The island measures north–south and east–west, and has an area of. It is densely wooded and very mountainous and is marked by ridges and numerous deep ravines lead to a steep, rugged coast on the east side of the island off which are tremendous depths. The coastal cliffs are up to high. The south, west and north sides of the island have sandy strips of beach which extend offshore in places. The highest peak, Cerro de Los Inocentes, rises to in the southwest part of Alejandro Selkirk Island, where there is also a prominent rock with a hole through it. Landing is possible near the center of the east shore at Quebrada Sánchez, and at the foot of Quebrada Las Casas, where there is a boat slip and buildings of the former penal colony. Anchorage can be taken about east-northeast of Quebrada Sánchez, in depths of to. The ravine is recognized by a white patch on a hill near it. Anchorage can also be taken, in depths of to, with sand bottom, off Rada de la Colonia. Dating of rocks on the island show that it is the youngest of the Juan Fernández Islands at an age of 1 – 2 million years old. Due to its young age, the island shows little evidence of erosion. According to some early reports, now disputed, about southwest of the island is the submerged Sefton Reef, almost reaching sea level, and to northwest, Yosemite Rock. Podesta Island, once reported to lay farther to the west, is a phantom island.
Climate
Alejandro Selkirk has a subtropical climate, moderated by the cold Humboldt Current and the southeast trade winds. Temperatures range from to, with an annual mean of. Higher elevations are generally cooler. Average annual precipitation is, varying from to year to year. Rainfall is higher in the winter months, and varies with elevation and exposure; elevations above experience almost daily rainfall.
Throughout much of its history, the island has been uninhabited. There was formerly a penal settlement along the middle of the east coast, at. About 20 buildings can be made out on detailed satellite images. The settlement was operative from 1909. It initially housed 190 criminals, while there were as many as 160 political prisoners from 1927 to 1930. The penal colony was abandoned in 1930. Recently, the island has seen regular settlement, with 57 people living on the island as of the 2012 census. In 1966 the Chilean government renamed Isla Más Afuera as Alejandro Selkirk Island, with Isla Más a Tierra becoming Robinson Crusoe Island. Alexander Selkirk was a Scottish sailor who was marooned as a castaway on Más a Tierra from 1704 to 1709. His story of survival likely inspired the 1719 novel Robinson Crusoe by Daniel Defoe.
The author Jonathan Franzen wrote an essay in The New Yorker about his brief stay on the island while mourning the suicide of his friend and fellow author David Foster Wallace, some of whose ashes Franzen scattered on the island.