted if they collide with each other, and why?
Plate B will subduct because it is older—and therefore colder—and more dense.
- The older an oceanic plate is, the colder it is, making it more dense.
In a collision between two continental plates, can a continental plate subduct? Why or why not?
Continental plates are too buoyant to subduct.
- Continental lithosphere has a density of 2.7 grams per cubic centimeter, making it unlikely to subduct.
Which type of tectonic boundary is the result of two tectonic plates sliding past each other?
transform boundary
- Transform boundaries are created through shear stress, which causes the two plates to slide past each other.
Which fault in Southern California was created by the sliding of the Pacific Plate past the North American Plate?
San Andreas Fault
- The San Andreas Fault starts at the Gulf of California and passes through southern California before leaving the continent near San Francisco.
The western edge of the Juan de Fuca Plate is a divergent boundary with a ragged, zigzag edge. (See the figure.) Which of the following describes the motion of the Juan de Fuca plate on its southern boundary at the Mendocino Fault?
The Juan de Fuca Plate has right-lateral motion with the Pacific Plate along the transform Mendocino Fault.
- As new seafloor moves away from the Juan de Fuca Ridge, it slides past the Pacific Plate at the Mendocino Fault.
What do you think will happen to the location of Los Angeles with continued activity along the San Andreas Fault over a long time span?
Los Angeles will slide to the northwest
- Los Angeles is on the Pacific Plate, which is currently moving toward the northwest relative to the North American Plate.
At a divergent boundary, what is the relative motion of the plates on each side of the boundary?
the plates away from each other
- At divergent plate boundaries, the plates move away from the boundary.
Which of the following statements about divergent boundaries is true?
new ocean crust is created at a divergent boundary
- At divergent plate boundaries, new ocean crust is created.
Movement of plates away from each other at divergent boundaries typically shows which of the following average velocities?
5 cm per year
- At divergent boundaries, plates typically move away from each other at a rate of about 5 cm per year.
When moving away from a divergent boundary in either direction, which of the following statements is true?
the rocks increase in age
- Since new ocean crust is created at the spreading center, as rocks move away, they are older.
Transform boundaries are classified under which type of fault?
strike-slip
What role do transform boundaries play?
Transform boundaries connect other segments of plate boundaries.
Do fracture zones near transform boundaries play a role in plate motion?
No, fracture zones play no role in plate motion
What is the relationship between transform boundaries and mid-ocean ridges?
Transform boundaries connect segments of mid-ocean ridges.
Which of the following lithospheric plates is not included among the seven largest?
the Philippine plate
- The Philippine plate is one of the intermediate-sized lithospheric plates.
Earth’s lithospheric plates are composed of crust and which of the following layers?
the outermost portion of the mantle
- Earth’s lithosphere consists of the crust and the outermost portion of the mantle.
Which of the following processes can be observed at the margins of lithospheric plates?
All of the responses are correct.
faulting + earthquakes + volcanoes + mountain building
- At the margins of lithospheric plates, earthquakes, faults, volcanoes, and mountains occur where the plate edges interact.
In studies of rock obtained from ocean basins all over Earth, the oldest ages obtained are approximately 200 million years before the present. Why have no older oceanic rocks been discovered?
Due to tectonic activity, rocks on the ocean floor are continually recycled.
- As the lithospheric plates move around, ocean floor is constantly being consumed.
Currently the Juan de Fuca plate is interacting with the North American plate where inland volcanoes and deep focus earthquakes are occurring. What type of plate boundary is this?
a convergent plate boundary
- The boundary that exists between the Juan de Fuca plate and the North American plate is a convergent plate boundary.
Which of the following is a layer of the Earth that is classified NOT by composition, but by physical properties?
asthenosphere
- The asthenosphere is a solid, but mobile part of the upper mantle.
Oceanic lithosphere contains oceanic crust and the __________.
outermost part of mantle beneath the oceanic crust
- Both oceanic and continental lithosphere contain material from the outer, rigid part of the upper mantle.
How is the thickness of the lithosphere going to change as it moves away from a divergent plate boundary?
