The islands are the peaks of gigantic undersea volcanoes, composed almost exclu­sively of basalt. Most of them rise from about 2,000 to 3,500 m above the surround­ing seabed, but over 7,000 m above the deepest parts of the adjacent ocean floor to the west of Fernandina. The highest summit is Volcano Wolf on Isabela island, 1,660 m above sea level. Eruptions have taken place in historical times on Fernandina, Isabela, Pinta, Marchena and Santiago. The most active today are Fernandina, Isabela, Pinta and Marchena, and fumarolic activity may be seen intermittently on each of these islands. The islands are also very gradually drifting eastward, due to the movement of the tectonic plate on which they rest. Hence the oldest islands lie to the east, including San Cristobal and Espanola which are approximately 3-3.5 million years old. The youngest islands, such as Fernandina and Isabela are in the west of the archipelago, and have been in existence for some 700,000 to 800,000 years.