layers called flood basalts. Flood basalts can be several thousand meters thick and
cover more than a half a million square km. The landscapes of the Deccan
Plateau in India and the Columbia Plateau of the northwestern United States have
been eroded out of these flood basalts.
Volcanoes are of four major types: (1) The most explosively eruptive are
composite volcanoes made of alternating layers of pyroclastic materials and lava
flows. These volcanoes have steep slopes (as great as 35°) and can be quite tall.
Famous examples include the Cascade volcanoes of the U.S. Pacific Northwest,
Mt. Vesuvius of Italy, and Mt. Fuji of Japan. (2) Shield volcanoes are named for
the fanciful resemblance to the cross section of a warrior’s shield. These volcanoes
are less explosive and have much gentler slopes (commonly less than 5°)sothe
shield volcanoes are taller than wide and are comprised of much more material
than composite volcanoes. Mauna Loa in Hawaii is the largest shield volcano on
Earth and has a relief of over 17 km considering its base rises from a deep sea
bottom. (3) Lava domes are composed of thick, viscous, high silica lava capable
of explosive eruptions. They are considerably smaller than the first two volcano
types and have heights less than 600 m; therefore they do not present threats over
large areas. (4) Cinder cones are smaller yet and usually less than 500 m in height.
They are made of ash and cinders (larger pieces) that hold slopes between 25 and
35 percent. Their smaller size bespeaks the relative brevity of their eruptive lives.
Volcanic eruptions can be of several types. Controlling factors appear to be the
amount of silica (occurring as silica dioxide), the pressure of the magma’s confine-
ment, and the strength of surrounding rocks. Gases in magma tend to be explo-
sively released if the magma has high silica content and an acidic chemistry.
Cooler, basic lavas have lower silica contents and lowertemperaturesthus
allowing gases to slowly escape making eruptions more placid.
Volcanoes arenatural hazardsto all manner of human endeavors. Agricultural
activities have long hugged the fringes of volcanoes because of fertile andisols of
volcanic origin. This symbiosis has been repeatedly broken. There have been
many deaths recorded through history. Some populations at risk from the explo-
sion of large composite volcanoes include: Naples, Italy; Tokyo, Japan; Mexico
City, Mexico; and Seattle, United States. Many other areas run significant volcanic
risk because they are in confined valleys near volcanoes or in narrow strips
between volcanoes and coastlines.
It is common to think of the hazard from a volcano confined to the flow of lava.
Although lava is a serious hazard, it pales in comparison to others because lava
moves by gravity to low areas and doesnot spew evenly around a volcano in all
directions. Lava flows at a variety of speeds depending on its viscosity. The most
viscous flows are a few centimeters per day while the fastest flows are on steep
Vulcanism 359