why do we have volcanoes and earthquakes

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.
You probably know that earthquakes can be huge natural disasters. But why do they happen? The Earth s surface is formed of massive slabs of rock called. These plates, also called tectonic plates, are always moving. Sometimes they just slide past one another. At other times they actually collide with one another. Plate movement causes the buildup of tremendous quantities of energy in the rock. When the energy is released, it produces vibrations that travel through the rock, leading to earthquakes. During earthquakes, or giant cracks, are produced by the pressure of the moving rock. Earthquakes and volcanoes occur along the edges of the plates. Scientists have developed a theory that explains how these giant plates move, thereby creating, destroying, and re-forming continents and oceans over long periods of time. This theory is called the. According to the theory, there are two types of plates, oceanic and continental.


Various types of movement occur along the different kinds of plate boundaries. Plate collisions create landforms such as coastal volcanoes, island arcs, and mountain chains. When plates move apart, they produce new ocean floor as magma from the mantle rises up through volcanoes and deposits new rock along the plate boundaries. In some areas plates slide alongside each other, neither creating nor destroying land. When the plates move, they cause vibrations that produce earthquakes. Thousands of earthquakes take place every year, but only a few of them are destructive enough to be considered disasters. Many earthquakes are so mild that few people notice them. Scientists have developed an instrument called a, which measures and records the intensity of earthquakes. Because seismographs are very sensitive, they help scientists predict earthquakes, too. They can record even minor shocks and changes in the Earth s layers, and this helps to monitor the build-up of stresses that lead to earthquakes. By comparing the data from seismographs in different locations over the world, scientists can get a better picture of the forces at work underground. To learn more about earthquakes, try the.

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