August 30, 2013
Today was an amazing dive at a lava pond complex at the southern end of the South Rift of Axial Volcano, 50 kilometers from the summit. We dove here in 2005 using the ROV Tiburon without the benefit of high-resolution maps, and mapped it in 2011 with the AUV at one-meter resolution, but did not have a chance then to do any more ROV dives. Having studied the maps, we realized we had so many more questions about the pond complex that we just had to come back. (After today’s dive, we know that we need to expand our mapping of the region and probably return again with the ROV.)
The pond complex is a six-kilometer-long cluster of six large, and many smaller, lava ponds perched on the edge of the spreading ridge’s axial valley. Outside the complex are still more, but shallower, ponded flows extending up and down the rift zone. The large ponds have vertical levee walls up to 107 meters tall (the AUV map was a challenge to design!), and the volume of the ponded lava was about 0.5 cubic kilometers (Kilauea’s typical output is 0.1 cubic kilometers per year). Among the questions are: “Where did the lava go?” and “How long ago did this happen?” Therefore, the primary goals of our dive today were to examine a suspected breach point in one wall and follow the lava that flooded out across the axial valley, and to take sediment cores for dating the underlying flows, because when we were here before we had not yet developed that technique. We also are trying to understand the mechanics of this style of eruption, which is unusual for mid-ocean ridges but must happen repeatedly here, and how it ties with activity at the summit.
We began inside the northeastern-most pond and climbed the nearest wall. Lava draperies, veneers, and lavacicles plastered the wall, left after the pond abruptly drained and the molten interiors of lobate pillows and caverns continued to percolate out. We passed through the breach and encountered enormous blocks of veneered levee wall that had been rafted by the violent outpouring of lava. We found the chaotic jumbled sheet flow ‘a’a-like material had ramped against the east wall of the axial valley (which was an ordinary, tectonically faulted wall exposing layers of lava pillows), coursed to the north past a large pillow cone, and also flowed west into another large collapsed lava pond floored by ropy sheet flow. At the end of the dive we climbed up the west wall of the axial valley and collected the last of our full compliment of 15 sediment cores for dating and our final rock of the expedition. Most of the 16 lava samples we collected on this dive were from the pond’s levee walls or jumbled sheet flow outwash, so from the same flow, but they may have subtly different crystal, chemical or gas compositions. They are plagioclase ultra-phyric, and the extreme crystallinity may have contributed to the lava drapery’s viscosity, persistence, and spectacular scenery.
— Jenny Paduan