After a volcano erupts a large crater forms at the top and after a long period of quiet, infrequent phreatic explosions remind us of what is occuruing below the surface. After many years rainwater collects inside the cone forming large pools or lakes. The cool air at the summits high altitudes, mixed with the hot air rising from the core, create conditions that are ideal for condensation and increased rainfall. Six of the active volcanoes have a lake, or lakes, in their craters. Some of that water will filter back into the volcano’s conduit’ and settle at a point near the magma load. Inside the volcano the water is heated and some will filter through and flow back out as hot rivers, whilst some is converted to steam that will lodge in the conduit and pressurize it.
The seal at the top of the vent retains the pressure, holding in the steam and gasses. This can go on for many years until the pressure can no longer be contained. The reaction, called a phreatic explosion, is not an eruption because there was no lava or magma discharged in the explosion. However there can be explosions of steam, water, ash, rock and volcanic bombs. These can be accompanied by carbon dioxide or hydrogen sulfide gas emissions.
Water seeping into the conduit of the Rincon de La Veija Volcano trickles down until it makes contact with the heated magma layer below the Earth's crust. The water is filtered and purified and the returned to the surface as a hot river of water. Blue River Resort tours in Rincon de la Vieja park beside the volcano include a dip in a warm water river. The minerals under the ground that make contact with the volcanic activity is what form the process for what creates hot springs and blue rivers. These blue rivers found on Rincon de la Vieja only flow down the north side of the volcano by Blue River Resort & Hot Springs (in Guanacaste). There are also other Costa Rica National Parks with blue rivers. Hot springs appear where these underground rivers break through to the surface of the land and the filtered volcanic water creates natural pools. Closer to the source, the water will be extremely hot. As it flows away from the heat source it cools, becoming pleasantly warm and therapeutic throughout Costa Rica.
On February 25, 2014 at about noon, there was a significant phreatic eruption (water, hot gases and lake sediments) in Laguna Caliente on Poas Volcano, Below is a picture:
More on the volcanoes of Costa Rica: