Predicting volcanic eruptions is notoriously tricky. In large part this is because volcanos are unique, each with their own quirks and personalities: the lessons learned from studying one volcano may not apply directly to another. Luckily, researchers are getting better at finding warning signs that they can apply broadly. Some of the most well-known are heightened seismic activity, rising temperatures, expanding magma pools, and the release of gases. New research using satellite imagery now offers a new warning sign for underwater volcanos: a change in the color of the ocean.

The idea is simple: it has long been known that as underwater volcanos prepare to erupt, the gases and compounds they release affect the composition of the surrounding seawater. Iron-rich water looks yellowish or brown, for example, while aluminum and silicon turn the water white. The challenge has always been in systematically applying this information to make useful predictions. Measuring these color changes accurately isn’t easy.

Yuji Sakuno, associate professor at Hiroshima University, has been working on this problem. As an expert in remote sensing, his key tool in this endeavor is the Japanese Space Agency (JAXA)’s Global Change Observation Mission – Climate (GCOM-C) satellite. GCOM-C observes the ocean every 2-3 days at 250-meter resolution, giving Sakuno reliable data about changes in water color over time.

By combining GCOM-C imagery with eruption information from Himawari-8 (a geostationary weather satellite) Sakuno was able to note changes in sea water colour about a month prior to volcanic activity on Nishinoshima Island.


This photo shows a sample of the (Fe + Al)/Si distribution as a volcanic activity index from May 16 to June 25 around Nishinoshima Island: (a) May 16-23, 2020, (b) May 24-31, 2020, (c) June 1-8, 2020, (d) June 9-16, 2020, (e) June 17-24, 2020, (f) June 25-July 2, 2020. It is mapped by applying the equation ((Fe+Al)/Si)=45.4(x)-13.3 to the SGLI (Second Generation Global Imager – an instrument onboard GCOM-C) data. From this, it can be seen that during this period, the distribution on the northeastern part of the island rises, and then the discoloration gradually progresses to the sea area around the island, before it disappears. Nishinoshima Island is located approximately 1,000 km south of Tokyo, Japan. The original data used for this product have been supplied by JAXA’s JASMES (JAXA Satellite Monitoring for Environmental Studies). Credit: JAXA/Yuji Sakuno.

One of the breakthroughs that made this possible involved finding a way to measure color accurately, despite the way that sunlight can distort and play tricks with apparent water color. Sakuno looked to other areas of research to find a solution: previous work done on hot spring water provided the tools needed to counteract the Sun’s distortions.

Sakuno has big plans for this technique: “In the future,” he said, “I would like to establish a system that can predict volcanic eruptions with higher accuracy in cooperation with the Japan Aerospace Exploration Agency (JAXA), the Maritime Security Agency, which is monitoring submarine volcanos, and related research.”


This image illustrates the colorimetric data of discolored seawater in four directions (north, east, south, and west) around Nishinoshima Island in 2020. The study investigated the color characteristics of the water to validate if the data obtained by SGLI accurately captures the actual conditions of the discolored seawater. It detected significant fluctuations in the distribution of chemicals in Nishinoshima Island, estimated from SGLI data, about one month even before the volcano became active. Credit: Yuji Sakuno.

This research also underlines the value of Earth
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