Volcan Alcedo is located in the center of Isabela, (also known as Albermarle Island) located between Volcan Darwin to the north and Sierra Negra to the south. The composition of its lavas differs from that of both its neighbors, indicating it has an independent magma plumbing system. It quite symmetrical and rises to an elevation of 1100 m (3650 feet). Relative to its size and height, Alcedo's caldera is large (7 to 8 km in diameter) and shallow (270 m deep). A steam vent, or fumerole, is located on the western side of the caldera. Only one historic eruption is known, a basaltic lava that erupted the southeast flank sometime between 1946 and 1960 (based on comparison of air photos). Volcan Alcedo is unusual in that its flanks are mantled by rhyolite pumice. Rhyolite is a light-colored volcanic rock that is produced by fractional crystalization. This occurs when basaltic magma intrudes into the crust and cools. As it does, it crystallizes, the crystals settling out. As a result, the composition of the magma steadily changes, becoming poorer in magnesium and iron and richer in silicon. The low iron content accounts for its light color compared to basalt. Alcedo is probably the oldest volcano on Isabela, and may be in the final phase of its evolution. This may account for its unusual lava chemistry and morphology.

Rhyolite is highly viscous, so unlike basalt it does not flow easily. Furthermore, rhyolite magma is often rich in water. These two factors combine to produce explosive eruptions, which generate ash rather than lava flows. At least two such explosive eruptions have occurred on Alcedo, the most recent of which was 100,000 years ago, producing the pumice that mantles it flanks. The eruption was of "Plinian" style (similar to Pinatubo or Mt. St. Helens), with the eruption column reaching heights of 20 to 30 km). Such eruptions are extremely unusual on oceanic islands. Once the gas pressure in the magma was releaved somewhat, a viscous rhyolite magma flowed out on the caldera floor, freezing as obsidian. The white of the rhyolite pumice stands in stark contrast to the black basalt.

The pumice covering the flanks makes the ascent to the summit comparatively easy, and Alcedo, along with Sierra Negra, are the only calderas that tourists may visit. Alcedo is home to the largest population of giant tortoises in the islands. Up until a few years ago, the caldera floor was lushly vegetated and giant tortoises not threatened. The tortoises were often found wallowing in temporary rain pools. However, in the last 10 years, the population of feral goats on the volcano has absolutely explosed. In 1983 there were no goats. It is estimated that there are now between 50,000 and 100,000 goats living on the volcano. The goats have now virtually defoliated the volcano, threatening one of the best tortoise habitats. Alcedo is also excellent habitat for land iguanas, as they can easily burrow in the rhyolite ash that mantles to volcano.

You can learn more about Alcedo and the goat crisis at the Alcedo Web Pages of the Geologic Atlas of the Galapagos.