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.
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.
- 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!
- 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.