Chapter 4- Plate Tectonics (Week 2) Flashcards
What observation led Alfred Wegener to propose the idea of continental drift?
In 1911, Wegener discovered a scientific publication describing matching Permian-aged terrestrial fossils in South America, Africa, India, Antarctica, and Australia. He concluded that the continents must have been joined in the past, allowing the organisms to move from one continent to another
Wegener proposed that the continents were once joined in a supercontinent called Pangea, allowing the organisms to move freely across land. He termed the process of continents moving and reconfiguring themselves as “continental drift.”
What does the term “Pangea” mean, and why did Wegener use it?
The term “Pangea” means “all land.” Wegener used this term to describe his vision of a supercontinent comprising all present-day continents.
How did Wegener support his idea of continental drift?
He relied on matching geological patterns across oceans, such as sedimentary strata, coalfields, mountain structures, and rock types.
evidence of the Karoo Glaciation from South America, Africa, India, Antarctica, and Australia, suggesting that these continents were once connected as a single supercontinent.
How did Wegener suggest the continents moved, and what criticism did he face?
Wegener proposed that continents were like icebergs floating on the heavier ocean crust, moved by Earth’s rotation and tidal forces. However, the main criticism was that he couldn’t explain how continents could move, given the prevailing view of Earth’s crust as continuous.
Wegener first published his ideas in 1912 and revised them up to 1929. The main criticism was the lack of a plausible mechanism for continental movement within the continuous crust model.
His ideas were tentatively accepted by a small minority and firmly rejected by most. However, within a few decades, plate tectonics emerged, validating many of Wegener’s ideas
What was the prevailing view on the origin of mountain chains at the beginning of the 20th century?
At the start of the 20th century, one prevailing view on the origin of mountain chains was contractionism, suggesting that Earth, slowly cooling, was also shrinking, leading to the formation of mountains as the Earth’s crust wrinkled.
What issues did the contractionism theory face?
The contractionism theory faced challenges such as Earth not cooling fast enough for the required amount of shrinking and the principle of isostasy, which prevented blocks of continental crust from sinking as needed for oceans to form.
The alternative view was permanentism, suggesting that continents and oceans have always been generally the same. The geosyncline theory proposed that geosynclines, thick deposits of sediments, could develop into fold-belt mountains through compression.
The idea that geosynclines develop into fold-belt mountains was first proposed by James Hall and later tested by Philip Kuenen in 1937 using layers of paraffin wax, causing layers within a geosyncline to fold up.
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What was the main problem with the geosynclinal hypothesis for mountain building?
The geosynclinal hypothesis lacked an adequate explanation for the lateral forces required to cause compression. While an experiment by Kuenen used pistons to compress layers, the main force in nature was suggested to be gravity pulling the geosyncline downward, drawing the sides together as it folded.
What challenge did proponents of the geosyncline theory face in explaining intercontinental terrestrial fossil matchups?
Proponents of the geosyncline theory struggled to explain intercontinental terrestrial fossil matchups. The proposed explanation was the existence of land bridges that once linked continents, allowing animals and plants to migrate back and forth.
What is paleomagnetism?
Paleomagnetism is the study of the record of Earth’s magnetic field in rocks through time. Rocks, especially those with magnetic minerals like magnetite, can retain a remnant magnetism aligned with the Earth’s magnetic field when they form.
How does paleomagnetism work in rocks?
Magnetic minerals in rocks, such as magnetite, become aligned with the Earth’s magnetic field during the rock’s formation. This alignment is locked in place as the rock cools, creating a remnant magnetism. By studying the horizontal and vertical components of this remnant magnetism, one can determine the direction to magnetic north and the latitude where the rock formed.
The vertical component of remnant magnetism points more sharply downward the closer it is to the magnetic north pole. By analyzing this component, researchers can determine the latitude where the rock formed relative to magnetic north.
What are apparent polar wandering paths (APWP)?
Apparent polar wandering paths are records of the apparent movement of the magnetic poles over time based on the remnant magnetism of rocks. In the early 1950s, geologists observed different magnetic pole positions for rocks of different ages in the same area, assuming Earth’s magnetic pole had shifted significantly. However, it was later realized that the paths were not true records of pole movement but rather reflections of continental drift.
Apparent polar wandering paths provided the first new evidence supporting continental drift in the 1950s. While not immediately convincing for all geologists, these paths indicated that continents had different magnetic pole positions in the past, supporting the idea that continents were not fixed but had moved over time.
What advancements occurred in understanding ocean basin geology and geography during the 20th century?
Before 1900, knowledge about ocean basin bathymetry and geology was limited. By the end of the 1960s, significant progress was made, leading to detailed maps of ocean floor topography, insights into ocean floor sediment and solid rock geology, and a comprehensive understanding of the geophysical characteristics of ocean rocks, rivaling the knowledge of continental rocks.
What are some important physical features of the ocean floor identified through bathymetric data?
Key ocean floor features include extensive linear ridges (at 2,000 to 3,000 m depths), fracture zones perpendicular to ridges, deep-ocean plains (4,000 to 5,000 m depths), relatively flat continental shelves (depths under 500 m), deep trenches (up to 11,000 m deep, mostly near continents), and seamounts and chains of seamounts.
What is seismic reflection sounding, and how does it contribute to understanding the ocean floor?
