Formation and evolution of Hule and Río Cuarto maars, Costa Rica.

ALVARADO, G.; SOTO, G.; SALANI, F. M.; RUIZ, P.; HURTADO DE MENDOZA, L.

JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH

ELSEVIER SCIENCE BV

Año: 2011 vol. 201 p. 342 - 356

0377-0273

The Hule and Río Cuarto maars are located 11 and 18 km northward respectively, of the active Poás volcano, on the Caribbean side of the Central Volcanic Range of Costa Rica. They lie along the northern part of a N-S trending volcanic fracture zone, ~20 km-long crossing Poás volcano.The volcanic products from Hule maar (2.3 km x 1.8 km, area ~3.5 km2) explosion are mainly pyroclastic surges (poorly vesiculated andesites with tiny plagioclases), acidic andesitic pumice flows, air fall deposits, ballistic blocks, and reworked deposits that overlie the regional volcanic basement. They were produced during three main explosive phases. Two intra-maar overlapping pyroclastic cones are into Hule maar, and at least 3 lava fields are related to them (high-Al basalt to basaltic andesite). Another explosion depression, Pata de Gallo (400 m across), is located less than 1 km off the SE rim of Hule. Río Cuarto is a nearly circular crater (700-850 m across) and an area of 0.33 km2. Río Cuarto products include surges, ballistics and air-fall tephra, produced as well during three main explosive phases. Its deposits show a narrow fan oriented westward, according to eastern-coming wind direction, indicating a directional surge (first 2 km), and then air-fall deposits (up to 5 km away), from a relatively low altitude explosive column. Radiocarbon dating has shown that Hule was formed ~6.2 calendar ka ago and Pata de Gallo probably formed ~2.8 ka, while the inner products could have ages of ~1.7 ka or ~0.7 ka. There are no radiocarbon ages yet for dating the formation of Río Cuarto maar, but archaeological data suggest that it erupted between 3-4 ka ago. The volume of pyroclastic deposits associated to Hule maar is estimated in 0.51-0.53 km3, from which ~20% are juvenile material, therefore 0.07-0.08 of new DRE magma. The tephra from Río Cuarto is estimated in 4.4 x 107 m3, which 0.008 m3 correspond to new DRE magma. There is knowledge of overturn events in both maar lakes, with mass death of fish and changes in the lake color, in cycles of 6-8 years. The most likely explanation could be that during long periods of relatively cold weather, combined with extremely strong winds, a physical and chemical mixture of hypolimnion and epilimnion happens. In these systems the hazard related to the possible occurrence of Nyos-type gas eruptions can be considered negligible or very local, but important if tourists camp in or near the lake shoreline.