What Lies Beneath: The Volume of Kīlauea’s Magma Chambers
The following is this week’s edition of “Volcano Watch” from the USGS Hawaiian Volcano Observatory:
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 now be well on his or her way to becoming a volcanologist, because one of the most fundamental questions of volcanology today is “How big is that red blob under the mountain?”
In fact, we know remarkably little about the volumes of magma stored beneath most volcanoes, including Kīlauea. This isn’t just an academic question, because the amount of lava or ash that a volcano can produce during a climactic (sudden, dramatic, and brief) eruption is limited by the size of its magma reservoir. Kīlauea’s eruption has been ongoing for more than 30 years, because the magma chamber is being recharged by new magma from below, but it, too, is capable of large, sudden, explosive eruptions, as detailed in previous Volcano Watch articles.
Volcanologists use a variety of techniques—none perfect—for estimating 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 these 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 that 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 Kīlauea, however, 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 Kīlauea’s magma chamber? There are at least two, and probably more, magma storage regions beneath the summit of Kīlauea. Let’s focus on the one closest to the surface, most likely connected directly to the lava lake currently active in Halemaʻumaʻu Crater. Current research suggests that the volume of this reservoir is roughly one cubic kilometer (about 0.2 cubic miles)—that’s about 15 times the amount of water in Hilo Bay, or enough to form a layer about 10 cm (4 inches) deep if poured evenly over the entire Big Island.
As volcanologists develop new techniques and obtain new measurements of activity at Kīlauea, 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!