ASK-A-GEOLOGIST by Jeff Wynn

Deformation, GPS, and GNSS. Wait! Is that volcano… swelling? Part 1.

We get a lot of questions about volcanoes, including how to “know if she’ll blow.” There are a number of ways we can track magma movement at depth, including monitoring deformation and tracking “LP’s” — long-period (low-frequency) seismic tremors indicative of deep magmatic fluid movement. At late stages of unrest, we will start seeing “Vt’s” — short-period volcanic seismic evidence of rocks breaking — and often dramatic increases in Carbon-14-depleted (ancient) CO2 and H2S (rotten egg smell) gases. There is a good possibility that we can detect very early movement of magma at 30–40 km depths using magneto-telluric systems, something I proposed in 2007, but so far there hasn’t been enough funding to try this.

As I write this, edifice deformation currently reaches out in time the longest of any of these detection systems to give us warning of an impending eruption — up to months ahead of time.

The term “deformation” is used by specialists in ground movement in the geosciences; these guys call themselves “geodesists”. Geodesists measure movement as a component of strain along an active fault, to try to get a sense of the elastic energy accumulating that could lead to an earthquake. Think: is that spring getting tighter? Deformation is also used in volcanology to look for — and then track — inflation in a volcanic edifice. Deformation is measured in a number of ways:

1. Surveying the ground with high precision. This has been done at Yellowstone since the mid-1920’s, and those early data have helped us get a much better sense of how the huge caldera moves and breathes over time. It’s not at all unheard of to find a section rising 20 cm (7.5 inches) in a few weeks at Yellowstone. Typically, another part of the caldera will be deflating at the same time.

A USGS geodetic survey team working on the Kilauea volcano southwest rift zone, Hawai’i. Photo by the author.

2. Deploying tilt-meters. Originally these were long tubes of water laid out over the ground. If the ground under the flank of a volcano started tilting, a very tiny vertical movement would show up in amplified displacement of water in vertical tubes at the end of the long horizontal tube. Modern tiltmeters are ultra-sensitive cylinders placed in a vertical hole in the volcanic rock, then packed in with sand. The signal from these devices and all the following systems (below) is generally telemetered back to a recording and monitoring station.

A modern tiltmeter being lowered into a borehole at Mount St Helens, WA. Photo by the author.

A tiltmeter station (electronics and telemetry) nearing completion, Mount St Helens, Washington (author on left). Note the telemetry antenna above the author. Photo using author’s camera.

Next: Volcano Deformation, Part 2.