Socorro Island (Spanish: Isla Socorro) is a volcanic island in the Revillagigedo Islands, a Mexican possession lying 600 kilometres (370 mi) off the country's western coast. The size is 16.5 by 11.5 km (10.3 by 7.1 mi), with an area of 132 km2 (51 sq mi). It is the largest of the four islands of the Revillagigedo Archipelago. The last eruption was in 1993.
Geology
The island rises abruptly from the sea to 1,050 meters (3,440 feet) in elevation at its summit. Socorro Island is the emerged summit of a massive, predominantly submarine shield volcano. The island is part of the northern Mathematicians Ridge, a mid-ocean ridge that became largely inactive 3.5 million years ago when activity moved to the East Pacific Rise. All four islands along with the many seamounts on the ridge are post-abandonment alkaline volcanoes. Socorro Island is unusual in that it is the only dominantly silicic peralkaline volcanic island in the Pacific Ocean. It most recently erupted in late January and early February 1993, which was a submarine flank eruption off the coast from Punta Tosca. An earlier eruption was on May 21, 1951; earlier eruptions probably occurred in 1905, 1896 and 1848. The initial volcanic event probably occurred in 3090 BC ±500 years. Mount Evermann (Spanish: Cerro Evermann) is the name given to the summit dome complex, in honor of ichthyologist Barton Warren Evermann. The island's surface is broken by furrows, small craters, and numerous ravines, and covered in lava domes, lava flows and cinder cones. There are eight bays surrounding Socorro: three on the southern end of island – Vargas Lozano, Braithwaite, and Lucio Gallardo Pavon; Ensenada Grayson on the southwest; Blanca on the northwest with two inlets separated by a rocky outcropping; and North Bay on the island's north end with two inlets separated by a volcanic formation. There is a naval station, established in 1957, with a population of 250 (staff and families), living in a village with a church, that stands on the western side of Bahía Vargas Lozano, a small cove with a rocky beach, about 800 meters (870 yards) east of Cabo Regla, the southernmost point of the island. The station is served by a dock, a local helipad and Isla Socorro airport, located six kilometers to the north. There is a fresh water spring about 5 kilometers (3.1 miles) northwest of Cabo Regla, at the shoreline of Ensenada Grayson (or Caleta Grayson). This is brackish and sometimes covered by the sea at high tide. In the 1950s, a small freshwater seep was known to exist some 45 meters (49 yards) inland at Bahía Lucio Gallardo Pavon (Binner's Cove), 800 meters (870 yards) northwest of the naval station. In 1996, multiple sources of surface water above 60°C were observed.
History
No evidence of human habitation on Socorro exists before its discovery by Spanish explorer Hernando de Grijalva and his crew on 19 December 1533, who named it Santo Tomás ("St Thomas Island"; French: Isle St Thomas). In 1542, Ruy López de Villalobos, whilst exploring new routes across the Pacific, rediscovered Inocentes and renamed it Isla Anublada ("Cloudy Island") due to the clouds frequently forming on the northern slopes of Mount Evermann, and again in 1608, Martín Yañez de Armida, in charge of another expedition, visited Santo Tomás and changed its name to Isla Socorro after Our Lady of Perpetual Help (Virgen del Perpetuo Socorro). At the beginning of the twentieth century, Barton Warren Evermann, director of the California Academy of Sciences in San Francisco promoted the scientific exploration of the island. The most comprehensive biological collections were obtained at this time. The volcano on Socorro was renamed in his honour. Archie Smith, an American labourer from San Diego, was marooned on the island for one month in 1929 before being rescued by a passing fishing boat. This was because the expedition that brought him to the island to shear wild sheep returned to port for supplies, but went bankrupt and could not return. In September 1997, Hurricane Linda passed near the island near peak intensity causing minor damage to meteorological instruments on the island.
Ecology
The lowlands of Socorro – except on the northern, more humid side – are covered with thick shrubland, consisting mainly of endemic Croton masonii and a cactus, probably Engelmann's prickly pear (Opuntia engelmannii). Above 650 metres (2,130 ft) and on the northern side, a richer vegetation occurs. This includes small trees such as Ficus cotinifolia, black cherry (Prunus serotina), and the endemic Guettarda insularis, which bear epiphytic orchids (Epidendrum nitens, E. rigidum and the endemic Pleurothallis unguicallosa). The native land fauna is depauperate, with birds predominating and mammals absent. There is one endemic species of iguanid lizard (Urosaurus auriculatus) and the land crab Johngarthia oceanica, which also occurs on Clipperton Island. Sheep, cats, and rodents were introduced to the island by human activity; more recently, the locust Schistocerca piceifrons has also established itself on the island. Unlike the mammals on Guadalupe Island or Clarión, their impact on the local flora was minor, but cat predation has had a drastic effect since the mid-1970s due to the fauna's island tameness, and the locusts that swarm twice a year seriously damage vegetation during that time. There have been no recorded extinctions of plants on Socorro; several birds have been drastically affected by cat predation however, and one taxon, the Socorro dove, has gone extinct in the wild. Socorro is an important breeding location for several seabirds, many of which have here one of their northernmost breeding colonies. The present status of these birds is not well known, and they presumably have suffered from cat predation. In 1953, the following taxa were present:
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