Science Web Assignment for Unit 41
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Since Wegener's theory won broader acceptance in the 1960s, geologists have focussed their efforts on mapping the current geological tectonic plates and using their current motions and ancient magnetic fields, plus surface features, to determine how the plates have moved in the past and how they will move in the future.
This diagram shows the mechanisms modern geologists use to explain the movement of plates on the Earth's surface:
Plates don't always map directly to the "land area" we call continents. As you can see in the diagram below, the South American plate not only includes the surface area of South America, but much of the South Atlantic Ocean floor as well...all the way out to the mid-Atlantic mountain ridge discovered by Matthew Maury in the mid-1850s. That ridge marks the current boundary between the African Plate to the east and the South American plate to the west. Geologists use evidence of sea-floor spreading to explain why these two continents are moving away from each other by a growing "ocean floor" region.
Read through the summary of evidence supporting sea-floor spreading.
If the continents are moving around now, one of the fascinating questions is how did they move in the past?. Using magnetic striping and surface features to map old alignments, geologicsts have put together a picture something like a complicated dance involving the the history of Earth's tectonic plates.
Read the description of Pangaea and the dispersal of the continents at the USGS Historical Perspective site, then watch the animation of the movement of the plates (a Quicktime movie) based on computer generated simulations.
Tectonic plates are almost unimaginably huge masses. To move them requires -- and releases -- tremendous amounts of energy. The friction along the edges of plates moving against one another generates heat, weakness in the crust, and results in earthquakes and volcanoes. Particularly around the Pacific plate, which is moving away from the Asia plates and colliding with the North American plate, there is so much volcanic activity that the border of the Pacific plate is called "the ring of fire".
We don't have enough time to spend on looking at volcanoes and earthquakes in detail, so we'll concentrate now on just looking at the types of volcanoes that have been classified by earth's geologists.
Read through each description of the six types of volcanoes at the Volcano World site.
Volcanoes and earthquakes are often related phenomena: volcanic eruptions cause earthquakes, and earthquakes can shift plate edges and allow the formation of volcanoes. Take a look at the map of the location of the Nisqually Earthquake based on the Seattle Times article of 1 March 2001 (the quake was the previous day, 28 February 2001). This was a 6.8 magnitude quake. Its proximity to several large cities (Olympia, Tacoma, and Seattle) made it dangerous: a number of buildings collapse and several people were killed. Note the location of the quake's epicenter: it's on the fault marking the separation of the Juan de Fuca plate and the North American plate. The Pacific plate is pushing the Juan de Fuca plate underneath the American plate in a process called subduction. As material in the Juan de Fuca plate heats up, the liquifying magma finds its way through cracks in the North American plate and flows out onto the surface of that plate, forming the Cascade volcanoes (Baker, Glacier Peak, Rainier, St. Helens, Adams, and Hood in Washington and Oregon).
While they cause widespread damage, earthquakes also allow us to map the earth's interior by detecting changes in direction of the seismic waves generated when plates slip.
Take a look at the map of the earth's interior and composition based on seismological information at the Nevada Seismology Lab.
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