Lithosphere will thicken as more mantle is added.
- Cooling of the underlying asthenosphere will add rigid material to the lithosphere.
What is the main material being subducted at an ocean-continent convergent boundary?
oceanic lithosphere
- Oceanic lithosphere is more dense than continental lithosphere, so when the two meet, the one that is denser will be subducted.
What are two differences between oceanic crust and continental crust?
The continental crust is thicker and less dense than oceanic crust.
- It is the differences in density and thickness that characterize the two types of crust and control that will be subducted.
What causes the volcanoes and deep valleys of East Africa?
A continental rift along parts of the African continent beginning to slowly separate
- A continental rift along parts of the African continent beginning to slowly separate causes the volcanoes and deep valleys in East Africa.
A modern example of an active continental rift is the East African Rift. The East African Rift illustrates the initial stages in the breakup of a continent. At the East African Rift, tensional forces have stretched and thinned the crust, allowing molten rock to ascend from the mantle.
Which of the following properly describes Mount St. Helens and the other Cascade volcanoes?
They are young, active volcanoes built on a continental margin above a sinking slab of oceanic lithosphere.
- The Cascade Range in Washington, Oregon, and California is referred to as an active continental volcanic arc that is produced, in part, by volcanic activity associated with the subduction of oceanic lithosphere. The Cascade Range consists of several well-known volcanic mountains, including Mount Rainier, Mount Shasta, and Mount St. Helens.
What type of boundary occurs along the Aleutian Islands in the North Pacific?
A convergent boundary on a volcanic arc above the northward-subducting Pacific plate
- Where two plates converge, one descends beneath the other. In this setting, volcanoes grow up from the ocean floor. When subduction is sustained, it will eventually build a chain of volcanic structures large enough to emerge as islands. The newly formed land consisting of an arc-shaped chain of volcanic islands is called a volcanic island arc, or simply an island arc. The Aleutian Islands are an example of relatively young volcanic island arcs located in the western Pacific.
What type of plate boundary is usually associated with pull-apart rift zones?
A divergent plate boundary
- At convergent plate boundaries, two plates move towards each other; the leading edge of one is bent downward as it slides beneath the other. At transform plate boundaries, plates slide horizontally past one another without the production or destruction of lithosphere.
Which of the following is an example of an active continent-continent collision?
The northward movement of India into Eurasia
- An example of an active, continent-continent collision began about 50 million years ago when the subcontinent of India “rammed” into Asia, producing the Himalayas—the most spectacular mountain range on Earth.
Where in the ocean does oceanic lithosphere sink into the mantle?
At subduction zones along convergent plate boundaries
- Subduction zones or trenches are sites where oceanic lithosphere is descending (being subducted) into the mantle. Subduction occurs because the density of the descending tectonic plate is greater than the density of the asthenosphere below. In general, oceanic lithosphere is denser than the asthenosphere, whereas continental lithosphere is less dense and resists subduction.
At transform plate boundaries, plates slide horizontally past one another without the production or destruction of lithosphere. Transform faults connect a divergent boundary to a subduction zone.
At divergent plate boundaries, called spreading centers, seafloor spreading occurs. Here, two adjacent plates are moving away from each other, producing long, narrow fractures in the oceanic crust. Along the axis of some segments is a deep structure called a rift valley. This structure is evidence that tensional forces are actively pulling the oceanic crust apart.
A hot spot is the surface manifestation of the upwelling of hot rock, called a mantle plume. As the hot, rocky plume ascends through the mantle, partial melting occurs. Volcanism, high heat flow, and crustal upwelling that is a few hundred kilometers across may occur.
What process is evident at oceanic trenches?
Sinking of oceanic lithosphere into the mantle at a subduction zone
- A continental rift results in the broad up warping of the overlying lithosphere. As a result, the lithosphere is stretched, causing the brittle crustal rocks to break into larger slabs. As the tectonic forces continue to pull the crust apart, these crustal fragments sink, generating an elongated depression called a rift valley. Rising hot rocks may produce an elevated area on the surface, perhaps surrounded or lined by rift valleys. Transform faulting is a sideways movement of plates, with some elevation changes on the moving plates.
