By Edwin Schiele
October 17, 1998. Imagine trying to find a tennis ball in a field. It is dark, and all you have is a flashlight that you must keep pointed down. Or imagine trying to decipher a giant mural in the dark. All you have is a small pen light with a battery that will soon go dead. Photographing features on Puna Ridge with ARGO II provides somewhat the same feeling. We are trying to interpret a massive volcanic ridge by shining a light on small pieces of it.
Last night I sat on my first watch monitoring ARGOs progress. The control van looked a bit different. Gone was the colorful sonar screen. Instead, large monitors on the wall displayed images taken by ARGOs three video cameras (two color and one black-and-white) and one still camera. Two of the video cameras and the still camera look straight down. The other video camera looks forward. Bright lights, including a strobe, in the back of ARGO illuminate the surface. ARGO has a 200 kHz sonar facing down that produces much more detailed images than the 220 kHz sonar on the DSL 120. It also has a 100 kHz sonar that faces forward. As before, the Raytheon printer in control slowly spits out a steady stream of images.
The ship tows ARGO at a speed of one-half knot. Any faster and we would have a difficult time keeping up with the video images. The flyer guides the fish within several meters of the ocean bottom. The flyer relies on the forward-looking cameras to see whats ahead. The watch leader provides guidance on the type of terrain coming up. The ARGO also has two thrusters that enable the flyer to adjust the fishs heading. With so little margin, accidents do happen. But unlike the DSL 120, ARGO is built to withstand the occasional scrape with the bottom.
I sit in a corner of the control van facing a bank of four small monitors displaying the video and still images, five cassette decks that record the video and still images, and a computer keyboard. My job is to watch the video monitors and log the features I see onto the computer. Meanwhile, the watch leader, Frank Trusdell, studies the video images on the larger monitors and reports features that I should log in.
During my watch, I got quite good at typing "pillow lava," "rubble," and "sheet" or "lobate flows." Pillow lavas are bulbous, pillow-shaped features that are abundant wherever lava erupts underwater. When the erupting lava contacts the cold water, a thin crusts forms over the surface almost instantly. The lava underneath this thin crust, however, remains molten. As pressure from the molten lava builds, the crust cracks, and lobes of molten lava bulge out. Think of forcing honey through a crack in a jar. The surface of these bulges then crusts over, and the process repeats itself. Often, another eruption or an earthquake can cause the pillow lavas to crumble into rubble. Faster moving lava hardens into smooth or undulating surfaces called sheet flows.
I also noted in the computer log if there was sediment covering any of the features, a sign that the eruption that created these features occurred long ago. Pillow lava on top of rubble indicates that lava from a more recent eruption has buried lava from an earlier eruption.
Amid the pillow lavas, rubble, and sheet flows, we find larger prizes such as faults, fissures, lava tubes, or grabens that can really tell us what has gone on along the ridge. Here is where it begins to feel like we are using a flash light to search an entire field. Based on what we see on the sonar images, we are targeting areas and features such as terraces, fissures, and craters that we think will tell us the most about the dynamics of the ridge. Using the sonar images, bathymetric maps, and sophisticated navigation equipment, we can almost always hit our targets. If the watch leader or one of the principle investigators wants to investigate a particular feature, he or she can instruct the flyer or bridge to change course. Yet we also rely on luck. We dont really know what we will find down there or if we will see features that will tell us what we want. The cameras illuminate an area that is approximately 15 by 20 meters. It is easy to miss things, even large features such as fissures. At the same time, we might fortuitously cross features that we did not expect.
So when we do find what we are looking for, or see things that give us insights about the ridge, or come across surprises, the feeling is that much more satisfying.