GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.2 175 et al. , 1996). Caldera formation occurred probably around 370 ka and was followed by up to 200 kyr of quiescence. The presence of a small summit caldera on Socorro suggests that the silicic magma reservoir was shallow, probably located within the volcanic edifice or the upper oceanic crust. Caldera formation was followed by a change in eruptive style from predominantly explosive to predominantly effusive (Bohrson et al. , 1996). Rocks of the postcaldera stage had been subdivided into the Cerro Evermann Formation and Lomas Coloradas Formation, which erupted between 180 and 15 ka (Cerro Evermann) and 150 and 70 ka (Lomas Coloradas), respectively (Bohrson et al. , 1996). Rocks of the Cerro Evermann Formation are pyroclastics, lava flows, and lava domes of peralkaline composition, exposed in the caldera and at the northern, western and southern flanks of the volcano. These are primarily trachytes, comendites and pantellerites (Carballido-Sanchez, 1994). The lava flows and cinder cones of the Lomas Coloradas Formation are exposed at the south-eastern part of the island and are composed of basalt, hawaiite and mugearite (Carballido-Sanchez, 1994). Eruption of peralkaline-rhyolitic and basaltic magmas during the same time interval of post- caldera phase has been explained by a stratified magma chamber composed of soda-rhyolite overlying basaltic magma (Siebe et al. , 1995). Morphologically, the subaerially exposed rocks on Socorro Island can be classified as lava-flows, domes, pyroclastic flows, air-fall pyroclastics and cinder cones. The lava-flows on Socorro Island can be of both basaltic and peralkaline composition, whereas the pyroclastic flows are exclusively peralkaline (Carballido-Sanchez, 1994). Cinder cones are generally basaltic in composition (except for one) and are confined to the Lomas Coloradas unit. The domes, on the other hand, are exclusively peralkaline. Air-fall pyroclastics are both peralkaline and basaltic, whereas the basaltic ash, cinder, spatter and bomb deposits are all (except for one deposit in the Bahia Academy area) restricted to the Lomas Coloradas area (Carballido- Sanchez, 1994). Bryan (1966) identified three major zones of fractures that intersect on Socorro Island. One zone has a north-south orientation and extends from the summit of Volcán Evermann to Bahia Academy, which is located in the northern part of the island. A second zone is orientated in east-west direction and extends from the western coast of the island (Cabo Henslow, Punta Tosca, Caleta Grayson) to the summit area of Volcán Evermann. The third zone is located in the southern part of the island and shows approximately NW-SE orientation. Bohrson and Reid (1997) found three different conditions that are required for the formation of silicic peralkaline magmas: a mildly extensional tectonic setting, a shallow magma reservoir and availability of parental transitional to mildly alkalic basalt. The formation of silicic peralkaline magmas can be explained by three different mechanisms: fractional crystallization of transitional to mildly alkalic basalt, partial melting of mafic intrusive rock and volatile complexing and transport in association with one of these (Bohrson and Reid, 1997). According to these authors, the most likely mechanism of silicic peralkaline magma formation for Socorro Island is a moderate degree of partial melting of intrusive alkalic basalt and associated crystal cumulates followed by crystal fractionation. Calculation of eruption rates for submarine and subaerial parts of the volcanic edifice suggests that either subaerial eruption rates are less than submarine eruption rates, or the growth of the volcano has continued to be dominantly submarine. Comparison of eruption rates of other isolated ocean islands suggests that the source of alkaline magmatism associated with Socorro may be consistent with a mantle plume (Bohrson et al. , 1996). Volcanic activity on Socorro Island has continued to the present. The first historically reported eruption took place in 1848, but no detailed description of the event exists. Another eruption was observed in 1896, but also no further information on the eruption is available. Further small eruptions were reported in 1947 and 1951. The so long last eruption occurred in 1993 and was submarine. At the moment, fumarolic activity can be observed near the summit of Volcán Evermann (Carballido-Sanchez, 1994).