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July 1, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


What is a volcano?

Many readers know that the island of Hawai`i 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 Hala`i Hills (on Mauna Loa in Hilo), Kulani Cone (on Mauna Loa), and Kapoho Cone and Mauna Ulu (on Kilauea) are places where eruptions took place in the past. But 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 meaning of the word, a volcano is a vent 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 dictionary 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 30 km (18 miles) and in Hawai`i more than 100 km (60 miles). Each of the five volcanoes on the island of Hawai`i 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 10 km (6 miles) deep and more likely about half that. In this respect, those cones are analogous to limbs on a tree, and the deeply rooted volcano is equivalent to the trunk of the tree. If by some miracle we could plug the deep conduit to Kilauea, the entire volcanic edifice, including Pu`u `O`o, would die. In reality, however, Kilauea will remain an active volcano long after Pu`u `O`o 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, rift vents. Sometimes "cone" is substituted for "vent." So, for example, Kulani Cone could be termed a flank vent on Mauna Loa, Mauna Ulu a rift vent on Kilauea, and the Hala`i Hills parasitic cones on Mauna Loa. Pu`u `O`o 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 that, for example, Pu`u `O`o is fed from shallow, not great, depth. With that evidence, though, a clear distinction can be made.

The second dictionary definition of "volcano"---that of 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 depressions are volcanoes.

A recent visitor to HVO remarked that Kilauea caldera "sure doesn't look like a volcano." This visitor is a geologist and should have known better, but the image of Fuji, Mayon, and pre-1980 Mount St. Helens is strongly entrenched as the classic stereotype of a "real" volcano. Had that visitor come to Kilauea in A.D. 1400, however, he would have seen a lava shield rather than a caldera. The caldera formed by collapse of the shield some time in the next 100 years or so. 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.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes from the vent to the sea. Lava is entering the ocean near Kamokuna and enlarging the bench. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

A resident of Pu`ukapu reported feeling an earthquake at 6:29 p.m. on Tuesday, June 29. The magnitude-3.5 earthquake was located 13 km (8.2 mi) southeast of Mana at a depth of 41.9 km (25.1 mi).

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Updated: 9 July 1999