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SOME lava flows are masquerading as cool rock, making them practically invisible to satellites trying to pick up 鈥渉ot spots鈥� on Earth.

Andy Harris from the University of Hawaii and his colleagues took a close look at active lava flows from the Santiaguito dome in Guatemala. Lava of this kind breaks off in chunks as it cools, leaving a rocky crust between 2 and 10 metres thick on top of the hot flow. This layer can keep the flow molten for up to a year, while leaving the surface at a relatively cool 30 掳C to 100 掳C. These flows can creep along for kilometres, or a build-up of hot gas can cause them to collapse suddenly, releasing blasts of hot ash and lava. About 70 per cent of volcano-related deaths worldwide are caused by this kind of explosion.

The team measured the heat coming from Santiaguito鈥檚 flows using hand-held thermal infrared detectors, while the Landsat 7 satellite took pictures in both the thermal and short-wave infrared. Surprisingly, they found that the short-wave infrared signals were swamped by reflected sunlight, making the lava practically invisible to the satellite. They could only see the flows if they added the thermal infrared data.

Short-wave infrared is the normal frequency used to search for hot spots, such as volcanoes and forest fires, from space. But Harris says volcanologists should look at thermal infrared data too, to avoid missing 鈥渟tealth鈥� lava flows. Rob Wright, who helps run a NASA 鈥渉ot spot鈥� website, adds that some satellites now use a longer wavelength that isn鈥檛 masked so much by sunlight. So volcanologists could restrict themselves to just that data. Or they could always hunt for volcanoes at night.