ISLAND FORMATION
Volcanic activities during the Pleistocene formed the four Revillagigedo Islands: Socorro, Clarion, San Benedicto, and Roca Partida. Socorro Island is the largest and tallest of the four and is located at the Clarion Fracture Zone in the east Pacific. Cerro Evermann is the summit of the volcanic island which is 1050 meters tall and is composed of tephra and lava dome complexes. There were two major periods of volcanic activity that can be recognized on the island. The first one is the pre-caldera period which was characterized by eruptions of olivine-poor alkali basalt, later followed by diffusion of soda rhyolite. This period of island volcanic activity was ended with the formation of a round couldron shaped indentation in the land, also called a caldera, which names the period. The following long dormant period was interrupted by rifting and down-faulting of the western side of the island along the north-south fracture system. This rifting event was accompanied by the eruption of soda rhyolite, which formed a large cone shaped tephra, on top of the old caldera. Due to change in the ground structure, the original central vent was blocked by dense obsidian. Eruptions in flank vents were still able to continue.
Silicic lava in the pre-caldera period flowed from the summit and had reached the coast and created an extremely irregular shoreline. Basaltic eruptions in later stages produced cones and flows near the coast. The oldest submarine lavas were estimated to be formed in the late Tertiary age. Some minor volcanic activities also occurred during the 19th century.
VOLCANIC ACTIVITY
Socorro Island is still an active volcanic island; in 1951, there was a submarine eruption. Another submarine eruption occured 3 km west of Socorro Island during 1993-94 which produced steaming lava balloons that rose to the surface. The Mexican Navy surveyed the island after the eruption and measured the temperature of the fumarole (a crack in the surface of the earth found near volcanoes that emits gases) and hot springs of the same location on both February 2, 1995 and February 5, 1993. The result showed that in 1995, almost all sites were above 90 degree Celsius. The temperature measurements in 1995 were several degrees higher than those in 1993. During 1995 visit, the team observed recently killed vegetation around some hydrothermally active pits. They also noted soft warm ground, dead vegetation, and newly formed pits. The majority of the fumaroles appeared similar to observation in 1993. Two out of three mud volcanoes were active in 1995.
SURVEY OF SOCORRO
Country: Mexico
Volcanic Region: Mexico and Central American
Primary Volcano Type: Shield
Last Known Eruption: 1994 CE
Latitude: 18.78 degrees North
Longitude: 110.95 degrees West
Summit Elevation: 1050 m (3444 ft)
Socorro Island is the peak of mostly submarine hills formed by volcanic activities. The summit is capped by a largely buried caldera called Cerro Evermann, which is 1050 meters tall. This picture was taken from the south side of the island.
Photographed by Steve Nelson, 1989 (Tulane University)
Cerro Evermann is a large tephra cone and lava dome complex which is 1050 meters high. Eruptions at Socorro have been limited to flank vents.
Photographed by Steve Nelson, 1989 (Tulane University)
The picture was taken from the Southeast side of the island. Cerro Evermann was formed about 370,000 years ago which was during Pleistocene period. The caldera rim is exposed at the south and east side of Cerro Evermann and it is estimated to be 600 meters tall.
Photographed by Martha Marin, 1998 (Mexican Navy)
Mountain surface is altered by the extensive hydrothermal vents near the summit of Cerro Evermann. The largest fumarolic vent is located near the origin of a large valley northeast of the summit. During a 1957 visit, vents were measured between 65 to 97 degrees Celsius. Similar measurements were taken during a 1993 visit. The temperatures were ranging between 61 and 101 degree Celsius.
Photographed by Martha Marin, 1998 (Mexican Navy)
Many mud-pots were found in the hydrothermal area near the summit of the island. The most active area is on the Southeast part of the northern dome as well as areas with vapor, boiling water, and mud.
Photographed by Martha Marin, 1998 (Mexican Navy)
Fumarolic areas are concentrated near the summit of Cerro Evermann. The most extensive hydrothermal alterations are located on the east, west, and southwest sides of North Dome.
Photographed by Martha Marin, 1998 (Mexican Navy)
Small submarine explosions were first observed on January 29, 1993, as rising hot rocks fractured as they reached the sea water surface. Large pieces of dark-gray basalt up to 5 meters in diameter rose to the sea surface. As the rock fractured and broke into small pieces, it emitted loud cracking noises. Pieces of rock cooled down and sunk back to bottom of the ocean.
Photographed by Hugo Delgado-Granados, 1993 (Universidad Nacional Autonoma de Mexico)