Science Lecture for Unit 41: Tectonic Plates and Geological Activity

For Class

• Topic area: Geology
• Terms and concepts to know: Continental drift, plate tectonics, subversion
• See historical period(s): All

Outline/Summary

• Plate Tectonics
  • Mechanisms and Earth's History
  • Volcanoes and Earthquakes
  • Earth's core
• Study/Discussion Questions
• Further Study

Plate Tectonics

The Mechanisms of Plate Tectonics

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 converging and creating uplift and strato volcanoes
• Ocean floor spreading
• Subduction at plate boundaries

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.

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, geologists have put together a picture something like a complicated dance involving the the history of Earth's tectonic plates.

Volcanoes and Earthquakes

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.

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).

Earth's Core

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.

Study/Discussion Questions

• How does the flow of heat energy explain most of the geological phenomena (volcanoes, earthquakes, continental drift)? Where does this heat energy come from?
• How does the sea floor give evidence of shifts in the magnetic fields?

Further Study On your Own (Optional)

• There is a good set of maps with more details about the possible past locations of the continents at the Paleogeography Through Geologic Time site.

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