Plate tectonics
Look at a map of the Ring of Fire: Hawaii's volcanoes, Mount St. Helens, and the earthquakes along California's San Andreas Fault. In the early 20th century, the almost perfect fit between the coasts of South America and Africa led Alfred Wegener to propose that the continents had once been joined together. Today that fit, together with evidence that Wegener could not provide, is explained by a single model: plate tectonics. By the end of this page you will be able to explain why California shakes but Washington's Cascade volcanoes erupt, and how a hot spot built Hawaii and fuels Yellowstone.
From continental drift to plate tectonics
If you mark on a world map the epicenters of earthquakes and the active volcanoes of the last decade, you will see that they are not scattered at random, but form continuous lines that enclose huge, almost quiet areas. Why is our planet's geological activity concentrated at the edges of large blocks, like the Ring of Fire around the Pacific?
Explain what a lithospheric plate is and name three plates: one containing Europe, an American one, and one that is almost entirely oceanic.
Explain what a lithospheric plate is and name three plates: one containing Europe, an American one, and one that is almost entirely oceanic.
State whether the following statement is true or false and justify your answer: "Continents move because the lithospheric plates on which they rest move."
State whether the following statement is true or false and justify your answer: "Continents move because the lithospheric plates on which they rest move."
The forces that move the plates
High-precision GPS stations measure that every year the distance between Europe and New York increases by about 1 inch (2.5 cm), about as fast as your fingernails grow. What internal mechanism is capable of pushing masses of rock covering millions of square miles across the planet?
State and briefly describe two forces that contribute to the movement of lithospheric plates.
State and briefly describe two forces that contribute to the movement of lithospheric plates.
Paleomagnetic and satellite measurements confirm that the Mid-Atlantic Ridge generates new oceanic lithosphere at a total spreading rate of about $2.5\ \text{cm/year}$,…
Paleomagnetic and satellite measurements confirm that the Mid-Atlantic Ridge generates new oceanic lithosphere at a total spreading rate of about $2.5\ \text{cm/year}$, which has enabled the widening of the Atlantic Ocean since the breakup of Pangaea.
- a) A student proposes the following deduction: "Since ocean ridges worldwide continuously generate crust, the volume and surface area of the Earth must have grown steadily over the last $200$ million years". Refute their hypothesis by explaining what process compensates for the creation of lithosphere and at what type of plate boundaries it takes place.
- b) If the current distance between the coasts of West Africa and North America at that latitude is about $4\ 500\ \text{km}$, estimate how many million years ago the Atlantic Ocean began to open in that region, assuming that the spreading rate remained constant.
- c) Predict and justify how the age of the rocks and the thickness of marine sediments will vary as you move from the ridge axis toward the African continental margin.
The three types of plate boundary
In Iceland the ground splits open along large fissures from which fluid lava pours, while in the Cascades (Mount St. Helens) volcanoes rise above a subduction zone, and in California two blocks grind sideways past each other along the San Andreas Fault without any volcanism. Why does contact between plates produce such different landscapes and geological hazards?
Hot spots and supervolcanoes
Not all volcanoes sit on plate edges. A hot spot is a fixed plume of hot rock rising from deep in the mantle. As the Pacific Plate drifts northwest over it, new volcanoes form and the old ones are carried away, leaving a chain: the Hawaiian Islands, with the Big Island over the hot spot today. The same kind of hot spot lies under Yellowstone. Its giant eruptions long ago were so huge that scientists call it a supervolcano.
The diagram shows the three types of boundaries that can exist between two lithospheric plates. State which type of plate boundary (divergent, convergent, or transform)…
The diagram shows the three types of boundaries that can exist between two lithospheric plates. State which type of plate boundary (divergent, convergent, or transform) corresponds to each number and explain what happens to the plates in each case.
In subduction zones, one plate slides beneath another. Describe two geological phenomena associated with these zones and give a real example of a subduction zone.
In subduction zones, one plate slides beneath another. Describe two geological phenomena associated with these zones and give a real example of a subduction zone.
The San Andreas Fault in California is a transform plate boundary. Explain the relative movement of the plates at this type of boundary, what hazard it creates, and why…
The San Andreas Fault in California is a transform plate boundary. Explain the relative movement of the plates at this type of boundary, what hazard it creates, and why volcanoes do not form there.
The evidence for seafloor spreading
Three lines of evidence support plate tectonics. First, the coastlines of South America and Africa fit together like puzzle pieces. Second, identical fossils, such as Mesosaurus, appear on continents now far apart, showing they were once joined in the supercontinent Pangaea. Third, magnetic stripes on the ocean floor show that new crust forms at ridges and spreads outward.