Socorro Island

This article is about the Mexican island. For the Chilean island, see Guamblin Island.

Socorro

Native name: Isla Socorro
Socorro Island, from satellite image
Geography
Location	Pacific Ocean
Coordinates
Archipelago	Revillagigedo Islands
Total islands	4
Major islands	San Benedicto, Socorro, Roca Partida & Clarión
Area
Length	16.5 km (10.25 mi)
Width	11.5 km (7.15 mi)
Highest elevation	1,150 m (3,770 ft)
Highest point	Mount (Cerro) Evermann
Country
Mexico
Demographics
Population	45
Density	0.34 /km2 (0.88 /sq mi)

Socorro Island (Spanish: Isla Socorro) is a small volcanic island in the Revillagigedo Islands, a Mexican possession lying some 600 kilometers off the country's western coast at 18°48'N, 110°59'W. The size is 16.5 by 11.5 km, with an area of 132 km². It is the largest of the four islands of the Revillagigedo Archipelago.

Geology

The island rises abruptly from the sea to 1,050 meters (3,440 feet) in elevation at its summit. Isla Socorro is a 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-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 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 Bahia Vargas Lozano, a small cove with a rocky beach, about 800 meters east of Cabo Regla, the southernmost point of the island. The station is served by a dock, a local helipad and airport Isla Socorro, located six kilometers to the north, at . There is a freshwater spring about 5 km northwest of Cabo Regla, at the shoreline of Ensenada Grayson (or Caleta Grayson), an inlet. This is brackish or even covered by the sea at high tide. Apart from some temporary pools and maybe one that is more permanent, a small freshwater seep exists most of the time some 45 meters (49 yards) inland at Bahia Lucio Gallardo Pavon (Binner's Cove), 800 meters NW of the naval station.

History

Offshore Socorro Island

No evidence of human habitation on Socorro exists before its discovery by Spanish explorers. Hernando de Grijalva and his crew discovered an uninhabited island on 19 December 1533 and named it Santo Tomás. In 1542, Ruy López de Villalobos, while 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 honor.

Archie Smith, an American laborer from San Diego, was abandoned on the island for one month in 1929 before being rescued by a passing fishing boat.

In September 1997, the island was struck by Hurricane Linda, one of the strongest hurricanes ever recorded.

Ecology

Map of Socorro Island

Location of Socorro Island and the rest of the Revillagigedo Archipelago, and extent of Mexico's western EEZ in the Pacific

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 Gecarcinus planatus which occurs on islands throughout the region.

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 north(east)ernmost 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:

• Wedge-tailed shearwater, Puffinus pacificus (or Ardenna pacifica)
• Western red-billed tropicbird, Phaethon aethereus mesonauta – breeding suspected but not verified
• Nazca booby, Sula granti – breeding suspected but not verified
• Northeast Pacific brown booby, Sula leucogaster brewsteri – breeding suspected but not verified
• East Pacific great frigatebird, Fregata minor ridgwayi – breeding suspected but not verified; a doubtfully distinct subspecies
• East Pacific sooty tern, Onychoprion fuscatus crissalis – a doubtfully distinct subspecies
• East Pacific brown noddy, Anous stolidus ridgwayi

Non-endemic landbirds and shorebirds occur mostly as vagrants or use the island as a stopover during migration; the northern mockingbird became established in the late 20th century. Among those that are recorded not infrequently are great blue heron, Hudsonian curlew, spotted sandpiper and wandering tattler. Unlike the situation on smaller and more isolated Clarión, wind-blown or vagrant birds seem to constititute the bulk of the recorded species, including brown pelican, osprey, peregrine falcon, semipalmated plover, willet, sanderling, belted kingfisher and buff-bellied pipit. It may be that this puzzling observation is due to the presence of the red-tailed hawks and cats, which has at least made the local Urosaurus more wary than its relative on Clarión, and might deter passing birds from stopping on Socorro.

Endemism

Being the largest of the Revillagigedo Islands and closer to mainland than Clarion, Socorro sports a rich array of endemic taxa, mainly plants and landbirds as well as lizards. Some are threatened due to the presence of feral cats.

Animals

The Socorro dove (Zenaida graysoni) only survives in captivity at present

• Socorro parakeet, Aratinga brevipes (endangered)
• Socorro red-tailed hawk, Buteo jamaicensis socorroensis
• Socorro common ground dove, Columbina passerina socorroensis
• Socorro elf owl, Micrathene whitneyi graysoni (probably extinct since c. 1970)
• Socorro mockingbird, Mimodes graysoni (critically endangered)
• Socorro yellow-crowned night heron, Nycticorax violaceus gravirostris (or Nyctanassa violacea gravirostris)
• Socorro tropical parula, Parula pitiayumi graysoni
• Socorro towhee, Pipilo (maculatus) socorroensis
• Townsend's shearwater, Puffinus auricularis (critically endangered; recently extirpated from San Benedicto and probably Clarión)
• Socorro wren, Troglodytes sissonii (near threatened)
• Socorro dove, Zenaida graysoni (extinct in the wild)
• Urosaurus auriculatus (endangered)

Plants

• Acalypha umbrosa
• Aegopogon solisii
• Aristida vaginata
• Aristolochia socorroensis
• Bidens socorrensis
• Botrychium socorrense
• Castilleja socorrensis
• Cestrum pacificum
• Coreocarpus insularis
• Croton masonii
• Erigeron socorrensis
• Eupatorium pacificum
• Guettarda insularis
• Hypericum eastwoodianum

• Ilex socorroensis
• Lantana involucrata var. socorrensis
• Lepechinia hastata ssp. socorrensis
• Meliosma nesites
• Paspalum longum
• Peperomia socorronis
• Physalis mimulus
• Pleurothallis unguicallosa
• Salvia pseudomisella
• Sida nesogena
• Sideroxylon socorrense (Vulnerable)
• Verbena sphaerocarpa
• Vernonia littoralis

Brickellia peninsularis var. amphithalassa, Cheilanthes peninsularis var. insularis, Nicotiana stocktonii, Spermacoce nesiotica and Zapoteca formosa ssp. rosei are near-endemics, being restricted to Socorro and Clarión. Whether Teucrium townsendii ssp. affine is the same plant as those on San Benedicto is not conclusively determined.

Visiting information

Socorro Island is a popular scuba diving destination known for underwater encounters with dolphins, sharks, manta rays and other pelagic animals. Since there is no public airport on the island, divers visit here on live-aboard dive vessels. The most popular months are between November and May when the weather and seas are calmer.

See also

• List of volcanoes in Mexico

External links

• Socorro Endemic Bird Area (BirdLife International)