Module 2 Plate Tectonics and Volcanoes Flashcards
Chapter 2 and 12 Understanding Earth
How and when did the idea of Continental Drift develop?
The idea that the continents appear to fit together and may have drifted apart has been around since the late 1500s when maps began to more accurately reflect the shape of the continents.
What was the initial evidence for continental drift?
The fit of the continents, the topography of the Appalachians, rock assemblages in South America and Africa, and the rock and fossil records - fossils of the same species and age are found on both sides of the Atlantic - Mesosaurus is a freshwater species, it couldn’t have crossed the ocean, but it still existed in both South America and Africa.
Why was continental drift initially rejected?
Wegeners estimates for the rate of drift were nearly a thousand times faster than the actual rate of seafloor spread, and he theorized the movement of continents was caused by forces outside the planet, rather than beneath the crust.
How and when did the idea of Sea-Floor spreading develop?
1928 British geologist Arthur Holmes proposed that convection currents “dragged the two halves of the original continent apart, with consequent mountain building in the front where the currents are descending, and the ocean floor development on the site of the gap, where the currents are ascending.”
Marine Geologist Maurice “Doc” Ewing showed that the Atlantic ocean is made of young basalt, not old granite.
What were key pieces of evidence that led to the development of the theory of plate tectonics over time?
Bruce Heezen and Marie Tharp, colleagues of Doc Ewing at Columbia university discovered a deep crack-like valley or rift running down the crest of the mid Atlantic ridge. The area tectonically active. 1960s Harry Hess of Princeton and Robert Dietz of Scripps Institute of Oceanography proposed earth’s crusts separates along the rifts in mid-ocean ridges and that new crust is formed by the upwelling of hot molten rock. New seafloor spreads laterally away from the rifts in a process of plate creation. 1965 Canadian geologist J. Tuzo Wilson described tectonics around the globe as rigid plates. Other scientists showed that almost all contemporary tectonic deformation is concentrated at these boundaries. They measured the rates and directions of crustal movements and demonstrated they were mathematically consistent with a system of rigid plates moving over the planet’s surface.
What are the major features associated with diverging, converging, and transforming plate boundaries?
Diverging boundaries include rifting and spreading along a mid-ocean ridge, creating new oceanic lithosphere. Convergent boundaries are where one plate subducts (dives under) the other creating ridge. The other plate folds creating mountains (Marianas Trench and Mariana islands are an example). Transform boundaries slide past each other horizontally. San Andreas fault is an example.
How does plate tectonics theory account for the distribution of most of the world’s volcanoes, earthquakes, young mountain ranges, and major sea-floor spreading?
Where plates meet is where the action is. The type of boundary determines the type of activity.
What is the general pattern of earthquakes around the world? How is it related to plate tectonics?
Plate boundaries are tectonically active. Divergent boundaries are marked by volcanism and earthquakes at the crest of the mid-ocean ridge. Convergent boundaries are marked by deep-sea trenches, earthquakes, mountain building, and volcanism. Transform faults can be recognized by earthquake activity and offsets in geologic features. Those processes combined with mantle convection and hot spots, or magma plumes explain Earth’s patterns of earthquakes and volcanoes.
How has plate motion been tested by studying marine magnetic anomalies?
Magnetic marine anomalies give us a measurable way to estimate the rate of sea floor spread
How do we measure plate motion today?
We measure plate motion with a combination of magnetic time (Speed=distance/time) and GPS (changes in distance between land-based GPS receivers on different plates and recorded over several years).
Why do the plates move? What drives plate tectonics?
Plate movement is driven by mantle convection.
Alfred Wegener
1880, a German with a degree in astronomy and meteorology who was obsessed with the idea that Africa and South America seemed to fit together. Theorized that North America and Europe were drifting apart at a rate of nearly 30m/year - a thousand times after than the north Atlantic seafloor is actually spreading
Lithosphere
Earth’s strong, rigid outer shell of rock
Asthenosphere
A weak, ductile layer of rock that constitutes the lower part of the upper mantle (below the lithosphere and over which the lithospheric plates slide.
Divergent boundary
Plates move apart and create new lithosphere.
Ocean spreading center - rifting and spreading along a mid-ocean ridge to create new oceanic lithosphere.
Continental rift zone - rifting and spreading zones on continents are characterized by parallel rift valleys, volcanism, and earthquakes.
Convergent boundary
plates collide and one is pulled into the mantle (subducted) and recycled.
Ocean-ocean convergence: where oceanic lithosphere meets oceanic lithosphere, one plate is subducted under the other, and a deep-sea trench and a volcanic island arc are formed. (Marinas trench and Marina Island arc)
Ocean-continent convergence: When oceanic lithosphere meets continental lithosphere, the oceanic lithosphere is subducted, and a volcanic mountain belt is formed at the continental margin (Peru-Chile Trench and Andes Mountains)
Continent-continent convergence: Where two continents converge, the Continental crust is too buoyant to subduct, so the crust crumples and thickens, creating high mountains and a wide plateau (the Himalayas and Tibetan Plateau).
Subduction
When one plate is recycled back into the mantle (the plate area decreases)
Transform boundary
Plates slide horizontally past each other.
Continental transform fault - Plates slip horizontally past each other (San Andreas Fault)
Mid-Ocean Ridge Transform Fault: Mid-ocean ridges are typically offset by transform faults.
Island arc
A result of Ocean-Ocean convergence. An arc of islands that form behind a deep sea trench. The subducting plate releases water, which super heats the overriding plate, creating magma that results in island arcs.
Magnetic anomaly
In many areas of the ocean floor the magnetic field alternates between high and low values in long, narrow parallel bands called magnetic anomalies that are almost perfect symmetrical with respect to the crest of a mid-ocean ridge. Magnetic anomalies are evidence that earth’s magnetic field does not remain constant over time. Magnetic North and South periodically “flip.”
Lava flows are iron-rich and become slightly and permanently magnetized in the direction of the Earth’s magnetic field when they cooled. This is called Thermoremanent magnetization because the rock “remembers” the magnetization.
These bands provide a history of magnetic revdersals of the past 5 million years called the Magnetic Time Scale. By dating magnetic reversals on land, geologists assigned ages to bands of magnetized rock on the sea floor and used magnetic anomaly patterns to calculate how fast the seafloor was spreading (speed =distance/time, where distance is measured from eh mid-ocean ridge axis and time equals seafloor age). On a divergent plate boundary, the combo of spreading rate and direction give us relative plate velocity, the velocity that one plate moves relative to another.
Magnetic field
Lava flows are iron-rich and become slightly and permanently magnetized in the direction of the Earth’s magnetic field when they cool. This is called Thermoremanent magnetization because the rock “remembers” the magnetization.
Mantle convection
The engine that drives plate tectonics. The energy come 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 the whole mantle is involved in the convention 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 hots spots in the middle of plates.
