The Hule and Río Cuarto maars, Chapter One: The beginning

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

San Jose Costa Rica

Workshop; 7th Workshop on Volcanic Lakes Costa Rica; 2010

IAVCEI Commission of Volcanic Lakes

The Hule and Río Cuarto maars are located 11 and 18 km northward 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. Along this “rift”, basaltic andesitic to andesitic aphyric lavas have been erupted through several vents along the last 200 ka. Both maars cut mid to distal volcanic facies. A prominent N70ºW fault scarp, 200 m high, is located between them, which is interpreted as a thrust propagation fault-fold. Hule is a sub-circular depression (2.3 km x 1.8 km, area ~3.5 km2). The volcanic products from the maar 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 basement and a first level of organic debris (BOD). 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, Los Ángeles (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. Its inner lake has a maximum depth of 66 m (the deepest natural lake in Costa Rica). Río Cuarto products include surges and air-fall tephra. Its deposits show a narrow fan oriented westward, according to eastern-coming wind direction, indicating a directional surge (first 2 km), and latter to a relative low altitude explosive column. The air-fall deposits are not traceable farther than 5 km away. The stratigraphic sequence shows three main explosive phases. Radiocarbon dating and field work have confirmed that Hule was formed ~6.2 calendar ka ago and Pata de Gallo probably formed ~2.8 ka. Younger pyroclasts ~1.7 ka and ~0.7 ka would be related to eruptions from elsewhere in the neighborhood. There are no ages available yet for dating the formation of Río Cuarto maar, but archaeological data suggest that it erupted between 3-4 ka ago. The minimum volume of pyroclastic deposits associated with Hule maar can be estimated in 0.37 km3 and up to 0.51-0.53 km3 (0.36-0.40 km3 DRE), 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 makes 0.008 m3 of new DRE magma.