On October 22, 2005, the Sierra Negra Volcano on Isla Isabela in the Galapagos Islands began erupting, sending ash as high as 7,600 meters (25,000 feet). By October 30, when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this image, the ocean to the west of the volcano was veiled in thick white volcanic haze. The haze formed as sulfur dioxide—a volcanic gas—mixed with oxygen and water in the atmosphere.
Red dots over the volcano mark where MODIS detected a region that was significantly hotter than its surroundings. The dots may indicate lava flows, hot ash and rock flows, or fires ignited by the erupting volcano.
The Galapagos Islands are a collection of volcanic summits poking their heads above sea level along the equator, west of South America. With the exception of Isla Isabela, each major island consists of a single volcanic summit. Isla Isabela, the largest in the archipelago, is composed of six volcanoes that merged above the water. The volcanoes are all shield volcanoes shaped like flattened domes with very gentle slopes. In contrast to volcanoes built from a combination of lava, rock, and ash flows, shield volcanoes are built exclusively from lava flows. Hardened black lava characterizes the islands, and the Sierra Negra’s name translates as “Black Mountain Range.”
Geologically speaking, the islands are young, ranging in age from hundreds of thousands to several millions of years old. Some volcanoes are still forming, and the Galapagos remain one of the world’s most volcanically active regions. No land bridges have ever connected these volcanic summits to another landmass, and the nearest continent is several hundred miles away. Puzzling over the plants and animals that populated these islands helped the naturalist Charles Darwin develop his theory of natural selection.