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Natural Science - Year II

Unit 41: Plate Tectonics

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Science Web Assignment for Unit 41

This Unit's Homework Page History Lecture Science Lecture Lab Parents' Notes

Science Lecture for Unit 41: Tectonic Plates and Geological Activity

For Class

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 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.

  • What are the mid-ocean ridges? why would these support the idea that the seafloor is spreading apart?
  • What proof is there that the earth's magnetic field changes? what does this indicate about the ages of rocks with different magnetic properties?
  • Where do deep sea trenches form? Why are they associated with island arcs?

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.

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 (CalTech)based on computer generated simulations.

  • Be sure to click on the small "Tectonic plates" link in the middle of the page. Which plate do you live on? Are you near the edge or center?
  • At what period, based on the maps or the animation, were all of the modern continents in one mass (more or less)?
  • How long ago (according to the simulation) did the continents of South America and Africa begin moving apart and into their modern relationship?
  • What implications does the theory that the continents have drifted apart have for explaining where why we find the same kinds of animals on widely-separated continents?

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.

Read through each description of the six types of volcanoes at the Volcano World site.

  • Which volcanoes are most dangerous?
  • What are the characteristics of volcanoes found mid-ocean ridges?
  • Where do you find shield volcanoes? strato volcanoes?
  • Can you use the differences in volcano types to map the edges of tectonic plates?

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).

2001 Nisqually Quake

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.

Take a look at the map of the earth's interior and composition based on seismological information at the Nevada Seismology Lab.

  • What are the crust, mantle and core layers? What is each made of?
  • What are the two kinds of waves generated by an earthquake?
  • Which travels through the earth's interior?
  • Which travels along the surface?
  • What happens when a seismology wave passes from the mantle into the core? How can that help us map the depth of the mantle?
  • What is convection? How does this type of heat movement drive the movement of earth's tectonic plates?
  • What are the asthenosphere and lithosphere? How can the lithosphere be deformed?
Take a look at this diagram of fault types from the Encyclopaedia Britannica, which shows the results of dip-slip faults and strike slip faults.

Study/Discussion Questions

Further Study On your Own (Optional)