By Edwin Schiele
October 20, 1998. Last night, we started surveying one of the most intriguing areas of the ridge. Earlier we had mapped an area along the south flank of the ridge filled with pits, craters, and terraces perched on top of steep towering slopes, and now we want to take a closer look.
The overriding question is whether the features on the south flank, particularly the craters, were created by primary or secondary eruptions. The scientists on board are split about which is more likely. During primary eruptions, the magma rises directly from a reservoir or dike. The eruption from the Puu Oo crater on Kilauea is an example. A secondary eruption occurs when lava from a primary eruption flows down the slope through an underground channel or lava tube, then breaks back through the surface. The difference between a primary and secondary eruption may be straight forward, but determining whether the craters on the south flank are products of a primary or secondary eruptions is not. If they were formed from primary eruptions, it would likely mean that a dike had to have branched off the main axis of the ridge and pushed under the south flank.
Towards the end of the previous line, the front-facing camera broke down and needed to be replaced. Because we were entering territory filled with towering bluffs and craters with overhanging rims, we need to see whats in front of ARGO II. We also wanted to study the walls of the pits and craters. In some instances, the patterns on the crater walls can help us determine whether these features formed from primary or secondary eruptions. So when ARGO II was pulled back on board the DSL group faced the wide-angle video camera forward and repositioned one of the lights so that it would illuminate farther ahead.
My watch got off to a typical start. ARGO II was in the middle of its 3,200 meter descent to the bottom, so there were no images to log. Frank passed around his weird dried fruit, and I prepared to wait. What saved me from tedium was a small, eyeball-shaped camera that takes pictures every second and displays them in real time on the computer monitor. Tim Dulaney (the logger from the previous watch) and I amused ourselves by pointing the camera at each person in the control van, then snickering at the images as they appeared on the screen. After 45 minutes, ARGO II reached the bottom, and the party ended.
First we explored a steep 500-meter bluff. Based on the sonar images, we expected to see lava flows covering the slope. Instead we found rubble. (Thats why we send down ARGO II rather than basing all of our conclusions on the sonar data.) We climbed to the terrace on top of the slope and found pillow lava extending right to the edge of the precipice. What we think may have happened was that lava erupted higher up on the ridge, flowed down, then accumulated on the terrace. Over time, edges of these flows crumbled away and cascaded down the slope. Its as if the slope is a great big garbage dump. The fact that sediment covers parts of the slope (old) and does not cover other parts (not so old) indicates that these slope failures happened several times.
The real prizes of the line were the pits, craters, lava channels, and collapsed tubes. This afternoon, I wandered into the control van at an opportune moment. ARGO II was hovering over a ten-meter deep pit that likely formed when a tube collapsed. The overhang was large, so lowering ARGO II into the pit was too risky. But soon we could see the opening of the tube. It was like looking into the mouth of a cave. A thin layer of lobate flow covered the top. We followed the tube to another area where it had collapsed. At one point, we could see a thin slab of lava arching over the collapsed tube. By this time, 13 people had crowded into the control van to watch the monitors.
From this second pit, we followed a lava channel, then descended down a rubble slope into a crater. Inside the crater we saw large cracks and lots of pillow lava. The trick is to determine whether lava flowed from the channels into the crater or from the crater into the tubes. If it is the former, we can rule out a primary eruption as being the source of the crater. If it is the latter, we can rule nothing out.
Tomorrow, we will continue our discussion of the features on the south flank.
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