The Galapagos Islands are thought to have formed about 3-5 million years ago, relatively recent in geological terms. They probably are the product of volcanoes that formed beneath the sea and which continued to rise until they broke through the water's surface and became islands. The islands on the east, especially the Island of Espanola, seem to be much older than the western islands by a million years or so. This is consistent with the "hot spot theory," which theorizes that the Galapagos are situated right over a "hot spot," an area of extreme heat within the earth's mantle. As the tectonic plates on which the islands are placed move (at a rate of about 3 inches a year), magma from these hot spots wells up through the new boundaries, forming a string of new islands as well as causing earthquakes and several volcanic eruptions. 

Today, the Galapagos are one of the world's most active volcanic areas, with over 50 eruptions in the last 200 years. The most active volcanoes are the ones on Fernandina, which last erupted in 1995, and the Cerro Azul on Isabela, which last erupted in October 1998. All of the volcanoes are still active (with the exception of the Ecuador Volcano on Isabela Island), but most of the eruptions have been small.

Other common geological activity includes uplifts, which are caused when magma beneath an island hardens and lifts the land through and above the waters surface, and large depressions called calderas, which happen when magma beneath the summit of a volcano cools and contracts, causing the entire peak to collapse inward.

For a closer look at volcanoes and their fascinating features, special tours are available to the rim of the Alcedo Volcano, with a spectacular view of the caldera, and a 1-2 day trip to the rim of the Sierra Negra Volcano, the oldest volcano on Isabela Island. There are also many hikes to and around different volcanoes, such through the Highlands of Santa Cruz Island or the Sugarloaf Volcano on San Salvador Island.