A very long-lived magma source located deep in the mantle is called a(n) __________.
hot spot
- A hot spot is the surface manifestation of the upwelling of hot rock, called a mantle plume. As the hot, rocky plume ascends (rises) through the mantle, partial melting occurs. In the area over the rising magma, occurs volcanism, high heat flow, and crustal upwelling that is a few hundred kilometers across.
An oceanic trench occurs when a subducting oceanic plate dives under another plate. A mid-oceanic ridge develops along a spreading center on the ocean floor. A batholith is a large area of rising granitic rock.
Which scientist argued forcefully for continental drift in the early part of the 20th century?
Alfred Wegener
- He was a German meteorologist and geophysicist, who wrote The Origin of Continents and Oceans. This book, published in several editions, set forth the basic outline for Wegener’s hypothesis called continental drift. Continental drift challenged the long-held assumption that the continents and ocean basins had fixed geographic positions. Wegener suggested that a single supercontinent consisting of all Earth’s landmasses once existed. He named the giant landmass Pangaea.
Harry Hess suggested his hypothesis of sea-floor spreading after WWII. James Hutton proposed the theory of uniformitarianism in the late 18th century. Wladmier Köppen, a noted German climatologist, was Wegener’s father-in-law.
What type of region is the modern-day Red Sea?
A rift zone that may eventually open into a major ocean if Arabia and Africa continue to separate
- The Red Sea, which formed when the Arabian Peninsula split from Africa, is a modern example of an active continental rift zone called the East African Rift.
Evidence for recent volcanic activity includes several large volcanic mountains including Mount Kilimanjaro and Mount Kenya, the tallest peaks in Africa. Research suggest that if rifting continues, the rift valley will lengthen and deepen, eventually extending out to the margin of the landmass. At this point, the rift will become a narrow sea with an outlet to the ocean.
The former, late Paleozoic supercontinent is known as __________.
Pangaea
- Alfred Wegener suggested that a single supercontinent consisting of all Earth’s landmasses once existed. He named the giant landmass Pangaea.
Which one of the following is an important fundamental assumption underlying the plate tectonic theory?
Earth’s diameter has been essentially constant over time.
- New lithosphere is constantly being produced at the oceanic ridges that are associated with most divergent plate boundaries; however, our planet is not growing larger; its total surface area remains constant. A balance is maintained because older, denser portions of oceanic lithosphere descend into the mantle at a rate equal to seafloor production. This activity occurs along convergent boundaries, where two plates move towards each other and the leading edge of one is bent downward causing it to slide beneath the other.
A transform plate boundary is characterized by __________.
a deep, vertical fault along which two plates slide past one another in opposite directions
- Stratovolcanoes are the manifestation of oceanic-continental convergence. Mid-ocean ridges are divergent plate boundaries where the seafloor is spreading apart. Divergent boundaries are the splitting of either continental or oceanic plates.
Why was Wegener’s continental drift hypothesis rejected?
Because Wegener could not identify a mechanism capable of moving continents
- Wegener proposed that gravitational forces of the Moon and Sun that produce Earth’s tides were also capable of gradually moving the continents across the globe. However, the prominent physicist Harold Jeffreys correctly countered that tidal forces of the magnitude needed to displace the continents would bring Earth’s rotation to a halt in a matter of a few years.
Wegener also incorrectly suggested that the larger and sturdier continents broke through thinner oceanic crust, much like ice breakers cut through the ice. However, no evidence existed to suggest that the ocean floor was weak enough to permit passage of the continents without the continents being appreciably deformed in the process.
In addition, Wegener used astronomical methods in an attempt to verify that Greenland had drifted westward with respect to Europe. After Wegener’s death, Danish workers continued to take measurements of Greenland’s position (1936, 1938, and 1948), but found no evidence of drift. Wegener’s final test proved to be a failure. Today, the Global Positioning System (GPS) allows scientists to measure the extremely gradual displacement of the continents that Wegener had worked so diligently, but unsuccessfully, to detect.
