why do volcanoes occur at 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. 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.

Have the students complete the worksheet.
Hot spots, Divergent plate boundaries (such as rifts and mid-ocean ridges), and The origin of the for hot spots is not well known. We do know that the magma comes from partial melting within the upper, probably from depths not too much greater than 100 km. The actual source of the heat that causes the partial melting (the actual hotspot itself) is almost certainly much deeper than that, but we really don't know how deep or even exactly what a hotspot is! At a divergent margin, two tectonic plates are moving apart, and magma that is generated in the upper mantle flows upward to fill in the space. This magma is probably generated at depths that are shallower than those for hotspot magmas. People argue about whether the magma forcing its way to the surface causes the plates to move apart or whether the plates move apart and the magma just reacts to that and fills in the space.

Perhaps it is a combination of these two. The most extensive example of this type of volcanism is the system of mid-ocean ridges. Continental examples include the East African Rift, the West Antarctic Rift, and the Basin and Range Province in the southwestern US. The final major place where volcanism originates is at convergent margins (subduction zones)--where an oceanic plate dives under either another oceanic plate or perhaps a continental plate. As the plate gets pushed further and further it starts to give off its volatiles (mostly water), and these migrate upwards into the mantle just under the overriding plate. The addition of these volatiles to this overriding mantle probably lowers the melting point of that mantle so that magma is generated. Part of the magma may also be generated by the downgoing plate actually starting to melt as it gets into the hotter and hotter interior.

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