Marchena, also known as Bindloe is a large shield volcano, of which only the upper 343 m (100 feet) is above sealevel. The island measures about 18 by 12 km and is one of the three small islands in the northern group (Pinta, Marchena, and Genovesa). While Marchena and Genovesa lavas are somewhat similar in composition, both having compositions close to that of mid-ocean ridge basalts, the composition of Pinta are quite different.
The many young flows and pyroclastic cones on Marchena testify to considerable volcanic activity in the recent geologic past. However, there is only one known historic eruption, which occurred in 1992.

Like many of the Galapagos volcanos, Marchena has a caldera. Marchena's caldera is roughly elliptical and measures 7 km by 6 km, within the range of caldera sizes of the large western volcanoes. Marchena's caldera is unusual, however, in that it has been almost completely filled with young lavas, some of which has spilled over and down the sides. The oldest lavas are 500,000 years old.

Marchena is rather desolate and has no fresh water and hence has never been settled, and its flora and fauna have not been disturbed by feral animals or introduced plants. Except for diving in the waters around it, it is off-limits to tourists and is therefore seldom visited. Tortoises have apparently never inhabited Marchena.