Volcano Watch — Underwater East Rift exploration reveals a few surprises

The East Rift Zone of Kīlauea Volcano is 130 kilometers long, but only 55 kilometers of the rift is above sea level. The lower 75 kilometers of the rift is submarine and extends east-northeast from Cape Kumukahi to a depth of roughly 5,400 meters (17,700 feet). The submarine portion, named the Puna Ridge, has been studied far less than the more accessible subaerial portion. In early October, we ran an oceanographic cruise to explore and sample the deeper submarine portion of the rift using the Navy's 6,000-meter submersible "Sea Cliff."

The dive part of the program was plagued by mechanical and electrical problems with the submersible, but we finally accomplished one complete dive on the axis of the rift zone at a depth of 2,900 meters. During the rest of the cruise, we collected swath bathymetric data of the entire submarine portion of the rift axis and its flanks and added survey data for the south flank of Kīlauea. This bathymetric data provides 100 percent coverage, so no features are missed. The map shows the general bathymetry of the Puna Ridge, the locations of the successful dive - sites where lava samples have previously been recovered by dredging from oceanographic ships - and several enormous lava flows that have ponded on the sea floor adjacent to the Puna Ridge.

Dr. Jim Anderson, a professor from the Geology Department at the University of Hawai`i at Hilo, was the observer for the successful dive on the rift axis. The submersible is a six-foot-diameter sphere filled with electronic equipment, two Navy pilots, and one observer. It has two manipulator arms to recover rock samples, external cameras to record what the bottom looks like, and thrusters to propel the submersible at speeds up to about 1.5 miles per hour. Generally, however, the bottom is rough and the submersible has to follow the bathymetry up and down, slowing progress to less than one mile per hour.

The 2,900-meter-deep section of rift was chosen for a dive target because a bottom photograph taken from a towed camera system showed flat, sheet-like flows in this area. Such flows interested us because we know that some lavas which erupted deep along the submarine rift have eruption temperatures much hotter than is known for any historic eruptions on land. These hot lavas would be more fluid and form flat, sheet-like flows instead of the more standard pillow lavas. They are important because they represent melt that has come directly and rapidly from deep in the Earth (in contrast with the usual lava erupted subaerially that has resided in the magma chamber beneath Kīlauea caldera and has cooled, partially crystallized, mixed with other magmas, partially melted the rocks around the magma chamber, and has otherwise been modified from the composition that originally arrived from deep in the Earth).

The submersible reached the bottom after a two-hour descent through the water. At the touchdown site, the sea floor consisted of abundant pillow lavas, with some sediment between the pillow-like lava lobes. The presence of sediment indicates that the flow was relatively old, thereby allowing time for sediment to accumulate. The area was delineated by a series of sharp ridges and troughs, but all consisted of pillow lavas. The dive proceeded uprift to locate the sheet-like flow seen in the bottom photograph. Within a few hours, the sediment between the pillow lavas was replaced by black sand, similar to that formed when lava flows into the sea. After a few more hours of searching, the sheet-like flows were found and sampled. At this point, the Navy used the remaining bottom time to test some equipment on the submersible before it returned to the surface nearly 12.5 hours after it was launched.

There were several surprises found on the dive. The first was that the bottom water in the area had an elevated temperature, indicating hydrothermal discharge on the sea floor. This is the first time that hydrothermal activity has been seen on the submarine rift zone of Kīlauea Volcano. The presence of sand formed along the submarine rift zone was also surprising. We have sampled submarine-erupted sands offshore, but most sand found offshore forms when lava, which has erupted subaerially, enters the sea. The abundance of sand suggests that the sheet flows were erupted rapidly and incorporated seawater within the flow. The seawater then expanded to steam, produced small explosions, and fragmented the lava to sand-sized particles.

The next surprise was that the sheet-like flow did not turn out to be unusual in composition; it looked just like all the pillow lavas that have been dredged from Puna Ridge. The sheet flow was, however, surprisingly unaltered and, therefore, young. As volcanic glass sits in seawater, it slowly alters to clay minerals and iron oxides. That the glass on the sheet-flow samples showed none of this alteration suggests that the flow is historic in age. There are only two times we suspect that lava has migrated down the East Rift Zone past the coastline: in 1790 and again in 1924. In 1924, the ground near Kapoho subsided as magma was withdrawn and migrated downrift. Since the sheet flows were also associated with active hydrothermal discharge, we suspect that the flow sample erupted in 1924, when the last steam explosions occurred at the summit of Kīlauea.

Understanding the entire volcanic system at Kīlauea requires work on that part of the volcano which lies beneath the sea. Later this month, another oceanographic ship - from the University of Washington - will continue to explore the area where this dive took place. They will be collecting samples of water to locate the site of hydrothermal discharge that we discovered. We hope to return to sea in late February or March on yet another ship to continue our samples of the lavas erupted under the sea.