The Revillagigedo Islands were discovered in 1533 by the Spanish conquistador Fernando de Grijalva; named by, and after himself, the Count of Revillagigedo who ordered the islands to be occupied in 1790. They are solitary remote islands, of interest only to military, scientists and adventurers; not much has been written about them over time. They gained some fame when Alexander von Humboldt mused about them in 1811, and in 1905 a ship reported that the islands had disappeared from sight. Whether that was due to a tsunami rolling over them, or perhaps to the collapse of some parts, was never found out. Today the islands are all accounted for; they are uninhabited except for a Mexican navy base on Sorocco and a small marine station on Clarion island. San Benedicto is an active volcano, and the fourth, Isla Roca Partida, is just a big double sea stack, home to zillions of sea birds.
All four are volcanic islands in the eastern Pacific Ocean, ~390 km southwest of Cabo San Lucas, the southern tip of the Baja California Peninsula, and 720-970 km west of Manzanillo, Colima, Mexico. The archipel was inscribed as a World Heritage Site for its unique ecosystem just recently, in July 2016. It is part of a submarine mountain range, with the four islands being the peaks of volcanoes emerging above sea level. Recent research suggests that island ages decrease from W to E – from Isla Clarion through Isla Roca Partida and Isla San Benedicto to Isla Socorro. Only two of them are presently volcanically active: Isla Socorro and the Bárcena volcano on San Benedicto island.
HUMBOLDT & SOME GEOLOGY of the Islas Revillagigedo
All four Revillagigedo islands are a product of intra-plate volcanism in the so-called Hotspot of Socorro. They are located at the northern end of the Mathematician ridge, a former mid-ocean ridge spreading center which was abandoned (stopped spreading) at ~3.5 Ma, when the spreading shifted eastwards to the East Pacific Rise. It is thought that a magma plume from the mantle is exploiting the crustal discontinuities and weakness inherited from the old ridge. This view is strengthened by the relatively high eruption rate of Socorro volcano, and by the chemical composition of its fumarolic gasses which is quite typical for intraplate hotspot oceanic islands. Moreover, the area is near a part of the Clarion fracture zone* where it is crossed by north- or northeast-trending fractures. The intersecting points of faults are often the weakest and allow magma to rise more easily.
Visiting the inland Mexican volcanoes in 1811, A. v. Humboldt had some foresightful ideas: “…suppose that there exists in this part of Mexico, at a great depth in the interior of the earth, a chasm in a direction from east to west for a length of 137 leagues, along which the volcanic fire bursting through the interior crust of the porphyritical rocks, has made its appearance at different epoquas from the gulf of Mexico to the South Sea? Does this chasm extend to the small group of islands called by M. Collnet the Archipelago of Revillagigedo, around which, in the same parallel with the Mexican volcanoes, pumice-stone has been seen floating?”
150 years later Menard proposed that the submergent features of the Clarion fracture zone, then known to extend a few thousand km west of Isla Clarion, might be related to the Islas Revillagigedo and perhaps the trans-Mexican volcanic belt. Today the Clarion FZ is thought to extend from near Hawaii at least to the Mexican coast at Puerto Vallarta. Its possible continuation through the volcanic belt has led some geologists to suggest that the Clarion FZ may extend into the Caribbean, as the Cayman Trench between Cuba and Hispaniola also lies along the same general trend. Isn’t it amazing how, 200 years ago when nobody knew anything of plate tectonics, spreading ridges or fracture zones, Humboldt could imagine a feature deep within the Earth’s crust that we only now begin to understand?
*Frakture zones are lines of submarine transform faults perpendicular to, and traversing, the mid-ocean spreading ridges. They occur where stresses from the seafloor spreading are being relieved. In this case, the Clarion fracture zone runs for 5200 km, crossing the northern part of the East Pacific Rise.
Isla San Benedicto, the NE-most of the Revillagigedo Islands, is an elongated island some 50 km north of Socorro and no more than 5 km long. Its southern part hosts the newly formed Volcán Bárcena and Montículo Cinerítico (“Ash Heap”), a smaller tephra cone that preceded and was largely overtopped by Bárcena. Montículo Cinerítico may be only a few hundred years old; it formed the high point of the island prior to the eruption of Bárcena. Growth of the Bárcena tephra cone was accompanied by strong explosive eruptions and pyroclastic flows. The eruption concluded with the emplacement of two small lava domes in the crater (one within the other) and extrusion of a prominent coastal lava delta at the SE base of the cone. Bárcena is now a palagonitic tuff cone with a central dome of trachyte, formed mostly by phreatomagmatic activity. The volcano is the most prominent feature of San Benedicto and a beauty to behold on aerial photographs:
THE 1952 ERUPTION
The birth of Bárcena has been well documented in the 1950s and is described, for example, in detail by A.F. Richards (1959) who also collected various eyewitness accounts. I just love those old reports from the times when volcanology was still in its infant shoes and more a matter of sharp observation and (geo-)logical thinking than number crunching and equations. If you are familiar with sci-hub.cc you can find the report using its doi-number 10.1007/BF02596580.
