By Laura Kong
October 29, 1998. As the sun rose today, Tom Crook was bringing aboard the last of the transponders we had deployed for acoustical navigation 32 days ago. Gigabytes of sonar data and digital photographic imagery are stored on our computers, and a ton of rocks and glass chips are on board. It is 8 a.m., Day 35, and soon we will be bidding goodbye to our home for the last month. Once again, we will sail past the coastal lava entry from the Pu'u O'o eruption, a last reminder of the dynamic interactions between water and magma that we saw frozen on the Puna Ridge seafloor. It is time to call it a cruise.
Over the last month, we collected an impressive amount of data to aid our understanding volcanic of the processes along a centrally-fed, submarine volcanic ridge. We had picked the Puna Ridge because it was the undersea extension of Kilauea volcano's East Rift Zone, a subaerial rift zone that has been well-characterized volcanically and tectonically. Previous reconnaissance geological and geophysical studies of the Puna Ridge had suggested that magmas erupting along the ridge axis had traveled over 100 km from Kilauea's summit caldera at 1220-m elevation down rift through dikes beneath the East Rift Zone and erupted on the seafloor along the Puna Ridge. We wanted to understand how and under what physical conditions this occurred, if eruptions were concentrated in particular along- or across-axis zones along the ridge, and what types of volcanic constructs were being erupted.
To do this properly, Debbie and I designed a survey to map the entire length of the Puna Ridge. We used the DSL-120 side-scan sonar vehicle to characterize the styles and amounts of volcanism and tectonism, and followed this with ARGO II video and camera lowerings to investigate geologically significant sites. This provided Kevin and Jennifer with a geological map of the entire Puna Ridge and detailed images to guide their rock sampling program. We collected altogether 323 hours of DSL-120 side sonar data, 125 hours of ARGO II photographic imagery, 22 rock dredges and 56 glass chip wax cores.
What did we find? We found a topography that none of us imagined. As the side-scan sonar images rolled out of the Raytheon plotter, we saw crater after crater, flat-topped seamounts with a characteristic crescent-shaped shadow indicating a deep hole, craters with jagged edges and channels emanating from them, lots and lots of hummocky mounds and ridges, hummocky pillow ridges that covered ridge-parallel cracks and fissure systems, and lots of smooth sonar textures caused by low-relief lobate pillows, rubble-covered seafloor, sheet flows, and sediment. When we put down ARGO II to see for ourselves what was down there, an exciting new world come alive before us.
Our expert ARGO flyers took us down into the craters safely past the overhangs at the rim where old seafloor had collapsed revealing sheared-off pillow cross-sections, along 40-100 wide lava channels filled with rubble and sheet flows, and over fields of collapsed lava tubes that we followed for 100s of meters peering down the sinuous networks through which lava had flowed only several meters beneath the seafloor. Will, Jim, Jennifer, and Kevin took all of us down 50-m high lava flows with pillow toes poking out at its base and up over 8-m long skinny pillows that had cascaded over and down slopes. We saw fresh-looking, almost-spatter like lava from near volcanic vents, 10-m high lava spires, crenulated sheet flows with exquisite examples of lava drapery, and exposed on the crater rim of a cone near to the shore, a 1-2m thick lava flow showing columnar jointing.
On the ridge axis, there was very little sediment, lots of sponges, anemones, and corals thriving on the black lava rocks. Near shore, we saw black sand or hyaloclastites in pockets between the pillow lavas, and wondered whether it was derived subaerially or had formed in situ. And yes, we did see rubble, rubble everywhere -- at the base of steep scarps, ramps of angular pillow fragments making up the slopes of seamounts, the bottoms of collapse pits and the walls of craters on seamounts; any place where the seafloor had suffered an instantaneous collapse or tearing, we saw results of the event.
What does this all mean? For one thing, compared to the older, sedimented and tectonized terrain we found over the southeast flank of the Puna Ridge, the upper half of the ridge axis appeared to be active with many recent episodes of volcanism. And, with so many samples of fresh glass recovered from the wax coring and dredging, we are anxious to see whether we can date the individual lava flows. By analyzing the rock chemistries of the samples, we will learn what they will tell us about the source of the eruptions and how far they might have traveled from the source. The task before all of us now is to sort it all out. What happened when, where, why, and how, and finally can we predict what future eruptions along the Puna Ridge will be like. These are questions that we'll be trying to answer over the next few years. So, stay tuned, the real story is still to come!
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