Magnetics, magma, and monitoring: new technology for old questions — USGS Volcano Watch

Earth’s magnetic field surrounds us every second of the day, everywhere on the planet. Anyone who has picked up a pocket compass and seen the magnetic needle quickly align itself has seen the action of this ever-present invisible field. But can we harness the magnetic field to forecast volcanic activity? Emerging technology in the field of “quantum” science may aid us in doing so. On a large scale, the structure of the Earth is divided into four main layers: the crust, the mantle, and the inner and outer core. The outer core, which starts at about 1800 miles (2,900 km) beneath your feet and extends for an additional 1,400 miles (2,260 km), is composed mostly of iron and nickel. At these depths the temperature (about 9,000 Fahrenheit or 5,000 Celsius) keeps the outer core fluid and constantly moving. This movement sets up a process a bit like an electrical generator you might be familiar with—the moving metallic fluid creates electrical currents. These currents generate the Earth’s magnetic field—the same one you use to orient a compass to North. At volcanoes, variations in the magnetic field arise primarily from four sources: long-term changes related to changes in the motion of Earth’s outer core, external electrical currents, space weather events (such as solar flares), and changes in magnetic properties of rocks due to volcanic activity. In general, the changes caused by the first three sources of variation can be considered relatively uniform over a small area, which allows us to correct for them using measurements at a remote, but still “local” and “magnetically quiet” reference site. Rapid magnetic changes associated with volcanic processes are usually very small, between 1 and 10 nanoTesla (nT) units. For context, a refrigerator magnet has a magnetic field strength of about five million nT! The sensitivity of magnetic measuring instruments (“magnetometers”) determines whether volcanic changes can be detected within the considerable noise produced through other, non-volcanic, electromagnetic fluctuations. These volcanic fluctuations can arise from variations in the magnetization of rocks induced by stress redistribution or changes in the thermodynamic state of the volcanic edifice. Italy’s National Institute of Geophysics and Volcanology (INGV) has had a magnetic monitoring network on Mount Etna for over 20 years. The network is comprised of eight stations on the volcano and a reference station. All stations measure the total local magnetic field every 5 seconds, and the network is designed to make it possible to easily distinguish deep sources of change from superficial ones. From their long time series of data, INGV has shown that the variations they observe in the magnetic field can often be attributed to shallow magmatic intrusions (when magma moves into a new area beneath the surface but fails to erupt). These intrusions cause permanent magnetic anomalies which, together with observed earthquake swarms and the ground deformation, generally precede and accompany magma moving towards the surface. For example, INGV has observed pre- and syn-eruptive magnetic anomalies during the eruptions of Mount Etna in 2001, 2002, and 2008. At Kīlauea, measuring changes in the magnetic field to monitor the volcano has only been briefly explored. Initial investigations were performed by USGS Hawaiian Volcano Observatory (HVO) geologist Gordon Macdonald between 1950–1951. Macdonald realized that conditions weren’t ideal, but he gave it his best attempt given available time, staffing, and the state of the current technology. Two decades later, in 1973, HVO geologist Paul Davis showed that daily averages of records from three synchronized magnetometers on Kīlauea revealed a 1.5 nT change in the local magnetic field during the ongoing Maunaulu eruption. A few years later, USGS volcano geodesist Dan Dzurisin operated a small network of magnetometers, but no results were ever published. Similar to recent instrument advances in measuring the absolute acceleration of gravity, newly available quantum-based sensors have opened the door on previous magnetic-monitoring challenges. These new quantum magnetometers provide comparable sensitivity and data rates to traditional technology but require significantly lower power and are simpler to install. The new sensors are also dramatically smaller, about the size of your thumb! While HVO does not currently operate any quantum-based magnetometers, the observatory is constantly testing the potential of new technology for volcano monitoring. The questions we ask about volcanoes fundamentally remain the same—where and when will a volcano erupt—and our ability to answer those questions only gets better as technology advances. Will magnetic monitoring have a future in monitoring Kīlauea volcano, as HVO scientists from 70 years ago hoped? We don’t know yet, but when we do, we’ll let you know! — Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates. Today's article is by HVO geophysicist Ashton Flinders.