On August 1, 1952, San Benedicto literally burst from obscurity to become known as the location of the first new volcano in the east Pacific Ocean whose birth was observed by human eyes. The renowned geologist and volcanologist Howel Williams (1898-1980) gave a description of it in his Volcano Letter (July-Sept. 1952, no. 517, p. 7, slightly shortened): The northern part consists of the remnants of a group of coalescing cones of scoria, now deeply eroded and craterless. After these had become extinct activity migrated southward to build a cone of white pumice known as the Ash Heap, to a height of 332 m.
Shortly thereafter a new vent opened in the saddle between the Ash Heap and the old cone cluster. First another pumice cone was built there, then a dome of columnar lava rose into the crater. Finally, owing to withdrawal of magma in the conduit, the top of the dome subsided to form a conical depression now known as Herrera Crater. The vent of the new volcano lies between those of the Ash Heap and Herrera Crater, on a north-south fissure that continues south under the sea towards the vents on Socorro Island.
Just when activity began is uncertain but probably during July of 1952. The first eruptions were reported on August 12, by which time a large cone had already been formed. News of the activity reached the mainland on August 27. During a visit on Sept. 13 the cone was about 330 m high and 1.5 km across at the base. The intensity of the eruptions had diminished greatly. Every 20 minutes or so, cauliflower clouds of steam and ash rose from the crater, and the odor of H2S was strong. On Sept. 20 the cone was some 500 m high. Discharge of ash had come to an end; instead, lazy puffs of steam, mingled with hydrogen sulphide, rose only a few hundred feet above the crater rim at intervals of a few minutes, and on the floor of the crater was a small biscuit-shaped mass of blocky lava: a miniature plug. Clearly, there had been no further explosive discharge of debris since the plug rose to the surface. Apparently the volcano was approaching extinction; not only was the throat sealed by the plug, but the vapor pressure was reduced to a few atmospheres.
The San Benedicto eruption is especially significant as being the first historic pumice eruption in the eastern part of the Pacific Ocean. Proper identification of the nature of the pumice must await detailed microscopic study, but the white color of the ejecta and the refractive index of the glass suggest that the material is either rhyolite or dacite. Almost surely most of the new cone was built during the first few weeks, before it was seen. The first explosions seem to have been of vulcanian type, the ejecta being hurled to great heights by strong gas pressure. Subsequently, as the vapor pressure diminished, the vulcanian explosions grew weaker and were accompanied by explosions that lifted the ejecta only a short distance above the crater rim so that they then swept down the outer flanks of the cone as glowing avalanches. Finally, as the vapor pressure continued to decrease, a mass of stiff, viscous lava slowly rose from the conduit onto the crater floor.
The lava eventually breached the base of the cone producing a fan-like flow and building a large lava delta into the sea. This extrusion produced withdrawal of magma from the crater, which resulted in subsidence of the dome. Only the margin of the dome had already solidified and consequently settled very little. It was left with a prominent positive relief, compared to the central part which sagged and became saucer-shaped. The Delta Lávico flow grew to a big “cow pad” the size of 1200 m width and jutting out to sea more than 700 m. By the end of February, 1953 all activity except solfataric steaming had ceased. Since then, wave erosion has worked on the “cow pad” to chisel out the individual lobes, giving it the lovely flower-like shape we see today.
The eruption had an VEI=3 (in the old classification its “index of explosiveness of about 90 per cent” was the highest of any oceanic volcano in the eastern Pacific Ocean at the time). The primary destructive force of Bárcena Volcano was the dense pumice dust, tephra and gas that covered the entire island, including a 0.5–1.0-m deposit of ash and pumice on the extreme north end of the island and on many of the small islets about the island. An estimated 20 000 sea birds were killed within hours when they refused to leave nests or were unable to fly due to the effects of the gas and dust.
