Volcan Alcedo

A Profile of the Volcano

Tortoises in the Mist

Alcedo has the largest population of giant tortoises in the Galapagos. The caldera is in in the background. Note the domal morphology of the tortoise's shell and how it is similar to the shape of the volcanoes.

Alcedo is in the middle of Isabela island, which is formed by the coalescence of 6 shield volcanoes. Alcedo is probably the most interesting petrologic feature in the Galapagos, because it has erupted an entire suite of diverse rocks, including basalt, icelandite, dacite, and rhyolite. A plinian, mixed magma deposit covers much of the eastern flank of the volcano, and several obsidian flows lie in the caldera and on the flank.

Here is a 5 m section of plinian airfall from the main phase pumice eruption.

• Geist DJ, Howard KA, Jellinek AM, Rayder S (1994) The eruptive history of Volcán Alcedo, Galápagos Archipeligo: a case study of rhyolitic oceanic volcanism. Bull Volcanol 56: 243-260
• Geist, D, Howard, KA, and Larson, P (1995) The generation of oceanic rhyolites by crystal fractionation: the basalt-rhyolite association at Volcan Alcedo, Galapagos Archipelago. J Petrol 36, 965-982.
• Nordlie BE, Colony WE (1973) Fumarole with periodic water fountaining, Volc_n Alcedo, Galapagos Islands. Geol Soc Am Bull 84: 1709-1720
• McBirney, AR, Cullen, AB, Geist, D, Vicenzi, EP, Duncan, RA, Hall, ML, and Estrella, M (1985) The Galapagos volcano Alcedo: a unique oceanic caldera, J Volcanol Geotherm Res 26, 173-177.

The Geologic Maps of the volcano are awesome!

Check out the excellent analyses from Alcedo - OR-

Download an Excel file of geochemical data.

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