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What is a volcano? The answer may surprise you

Updated: 
December 13, 2015 - 1:30am

Many readers know that Hawaii Island is made of five volcanoes — Kilauea, Mauna Loa, Hualalai, Mauna Kea and Kohala. Those same readers know that such obvious features as the cones that dot Mauna Kea, the Halai Hills and Kulani Cone on Mauna Loa, and Kapoho Cone, Puu Oo and Mauna Ulu on Kilauea are places where eruptions took place. If that’s the case, then why aren’t they called volcanoes? Isn’t a volcano a place where lava reaches the surface of the Earth? Why doesn’t the island have hundreds of volcanoes instead of only five?

In one dictionary definition, a volcano is a vent or opening in the Earth’s crust through which rock or lava is ejected. In another, a volcano is a cone-shaped hill or mountain built around a vent. Most volcanologists disagree with both of these definitions.

To a volcanologist, a volcano is a structure containing a vent or cluster of vents fed by magma rising directly from great depth within the Earth, generally more than 18 miles and in Hawaii about 60 miles. Each of the five volcanoes on Hawaii Island has such a deeply rooted feeder conduit.

In contrast, all of the cones mentioned above, and most others on the island, are supplied by magma that branched off the main conduit at a shallow depth, probably less than six miles deep and more likely, less than half that. These cones are analogous to limbs on a tree, and the deeply rooted volcano is equivalent to the trunk of the tree.

If we could plug the deep conduit to Kilauea, the entire volcano, including Puu Oo, would die. In reality, however, Kilauea will remain active long after Puu Oo stops erupting, because the main feeder conduit will still be intact.

Several terms are used to describe the vents that lack deep roots and get their magma from the main feeder conduit — flank vents, parasitic vents and rift vents. Sometimes “cone” is substituted for “vent.” So, for example, on Mauna Loa, Kulani Cone could be termed a flank vent and the Halai Hills parasitic cones. Puu Oo is an active flank or rift vent on Kilauea.

Physical appearance cannot be used to make the distinction between a volcano and a subsidiary vent on that volcano. Lacking geophysical evidence, it would be nearly impossible to know, for example, that Puu Oo is fed from shallow, not great, depth. With that evidence, though, a clear distinction can be made.

The second dictionary definition of “volcano” — a cone-shaped hill or mountain built around a vent — does not account for volcanoes such as Kilauea, whose shape is far from that of a cone. Another type of volcano lacking a cone shape is a large caldera, such as Long Valley in eastern California or Yellowstone in Wyoming. No one would guess, without doing some geologic sleuthing, that these wide shallow depressions are volcanoes.

Visitors to Hawaii Volcanoes National Park often remark that Kilauea Crater (the official name of the summit caldera) “sure doesn’t look like a volcano.” Even visitors trained in geology make that comment, because the image of Mount Fuji in Japan or Mayon in the Philippines is strongly entrenched as the stereotype of a “real” volcano.

Had these visitors come to the summit of Kilauea in 1400 CE, however, they would have seen a lava shield rather than a caldera. The caldera formed by collapse of the shield about 100 years later.

This illustrates another point about volcanoes — the shape can change drastically and quickly, and one year’s cone or shield can be next year’s caldera. So, shape is an unimportant and even misleading basis for defining a volcano.

Finally, the distinction between a caldera, such as Kilauea’s, and a crater, such as Halemaumau, is both arbitrary and meaningful. A caldera is a depression more than one mile in diameter, and a crater is smaller. Pretty arbitrary. A more important distinction is that a Hawaiian caldera forms by collapse of the volcano’s summit and has deep roots, whereas a crater, no matter where it forms, has shallow roots. In a perfect world, the term Kilauea Crater on maps would be replaced by Kilauea Caldera.

Volcano activity updates

Kilauea continues to erupt at its summit and East Rift Zone. The summit lava lake level varied between about 138 and 203 feet below the vent rim within Halemaumau Crater. On the East Rift Zone, scattered lava flow activity remained within about four miles of Puu Oo.

Mauna Loa is not erupting. The seismicity rate is elevated above background levels, but has not increased over rates observed in recent months. Continuous GPS measurements continue to show deformation consistent with inflation of magma reservoirs beneath Mauna Loa.

There were no earthquakes reported felt on Hawaii Island this past week.

Info: http://hvo.wr.usgs.gov, 967-8862 (Kilauea updates), 967-8866 (Mauna Loa updates); askHVO@usgs.gov.

Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.

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