(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)
This month, researchers from the University of Hawaii teamed up with the Hawaiian Volcano Observatory staff to use laser-based technology to peer into the depths of a new pit in Halemaumau. What they saw was startling.
The look revealed that the vent has a cavernous interior that dwarfs the size of the opening that we see at the surface. The cavern is approximately 650 feet below the floor of Halemaumau, making it a dizzying 935 feet below the now-closed Halemaumau overlook area.
To obtain the image, the UH researchers were using a technology called Light Detection and Ranging, or LiDAR, for short. By bouncing pulses of lasers off objects, LiDAR can determine the distance to out-of-reach objects.
Readers may be familiar with similar technology used in the laser range finders employed by golfers, hunters, and builders. Simple laser range finders measure a single distance to an object. In contrast, scientist using LiDAR data can produce entire images, or models, of distant surfaces based on thousands of individual laser bounces.
The remarkable part of the image is that it reveals a very deep pit crater with dramatic overhanging walls near the surface. This is different from the steep and vertical walls of the larger Halemaumau and Kilauea caldera.
What the image actually reveals is a snapshot in the evolution of a Hawaiian pit crater.
“Pit crater” is a general term used to describe the steep-sided, semi-circular, and flat-bottomed craters that are characteristic of the summit and rift zone areas of Kilauea and Mauna Loa volcanoes.
Interchangeable terms for these features include, “collapse crater,” and of course the eloquent Hawaiian name for a pit, “lua.” But how are these craters created?
Looking into this deep hole created by a volcano may lead one to think that all the material was blasted out. But if this were the case, we would be able to see a vast amount of excavated material deposited around the rim of the pit.
In fact, the amount of ejected material, or ejecta, surrounding Hawaiian pit craters is relatively small — far less than the amount of material required to fill the hole back up. And, some Hawaiian pit craters do not have any ejecta surrounding them.
The answer to how pit craters form is being witnessed right now as the new vent evolves. Hawaiian pit craters form primarily through the evacuation, or loss, of magma somewhere beneath the vent.
As magma evacuates from the plumbing system below, the conduits collapse and the vent floor sinks away. The walls may become overhung, since the interior of the pit expands as it drops.
Eventually, the overhung portions of the rim collapse into the pit, and the characteristic near-vertical sides of the pit crater are formed.
Our observations of lava within the vent, data from instrumentation, and measurements of magma flux at the east rift zone near Kalapana, suggest that the amount of magma within the plumbing system of Kilauea has been reduced.
This ebbing has caused the drop in the lava level and the growth of the cavern we now see. The recent pulses of ash-laden plumes issuing from Halemaumau have been caused by collapses of the pit’s overhanging rim.
Of course, the drop in the vent floor, and ebb in the flow of magma within the system, does not necessarily mean the eruption is winding down. A characteristic behavior of Hawaiian pit craters is that the floor may refill with a lava lake, only to drain and collapse again and again.
So the laser’s look into the lua is certainly only a snapshot of the on-going evolution of Halemaumau.