The Juan Fernandez archipelago, located off the coast of Chile, comprises a group of three small, considerably eroded volcanic islands, probably of late Tertiary age. Traces of old craters or calderas have been recognized on Robinson Crusoe island which is built mainly of basaltic lava flows, many of them picrite-basalts. Coarse-grained ejected blocks, in which olivine predominates, are fairly common on Robinson Crusoe and there are also a number of exposures of dolerite showing pronounced banding. The only acid rock reported from the group is a soda-trachyte from Alexander Selkirk island. Pumice washed ashore on the islands during the past year is possibly from a submarine eruption near the South Sandwich islands in 1962.

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