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December 3, 1998

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

Why study the Puna Ridge?

Scientists have learned a lot about the on-land part of Kilauea's east rift zone, but little is known about its underwater part, the Puna Ridge. A research cruise late in September and October was the first to explore the ridge in detail.

What did we learn? At a scale of 1-2 km, the Puna Ridge looks very different than the on-land rift zone. Large volcanic terraces, up to hundreds of meters (yards) high and several kilometers (miles) across, cover its axis and flanks. By comparison, the on-land rift zone is typically covered by low lava flows and low cones.

At a scale of tens to hundreds of meters (yards), however, we sometimes felt that we were walking along the east rift zone on land. We found fissure systems with volcanic vents along them that resemble Mauna Ulu and Pu`u `O`o, with strings of spatter cones built over the fissures. We mapped large collapsed craters similar to those along the Chain of Craters. We photographed lobate flows similar to pahoehoe flows on land. We also photographed skylights and looked into subterranean lava tubes, resembling those currently carrying lava to the shoreline from Pu`u `O`o.

From the description above, you may think that it will be easy to interpret all of the data that we collected. That isn't the case. While, in some places, it was easy to identify a volcanic vent, in most places it was not. What are the sources of those large terraces? Are the terraces located over an eruptive fissure or are they large flow fronts fed from a vent further up the ridge? What do the multitude of large collapse craters tell us about how magma and lava moves beneath the Puna Ridge. We are not yet sure.

The south flank of Kilauea above sea level has many stair-step faults and steep slopes covered by rubble and draping flows. The south flank of the Puna Ridge does, too. High-angle slopes and talus fans are evidence for large-scale mass wasting events.

An important finding is that lava flows in deep water are richer in olivine (a heavy mineral that the Green Sand Beach is made of) than are flows in shallower water. A simple explanation for this is that the heavy olivine settles out of the magma during transport and storage. Imagine that we have a column of magma with olivine crystals evenly distributed through it. Then we allow the magma to sit. After a period of time, the olivine would start to settle down toward the bottom of the column.

Well, the depths of the Puna Ridge span from sea level to 5,000 m (16,000 ft) depth at its distal end. Therefore, this simplistic model can explain--and allows us to predict--why the deeper lava flows have more olivine. Geochemical analysis of the lava flows will test and refine this and other ideas about magma movement and storage.

These are just some of the preliminary findings from the Puna Ridge cruise. In the coming months, we will close in on new discoveries as we analyze more than 600 km (400 miles) of high-resolu-tion side scan imagery, 29,000 photographs, many kilometers (miles) of video footage, scads of magnetics data, and rock samples from 21 dredge sites and 56 wax cores. Some of the discoveries will reconfirm hypotheses that currently exist; others will lead to new ones. Ultimately, the data from this cruise, will lead us to ask many new questions about the Puna Ridge and the formation of submarine volcanoes. Although we got a really good look at the Puna Ridge, we must infer many things. If we missed something, we will not be able to go back to fill in the missing pieces. If only we could drain away the water and walk along the 70 km (42 miles) of the Puna Ridge just like we do along the on-land part of the east rift zone. . . .

Eruption and Earthquake Update

The east rift zone eruption of Kilauea Volcano continued unabated during the past week. Lava was still flowing through a network of tubes from the vent at Pu`u `O`o to the sea coast, where it enters the ocean near Kamokuna. 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.

There were no felt earthquakes since November 22.

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Updated: 08 Dec 1998