Plate Tectonic - Plate Tectonics (4)
Post Lab 

  • Exploring the reasons for earthquakes and volcanoes.
  • Discovering how converging plates can create volcanoes.
  • converging
  • diverging
  • subduction
  • transform
  • worksheet

Students explore geologic events at converging plate boundaries.


Most earthquakes and volcanoes occur because of the movement of the plates, especially as plates interact at their edges or boundaries. At diverging plate boundaries, earthquakes occur as the plates pull away from each other. Volcanoes also form as magma rises upward from the underlying mantle along the gap between the two plates. We almost never see these volcanoes, because most of them are located on the sea floor. 

At converging plate boundaries, two situations are possible. First, both volcanoes and earthquakes form where one plate sinks under the other. This process, called subduction, takes place because one plate is denser than the other. The denser plate, which invariably has oceanic crust on its top, does the sinking. Second, only earthquakes occur when two plates collide (obduct), building a mountain range. The density of continental crust is too low for it to subduct, like wood floating on water. Instead, the two plates have a head on collision - building a mountain range. The Himalaya Mountains in Asia formed this way, from a collision between the Indian and Asian Plates. At transform plate boundaries, the two plates slide by each other. This generates little volcanic activity (there is no "gap" between the plates) or mountain building. Earthquakes, however, are common.

Much magma is generated at a converging plate boundary where subduction is occurring. The sinking plate melts as it descends into the asthenosphere; this generates magma, which rises through the other plate to form volcanoes. As it rises, more melting takes place in the rocks it travels through, generating yet more magma.

The volcanoes that form in areas of subduction form linear volcanic ranges. The Cascade Range in the Pacific Northwest of the United States is a good example. The volcanoes Mt. Lassen, Mt. Shasta, Mt. St. Helens, Mt Hood, and many others are all part of this chain. They have formed, and continue to erupt, as a small plate subducts underneath the North American Plate.

  1. Review the three types of plate boundary motions with the class. Emphasize that a plate has different pressures on it in different places. These may create a volcano, an earthquake, or both. You can make the analogy that people burp because they have pressures inside them... well, the Earth has pressure inside too! The Earth spells relief ... EARTHQUAKE or VOLCANO!
  2. Introduce the students to the concepts of subduction and collision (obduction). Draw the pictures on your board. You may wish to explain these motions in terms of "stronger" and "weaker" plates. The "weaker" plate is the one that is subducted. If both plates are the same strength, a collision is more likely. Note that the word "obduction" is somewhat out-of-date among geologists; they use collision instead. However, we have found that students really like the word "obduction," so you may want to introduce both terms.

A converging plate boundary where subduction occurs. 

A converging plate boundary showing collision (obduction) and mountain range formation.

  1. Have the students complete the worksheet.

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