West of Genovesa, Marchena is an active shield volcano with its last eruption in 1991. There are no land sites with three quarters of the island are covered with black lava and several fumaroles (steam vents).
It is very dry and hot here. Except for diving it is seldom visited. Underwater you might see cownose rays, turtles, schooling hammerheads and schools of mullet snappers, grunts, surgeonfish, spotted morays, scorpionfish, and the endemic Galapagos garden eel.