The Poas Volcano of Costa Rica is one of the most active of the world. Over 200,000 tourists visit the site each year. However, there is something else that makes this volcano so special and that is the lake that site at the bottom of the central crater. Of the 714 volcanoes that dot the planet, only 85 have crater lakes. But, more impressively, only 40 of those lakes have aguas calientes (or water heated by the volcano). The Poas Lake has a diameter of 300 meters and a depth of 45 meters. It sits in the middle of one of the largest active craters of the world. The active Poas Crater has a diameter of 1,320 meters.
At the time of this writing the water in Poas Lake is 35 degrees Celsius (or about 95 degrees Fahrenheit). The highest temperature ever recorded from the lake was around 94 degrees Celsius (or about 203 degrees Fahrenheit) and the lowest was 22 degrees Celsius (or about 72 degrees Fahrenheit). What amazes tourists who see the lake is its deep turquoise blue color. The reason for the brilliant colors is the density of chemicals in the lake, primarily sulfur, silicone and iron that when the sun’s rays hit these particles, they reflect the colors. Poas Lake is believed to have the highest concentration of these chemicals than any other volcanic “hot” lake in the world. The lake has more such chemicals than the largest such lake in the world, which is in the crater of the Kawah Ijen Volcano in Indonesia (with a diameter of 2,799 meters).
Changes in the chemicals, which reflect in a change in the lake’s color, are indicative of changes in the volcanic activity under the surface. When the color is the normal turquoise blue, the crater is moderately or normally active, but when it becomes a milky white it indicates a rise in such activity. Why is the lake there in the first place. Scientists point out several reasons. One of the most logical is the amount of rain that the area gets (some 3,800 millimeters per year). The surface of the crater below the lake also consists of materials that do not allow the water to be absorbed. Other volcanoes with hot lakes include the Rincon de la Vieja (also in Costa Rica), Santa Ana (in El Salvador), Chichon (in Mexico) and Copahue (in Argentina).