After maintaining a low simmer for 10 months, Italy’s Etna Volcano boiled over on February 19 and 20, 2013, with three outbursts in 36 hours. Each outburst, or paroxysm, featured “emission of lava flows, pyroclastic flows, lahars,
and an ash cloud,” according to the Italian Istituto Nazionale di
Geofisica e Vulcanologia (INGV).
The Advanced Land Imager (ALI) on the Earth Observing-1
(EO-1) satellite captured Etna on February 19 at 9:59 a.m. Central
European Time, about 3 hours after the end of the first paroxysm. The
false-color image combines shortwave infrared, near infrared, and green
light in the red, green, and blue channels of an RGB picture. This
combination differentiates the appearance of fresh lava, snow, clouds,
and forest.
Fresh lava, erupted hours earlier, is bright red—the hot surface emits enough energy to saturate the instrument’s shortwave infrared detectors, but is dark in near infrared and green light. Snow is blue-green, because it absorbs shortwave infrared light, but reflects near infrared and green light. Clouds made of water droplets (not ice crystals) reflect all three wavelengths of light similarly, and are white. Forests and other vegetation reflect near infrared more strongly than shortwave infrared and green light, and appear green. Dark gray areas are lightly vegetated lava flows, 30 to 350 years old.
NASA Earth Observatory image by Jesse Allen and Robert Simmon, using EO-1 ALI data from the NASA EO-1 team. Caption by Robert Simmon. Instrument: EO-1 - ALI
