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February 18, 1999

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


Kilauea Volcano and HVO in the spotlight

This past week, the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) hosted 68 Japan Broadcasting Company (NHK) personnel and associates. NHK is scheduled to transmit a live 90-minute program from HVO to Japan on Saturday, February 20. Kilauea Volcano and HVO are featured to demonstrate the quality of high definition TV and to attract a large audience.

Standing on the rim of Kilauea Caldera, the host asks HVO Scientist-in-Charge Don Swanson a series of questions. Students of Floyd McCoy's geology classes, who use this column for information, may be interested in the answers.

The first question was, "Why is there a caldera?" The answer is that a caldera is a large collapse feature which forms over a magma storage area. When there is a rapid and massive withdrawal of magma from the storage area, the roof of the chamber is left unsupported and falls. The extent of the surface area affected by the roof collapse is dependent upon the size and depth of the magma body. The last caldera-forming event at Kilauea Volcano was sometime before an explosive eruption in 1790.

The next question was, "With HVO located next to the caldera, is there any danger?" The answer is a qualified "yes." If an event similar to the 1790 eruption occurred today, HVO would be destroyed. However, these events are very infrequent, and if one were to happen, we would be able to predict it and evacuate.

Another question was, "Why are Kilauea eruptions different from those in Japan?" The answer is that the chemical compositions of the Hawaiian and Japanese magmas are different, and this causes a difference in physical properties. Hawaiian basaltic magmas are very fluid and allow volatiles (gases) to escape readily. Japanese andesitic magmas are more viscous and tend to retain the volatiles. The higher gas content produces a higher pressure which results in more explosive eruptions.

The program moved indoors into the observatory to show the various volcano monitoring techniques. One of the questions posed by the host was, "Why are earthquakes important in the monitoring of the volcano?" The answer given is that earthquakes can indicate the expansion or overpressurization of a magma body or track the movement of magma within the volcano.

The scene shifts to the caldera floor near the 1982 spatter ramparts. Looking at the 1982 lava flow, Don is asked, "Why is lava black?" The answer is that the dark color of the shiny surface of the flow is from the glassy layer that forms when molten lava is chilled rapidly. If the melt were allowed to cool very slowly, minerals would be able to form and grow. The resulting color of the rock would be much lighter.

Another question was, "What causes the older rock surfaces to turn white?" The answer is that the action of hot, sulfurous liquids and gases alters the glassy surfaces, and secondary white (sulfide) minerals are formed.

The line of spatter ramparts on the caldera floor resemble the outline of the back of a giant stegosaurus. The question was asked, "How was this feature formed?" The answer is that this is the trace of the fissure that was active in 1982. The eruption was short-lived and, toward the end of the eruption, activity became sporadic and was confined to a few vents, but did not include the entire fissure. Occasionally, blobs of molten material were thrown out from the vents, and they coalesced to form the small cones.

The host asked, "How far down below our feet is the magma?" The answer is that seismic and deformation data suggest that the magma is two to four kilometers (1.2 to 2.4 miles) below the caldera floor, but on occasion, it can intrude up to a depth of only a few hundred meters (yards).

The show will be seen live in Japan and, it is hoped, will be rebroadcast in a format that can be viewed in Hawaii.

Eruption Update

The eruption of Kilauea Volcano continued unabated during the past week. Lava flows from the Pu`u `O`o vent to the ocean near Kamokuna in a network of tubes. Breakouts from the tube system continue to occur occasionally. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

No felt earthquakes were reported during the week ending on February 18.

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Updated: 23 Feb 1999