Mexico - the volcanic islands in the Pacific - 3. Revillagigedo Archipelago

After Isla Benedicto , the other islands of the Revillagigedo Archipelago :
 
Socorro is the top of a large basaltic shield volcano, for the most part below the sea level, and capped by a summit caldera of 4.5 x 3,8 kilometers, dated of the Pleistocene.

Cerro Evermann, a large complex of dome lava and tephra cone,  1050 m. high, filled with other cones and vents, a large part of the caldera . Rhyolitic lava domes were established on the flanks along rifts . The lava flows coming from the summit and flank vents have reached the coast and created an irregular shoreline. Cones and flows of the following eruptive phases dot the coast.

                      Partial view of the rugged coastline of Isla Socorro - Photo CTBTO
 

The latest activities date back to 1951 when a short phreatic eruption ejected blocks and a plume that rose to an altitude of 1200 meters .
Cerro Evermann still has an active fumerollien field.
 

A submarine eruption, located on a vent 3 km . west of the island off the coast of Punta Tosca , occurred in 1993;

the explosive activity is accompanied by an issuing of large blocks of slag  , some exceeding 5 m. wide, which floated at the surface.

Socorro island - Simplified geological map and position on the Northern Mathematician ridge - doc . Bohrson et al 1996

The island-volcano Socorro is part, like the other islands of the Revillagigedo Archipelago, of the northern part of the Northern Mathematicians ridge, a mid- ocean ridge become largely inactive there are 3.5 Ma when activity shifted to the East Pacific Ridge.

Socorro is an exception , because it is the only volcanic island in the Pacific where a peralkaline silicic volcanism dominates .

                       Isla Roca Partida - the visible part - Photo DiversOne

The Roca Partida Island is the emerged summit of a submarine volcano. The erosion over millennia reduced Roca Partida to a ledge of 0.003 km ². Two peaks , respectively, 25 m high and 34m ( in 1953 ) are separated by a strip of rocks, 100m long and 8 m. wide and 6 m. relative elevation above sea level

Isla Roca Partida - doc . Solmarv

Without water or vegetation , however, it serves as a nesting place for some pelagic birds : brown and Grant bobby, brown noddy and sooty stern. Marine life thrives against all around.

                 Isla Roca Partida , from another angle - Photo Lasertrimann

Away from the three other islands of the archipelago, the Clarion Island , formerly called Santa Rosa, measuring 19.8 km² and has three peaks: Monte Gallegos ( 335 m) to the west, in the center, Monte Marina (280 m) and to the east, Pico de la Tienda ( 292 m).  

The coast is lined with cliffs from a height between 24 and 183 m., with the exception of the central part of the southern coast near Bahia azufre (Sulphur Bay ). Only two brackish water ponds are a source of temporary water.
                      Clarion island's cliffs - Photo Chris Bennett / Ladybugcruise.blogspot

Clarion's burrowing owl- Athene cunicularia rostrata : an endemic of the island of Clarion - Photo ecsc.edu

Sources :

- Global volcanism Program - Socorro

- Geochemistry of hydrothermal fluids from an intraplate ocean island: Everman volcano, Socorro Island, Mexico – By Y.A.Taran & al. 

- CNRS - Comment une dorsale interagit-elle avec un point chaud ? (La campagne océanographique PARISUB (PAnache-RIde SUBmersible), qui se déroule au large du Mexique du 19 mars au 17 avril 2010) 

- Geology - Active rifting in southwestern Mexico: Manifestations of an incipient eastward spreading-ridge jump – by J.F.Luhr & al.

- WWF - Islands of Socorro, Clarion, San Benedicto, and Roc Partida in the Pacific Ocean off the coast of Mexico