Sierra Negra Volcano
A large plume of ash from the Sierra Negra volcano is clearly visible in this image of the Galapagos Islands.
The ash appears hazy, as opposed to the more solid-looking white and whitish-grey features (which are clouds). Sierra Negra is a shield volcano, which means that is wide with gently sloping sides, formed by the flow of very fluid lava.
The Galapagos are a part of an island arc, a string of volcanoes that form above what geologists call a "hot spot". As the plate (on which the islands currently rest) moved over a column of hot magma or liquid rock, the islands were formed as the magma cooled into volcanic rock.
The islands are located in the Pacific Ocean, about 1,000 kilometers (621 miles) west of South America; they belong to Ecuador. The Galapagos are famous for their unique biodiversity and for a visit by the HMS Beagle in 1835. On board was Charles Darwin, then a young naturalist eager to collect specimens in exotic locations.
His observations of how variable individuals within a population (of a single species) are and how these variations are distributed in space led, in part, to his conception of natural selection as a mechanism for the "descent with modification" - or what we now call evolution - of species.