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Saturday, December 8, 2012

Volcano Roundup

When speaking yesterday with North Reading students about my 1989 visit to the world's smallest volcano, one student asked about the world's largest volcano. By a variety of measures, Mauna Loa holds this distinction. It rises 13,677 feet above sea level, which is surpassed by many volcanoes, but nearly 30,000 feet from the sea floor, and 56,000 feet (eleven miles!) from its base within the ocean crust. It is so massive that the crust dips more deeply into the mantle below it. Mauna Loa is also a fairly active volcano, with 33 documented eruptions since 1843.

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Although located in the middle of the Pacific Ring of Fire, Mauna Loa is not part of the Ring of Fire. Rather, it is a hot spot volcano, similar to those that have formed Iceland and Cape Verde. Such volcanoes form in the middle of ocean plates, as they pass over hot spots deep in the earth. If the spot alternately hot and cool, the result can be a line or arc of separate volcanoes, with the most recent activity at one end of a long line of older volcanoes.

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More than three quarters of the world's 15,000 active volcanoes are found along the edges of the Pacific Ocean, where convergence between oceanic and continental plates results in both earthquakes and volcanic eruptions. (Fourteen of these are in Nicaragua, including the recently -- and beautifully -- erupting San Cristóbal.)

By some measures, the Yellowstone volcano is much larger, and is also considered active. A catastrophic eruption is theoretically possible, but the risks are considered remote by park officials.

When measured in terms of elevation above sea level, Mauna Loa is not even close to the top-ten list, all of which are Pacific-Rim volcanoes in the Andes of South America. The tallest volcano in the solar system, by the way, is a basaltic shield volcano on Mars. Olympus Mons towers more than 85,000 feet above the surface, with a base about the size of New York -- the state, not the city!

During our visit to North Reading, we also discussed some of the beautiful volcanic craters that had just been listed on the web site World Geography the day before. The first of these is Diamond Head, which is on the Hawaiian island of Oahu. The page lists a total of ten craters, each of which has a unique form that results from the volcanic events that created it, erosion and human activity since its creation, and climatic factors. The story of each is included, with a link to further information and an interactive map.
 Top-Ten List of Craters from World Geography
Seeing such fantastic images reminds us of other volcanic craters and calderas we have visited or hoped to visit. One of my favorites is Lake Atitlan in Guatemala, where I visited coffee farmers in 2008. A friend with whom I shared the Diamond Head photo told me that he had climbed the outer edge (it is very steep!) and had also visited the nearby Punchbowl/Puowaina crater, which has been both an ancient and a modern burial ground.

The United States Geological Survey is a good source of detailed  information about volcanoes in the United States and earthquakes worldwide. Its web site includes nearly instantaneous mapping of earthquakes, wherever in the world they may occur. The main USGS employs many geographers -- along with other scientists -- engaged in mapping and studying biological, water, mineral, and other natural resources.

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