Costa Rica's Rincon de la Vieja volcano is showing signs of increased activity since September 17, 2014. Phreatic explosions have regularly occurred from the main crater which contains an acid lake. An aerial survey on Friday showed no fresh deposits, which suggests that the eruptions were rather small and did not eject material beyond the lake. Although none was observed directly, at least 6 explosions could be identified by seismic signals.
Based on the observations by park rangers the volcano is currently experiencing strong bubbling at the lake and floating sulfur rafts on the surface. The water temperature has increased significantly by 15 degrees Celsius, compared to measurements made in 2013.
Hot, acid mud flows (lahars) if larger explosions cause the lake to drain or eject larger quantities of water would be the main hazard from the volcano at present. Such lahars could affect all sides of the volcano, but the areas to the NE around the Azul and Pénjamo river beds would be most at risk. They have already been affected by lahars during previous eruptions.
At the moment, it is impossible to know whether the current activity is a precursor of a new magmatic eruption or will remain a more or less intense phase of phreatic explosions, caused by readjustments in the shallow hydrothermal system rather than fresh rising magma. (VolcanoDiscovery)
The volcano has had similar crisis as recently as 1998, 2011, and 2012. None of them was followed by the eruption of magma, and only phreatic (steam-driven) explosions occurred. The last magmatic eruption was in 1995.
Rincon de la Vieja last appeared in the GVP's weekly volcanic report during the week of February 27 - March 5, 2013:
OVSICORI-UNA received reports at 05:30 on February 26 of pulsing white plumes rising from Rincon de la Vieja's active crater about every four minutes. The seismic records showed no signals associated with a phreatic eruption or sudden gas output. Cloud cover prevented views of the active crater during an overflight later that day, however clear views of the N and S flanks and areas SW showed no changes.
Rincón de la Vieja is the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge that was constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Rincón de la Vieja, sometimes known as the "Colossus of Guanacaste," has an estimated volume of 130 cu km and contains at least 9 major eruptive centers.
Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of 1916-m-high Santa María volcano, the highest peak of the Rincón complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A plinian eruption producing the 0.25 cu km Río Blanca tephra about 3500 years ago was the last major magmatic eruption from the volcano.
All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent crater containing a 500-m-wide acid lake (known as the Active Crater) located ENE of Von Seebach crater.
Featured image credit: GVP