What lies beneath: The volume of Kilauea’s magma chambers


Ask a child to draw a volcano, and he or she will likely sketch a cone-shaped mountain erupting lava high into the air — with possibly a dinosaur or two thrown in for good measure. An older child might include a red blob under the mountain representing the volcano’s magma chamber. This child may be on his or her way to becoming a volcanologist. One of the most fundamental questions of volcanology today is: “How big is that red blob under the mountain?”

We know remarkably little about the volumes of magma stored beneath most volcanoes, including Kilauea. This isn’t just an academic question, because the amount of lava or ash a volcano can produce during a climactic — sudden, dramatic and brief — eruption is limited by the size of its magma reservoir. Kilauea’s eruption has been ongoing for more than 30 years, because the magma chamber is being recharged by new magma from below, but it is capable of large, sudden, explosive eruptions, as detailed in previous Volcano Watch articles.

Volcanologists use a variety of techniques — none perfect — to estimate the volume of magma storage beneath volcanoes. An upper limit may be obtained by measuring the volumes of material produced during individual climactic eruptions in the past, since the volume of erupted material must presumably be smaller than the volume of the reservoir. Another technique involves recording changes in seismic waves as they pass through magma in the crust; by examining many such waves, seismologists can infer where magma is stored and make rough estimates of its volume.

We can also estimate reservoir volume by looking at geophysical data recorded during eruptions. Imagine releasing the air from a balloon. The air will at first exit very rapidly, then more slowly as the pressure drops. How rapidly the pressure drops will depend on, among other things, how quickly the air is released and, most importantly here, the size of the balloon.

Remarkably, during eruptions, magma chambers embedded in solid rock can behave a bit like balloons, and, as a result, we observe decreasing rates of lava eruption and deformation of the surrounding rock. Although rock seems very solid, it can actually behave like an elastic material, stretching and bending in response to forces such as those exerted by magma. If we know a little bit about the properties of the magma and the conduit linking the chamber to the surface, we can estimate the volume of a magma chamber by looking at how quickly rates of lava eruption and ground deformation change during an eruption.

Additionally, we know the magnitude of ground deformation we observe at the surface depends on the volume of the reservoir and the pressure change that occurs within it. Unfortunately, small pressure changes in big reservoirs and big pressure changes in small reservoirs can produce almost exactly the same amount of ground deformation at the surface, making it very hard for us to independently estimate the chamber’s volume using ground deformation data. At Kilauea, we are lucky to have a summit lava lake, and we are able to independently estimate pressure changes in the reservoir by relating fluctuations in the level of this lava lake to eruptive activity. With this information, we can then estimate the volume of the chamber.

So, how big is Kilauea’s magma chamber? There are at least two, and probably more, magma storage regions beneath the summit of Kilauea. Let’s focus on the one closest to the surface, most likely connected directly to the lava lake currently active in Halemaumau Crater. Current research suggests that the volume of this reservoir is roughly 0.2 cubic miles — that’s about 15 times the amount of water in Hilo Bay, or enough to form a layer about 4 inches deep if poured evenly over the entire Big Island.

As volcanologists develop new techniques and obtain new measurements of activity at Kilauea, we’ll have a better idea of how much magma is stored within the volcano. Until then, kids should feel free to draw those magma chambers with wild abandon.

Kilauea activity update

A lava lake within Halemaumau produced nighttime glow visible via HVO’s webcam during the past week. The lava level fluctuated with several deflation-inflation cycles and was about 115 feet below the rim of the overlook vent Thursday.

On Kilauea’s east rift zone, the Kahaualea 2 flow continued to be active northeast of Puu Oo. After the flow front stalled several weeks ago at a distance of 4.8 miles northeast of Puu Oo, surface flows have been active behind the stalled flow front, up to 4.6 miles northeast of Puu Oo. Webcam images indicate that small forest fires are continuing.

There were no earthquakes reported felt on Hawaii Island in the past week.

Visit the HVO website, hvo.wr.usgs.gov, for Volcano Awareness Month events and current Kilauea, Mauna Loa and Hualalai activity updates, recent volcano photos, recent earthquakes and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.

Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.