Seismic reflection sounding involves transmitting high-energy sound bursts and measuring the echoes with geophones towed behind a ship. This technique, more advanced than acoustic sounding, allows mapping of the bedrock topography, crustal thickness, and sediment thickness. Seismic studies revealed that ocean sediments are thin or absent along ocean ridges and provided insights into the oceanic crust’s composition, which is mainly basalt.
What did Bullard and his colleagues find regarding heat-flow rates along the ocean floor?
Edward Bullard developed a heat flow probe in the early 1950s.
They found higher than average heat-flow rates along the ridges and lower than average rates in trenches.
The data were interpreted as evidence of mantle convection, where areas of high heat flow correspond to upward convection of hot mantle material, and areas of low heat flow correspond to downward convection.
The data suggest that the observed higher heat-flow rates along ridges and lower rates in trenches provide evidence for mantle convection, with upwelling and downwelling of hot mantle material, respectively.
What became possible with the advancements in seismographic networks?
It became possible to plot the locations and depths of both major and minor earthquakes with great accuracy.
There was a remarkable correspondence observed between earthquake locations and both the mid-ocean ridges and deep ocean trenches.
What did Gutenberg and Richter show in 1954 regarding ocean-ridge earthquakes?
They showed that ocean-ridge earthquakes were all relatively shallow.
What did Benioff show in the 1930s regarding earthquakes near ocean trenches?
Benioff, in the 1930s, demonstrated that earthquakes in the vicinity of ocean trenches were both shallow and deep. The deeper ones were situated progressively farther inland from the trenches.
What did Harold Hess propose in 1960, and what elements of plate tectonics did it include?
In 1960, Harold Hess proposed a hypothesis with elements now accepted as plate tectonics. He suggested that new sea floor is generated at ocean ridges and that old sea floor is dragged down at ocean trenches, driven by mantle convection currents. He also proposed that continental crust does not descend into trenches but colliding landmasses are thrust up to form mountains.
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Who provided the first understanding of magnetic stripes, and what was their interpretation?
Fred Vine suggested that magnetic stripes were related to magnetic reversals, with symmetrical patterns on either side of ridges.
What did researchers studying magnetic reversals discover?
Earth’s magnetic field periodically weakens, becomes non-existent, and re-establishes with reverse polarity during reversals.
What did the Vine-Matthews-Morley (VMM) hypothesis propose?
It suggested a link between magnetic patterns at ridges and magnetic reversals, with positive anomalies during normal events and negative anomalies during reversed events.
In 1963, who proposed the idea of a mantle plume or hot spot, and what evidence did they cite?
J. Tuzo Wilson proposed the idea based on the distribution of Hawai’ian and Emperor Seamount island chains, where volcanic rock becomes progressively younger toward the southeast.
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What is a mantle plume?
A mantle plume is a stationary, semi-permanent upwelling of hot mantle material beneath Earth’s surface.
How does the Hawaiian volcanic chain relate to mantle plumes?
J. Tuzo Wilson proposed that the Hawaiian volcanism results from a mantle plume. The Pacific Plate moves northwest over the stationary plume, forming the Hawaiian Islands. The change in direction of the volcanic chain near Midway Islands is attributed to the Pacific Plate’s movement over the plume. Mantle plumes, like the one at Yellowstone and the Anahim Volcanic Belt, are long-lived phenomena, lasting for tens of millions to possibly hundreds of millions of years.
Where are most mantle plumes found, and can they be under continents?
Most mantle plumes are within ocean basins (e.g., Hawaii, Iceland, Galapagos Islands), but some are found under continents (e.g., Yellowstone hot spot in the U.S., Anahim Volcanic Belt in central British Columbia).
How long do mantle plumes typically last, and can they endure for hundreds of millions of years?
Mantle plumes are very long-lived, lasting for at least tens of millions of years, and possibly for hundreds of millions of years in some cases.
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What are transform faults associated with oceanic spreading ridges?
Transform faults are faults perpendicular to oceanic spreading ridges, composed of a series of straight-line segments. They offset the ridge at intervals.
Who introduced the concept of transform faults and the idea that Earth’s crust is divided into rigid plates?
Tuzo Wilson introduced the term “transform faults” in 1965, describing faults associated with oceanic spreading ridges. He also proposed the concept of Earth’s crust being divided into rigid plates, coining the term “plate tectonics.”
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When did the ideas of continental drift and sea-floor spreading become widely accepted?
By 1965, the ideas of continental drift and sea-floor spreading were widely accepted, leading more geologists to think in these terms.
By the end of 1967, how had Earth’s surface been mapped?
Earth’s surface had been mapped into a series of plates, including major plates like Eurasian, Pacific, Indian, Australian, North American, South American, African, and Antarctic plates, as well as numerous smaller plates and sub-plates.
How can plate motions be tracked, and what are the rates of motion for major plates?
Plate motions can be tracked using Global Positioning System (GPS) data. Rates of motion for major plates range from less than 1 cm/year to more than 10 cm/year. The Pacific Plate is the fastest, followed by the Australian and Nazca Plates.
What explains the variation in motion rates within a single plate, such as the North American Plate?
Plates move as rigid bodies, but they also rotate. For example, the North American Plate rotates counter-clockwise, leading to different rates of motion in different places.
What are the three types of plate boundaries, and what materials make up the plates?
Plate boundaries are divergent (moving apart), convergent (moving together), and transform (moving side by side). The plates are made up of crust and lithospheric mantle.
