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
. The most active today are Fernandina
, 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
are in the west of the archipelago, and have been in existence for some 700,000 to 800,000 years.