Much of the ejected pumice began a trans-Pacific journey with the ocean currents, and samples were collected on Hawaii, Johnston, Wake, and the Marshall Islands. The pumice is believed to have even reached the Palau Islands in the western Pacific Ocean.
ROCK TYPES AND FURROWS
Bárcena rocks are a medium gray or black porphyritic sodic trachyte. Some fragments are composed of over 50 per cent glass and have a subconchoidal fracture. The sequence of rocks on the island is, from earliest to latest volcanoes: Trachybasalt (alkali basalt) • trachyandesite (mugearite) • sodic trachyte – sodic rhyolite. The first two were sampled on pre-Bárcena sites while the last pair, from highest evolved magmas, came from the Bárcena eruption.
Not much is known of the volcanic history of Isla San Benedicto. There is some evidence for the existence of one or more large earlier volcanoes associated with the eruption of trachybasalts, trachyandesites, and plugs of trachyte. Conical geomorphic forms that are believed to be parasitic volcanoes are located below sea level north and south of the present island. They may have formed during Pleistocene lower sea levels or it is equally possible that they were submarine volcanoes at the time of their formation. The size of crater Herrera and the thick deposits of pyroclastics underlying the dome to the north suggests that the tephra cone of Volcan Herrera was very large at the time that it was formed. The appearance of nearly all sides of Crater Herrera indicates that an extended period of wave erosion followed the cessation of Herrera activity.
With regard of its geological history, Isla San Benedicto can be divided into three regions:
– North: is the oldest, extending beyond the northern shore as a submarine mountain range.
– Center: the northern part of the subaerial island is a region of eroded domes, lava flows and volcanic rocks originating from the Herrera crater to the northern shore which includes La Trinidad and the Challenger Rock. Herrera crater consists of porphyritic trachytic lava. From there and Lopez de Villalobos hill there are also flows of trachybasalts. Cerro López de Villalobos has a particularly complex petrography, it might once have been a volcano in its own right. In the area of Punta Ortalán streams of trachyandesite and trachybasalts can be found – indicating early volcanism.
– South: the youngest part consists mainly of not yet consolidated fragmented rock material from latest eruptions.
Much thought has been given to the furrowed slopes of the cone. Those furrows have been created after the explosive phase of the eruption which had covered everything evenly in thick ash. Erosion by rain, avalanches of bombs or blocks, tephra avalanches and ash landslides from the crater rim had been suggested causes. It is interesting to follow the considerations that dismissed one cause after the other – arriving at the assumption that tephra avalanches (nuees ardentes, or pyroclastic flows) had created the furrows. Tephra avalanches usually deposit material rather than erode, but avalanche furrowing had also been found on Montagne Pelee. The avalanches tended to descend from the lowest parts of the crater rim and furrowing was most intense below these areas. By September the tephra avalanches were less powerful; instead of rolling and gliding they probably slid and eroded, thereby causing the furrows.
Today, Isla San Benedicto is a great diving paradise as well as a Biosphere reserve. Visitors would need special permits, limited sportfishing is allowed, and commercial fishing in the area is banned altogether. The Mexican government has given the job of enforcing those laws to the naval base on neighboring Isla Socorro, so there’s hope that the local Fauna will recover in due time and that this beautiful island will stay protected forever.
Here is a nice personal video of a sailing visit to San Benedicto by Vince Radice on YT. If you are not interested in sailing and fishing you can skip the second half:
While the north half of the island is green with vegetation again, “walking on the southern flanks of the island as we did in the video above must be not unlike walking on the moon. There is virtually no sign of life what so ever on the southern beach. The southeastern beach does have a small population of boobies and albatross and green vine like vegetation. […] The islands are not for the faint of heart, that is for sure but those who venture there will reap the rewards of a life time of memories. For a trip to the Island is truly a trip back in time.“ (from World’s Aquarium)
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– GVP, Bárcena
– Острова Ревилья-Хихедо. “Горячая точка” Сокорро ( Projects FEB RAS, in Russian)
– Prolonged history of silicic peralcaline volcanism […] Eastern Pacific (1996, PDF)
– The Electronic Volcano
– Rock Stars: Howel Williams (PDF)
– Bárcena Volcano, 1952: a 60-year report on the repopulation […] (2014)
– Geology of the islas revillagigedo, mexico. 1 (1959, paywalled)
– Geology of the islas revillagigedo, mexico. 2 (1966, text only)
– La Jornada Ecológica article about Archipiélago de Revillagigedo (PDF, in Spanish)
– The Humboldt Digital Library