Geologists measuring lava temperatures alongside a lava river. The intense heat (over 2,000 degrees Fahrenheit) requires use of heat shields and protective clothing. (U.S. Geological Survey photograph by T. A. Duggan.)

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3,000-foot elevation, destroying several square miles of native rain forest and moving to within 5 miles of the nearest homes on the outskirts of Hilo. Public concern mounted within that city, and HVO scientists continued to maintain round-the-clock surveillance.

Temperatures of the molten lava were measured repeatedly during the eruption because temperature changes would indicate changes in lava composition and fluidity, which could cause significant changes in the style and potential hazard of the eruption. Lava temperatures remained fairly constant during the eruption, around 2,080 degrees Fahrenheit, which is relatively "cool" for Hawaiian lavas.

On March 29, a levee of the principal flow channel collapsed at the 5,700-foot elevation, 8 miles upslope from the flow front that was threatening Hilo. Lava was diverted northward at this point to form a second flow parallel to the first, and most of the supply to the initial lava flow was cut off. This second flow also moved rapidly downslope at first but began to slow and did not overtake the first flow until April 4. A second channel collapse and diversion occurred at 6,800 feet on April 5, and a third flow headed rapidly downslope north of the others.

This flow moved only a short distance, however, as the rate of lava extrusion at the 9,400-foot vents began to decrease slightly, the lava feeding the flows appeared to become steadily more viscous, and numerous channel blockages and levee collapses occurred further and further upslope. This completely cut off lava being supplied to the lower flow fronts, easing the threat to Hilo. By April 14 no active lava flows extended more than a mile from the 9,400-foot vents, and the eruption ended on April 15.

About 250 million cubic yards of lava were erupted by Mauna Loa during this eruption, which buried about 12 square miles of the volcano's surface. This was the first eruption of Mauna Loa for which detailed monitoring was possible, and much more has been learned during this eruption than in all previous historic eruptions combined. Geological Survey crews spent the remainder of 1984 compiling eruption data and making detailed field observations of what had actually transpired as well as continuing studies of ongoing Kilauea activity. This information will be used to refine our understanding of Mauna Loa and to provide more acurate eruption predictions in the future at this and at similar volcanoes.

Io is a strange and facinating world. Tidal heating caused by the gravitational pulls of Jupiter and the other large Jovian moons result in Io's having the greatest volcanic activity and heat flow that is known in the solar system today (fig. 1). The average heat flow from its surface is about 19 times greater than that of the Earth and 50 times greater than that of the Moon. How is this tremendous amount of energy carried from within the moon to the surface of Io? Research by scientists from the U.S. Geological Survey and the Jet Propulsion Laboratory in Pasadena, California, supported by funding from the National Aeronautics and Space Administration (NASA), leads them to believe that the heat is transferred primarily through lava lakes composed of liquid sulfur. The studies by these scientists are based on a continuing analysis of the data returned by the

Voyager spacecraft and on new data
gathered at NASA's Infrared Telescope
Facility atop Mauna Kea Volcano in
Hawaii.

Seven hot spots on Io were located by the infrared spectrometer on the Voyager 1 spacecraft, and all correspond to relatively dark features on the moon's surface, much darker than the average reflectivity of the surface as a whole. These dark features make up only about one-tenth of 1 percent of the total surface area of Io, but they coincide with at least part of the seven hot spots seen by the Voyager infrared spectrometer. Earth-based observations from the Infrared Telescope Facility have enabled scientists to map the heat from the hot spots as Io rotates, so that the heat measured when the moon is in a certain position can be compared with the amount of dark areas showing at that same position (fig. 2). The

PERCENTAGE OF DARK

AREAS SHOWING ON IO

comparison shows that at any given position during rotation, the heat is greatest when the proportion of viable dark areas is greatest (fig. 2). If these dark features are assigned temperatures ranging from 242 degrees to 350 degrees Fahrenheit (appropriate for liquid sulfur), then the calculated heat is consistent with the Earth-based measurements. Furthermore, the albedos (relative ability to reflect light) and colors of the dark features match the reflectance spectra of liquid sulfur (fig. 3). Where high-resolution images are available, many of these dark features have characteristics suggestive of lava lakes, such as islands, floating debris, and chilled crusts (fig. 4). The most

energetic of these dark features has been named Loki Patera. These probable lava lakes may be efficient convecting systems for the transfer of tidally generated heat to the surface, where the energy is radiated into space. The stage is now set for an exciting scientific mission when NASA's Galileo orbiter spacecraft arrives at Jupiter in 1989. After delivering a probe into the Jovian atmosphere, the spacecraft will spend several years in orbital tour of the Jovian system. The Galileo Solid State Imaging Experiment, involving several Geological Survey scientists, will record changes in Io's appearance since Voyager and may even produce movies of active eruptions. Also on board will

Figure 2. Ground-based measurements of Io's heat flux made as the moon rotates (top) coincide with the concentration of dark features exposed at the same rotation position (bottom). Note the similar form of the two graphs; where the highest heat

value is measured, the

greatest percentage of dark areas is showing.