Eruption of Costa Rica 's Poas volcano in April and May was followed by 10 years of increasing gravity , thought to result from the intrusion of subsurface magma into the volcanic lake . After Poas 's last eruption in 1953 , lake depth stayed the same until 1983 when it began decreasing as heating magma caused water to evaporate . The Rymer and Brown study, in which microgravity was measured at regular intervals over a 10-year period, "represents the first time we can see a progression in microgravity over a long period of time" so that the results are much clearer, said geologist Al Eggers of the University of Puget Sound in Tacoma, Erupting water geysers in June 1987 brought levels down faster and water was gone by January 1989 when Poas erupted in a mile - high cloud of ash . Eruptions subsided when rains refilled the lake . Nicaragua 's Pacayo volcano , which erupted 10 years ago , has a different gravity change pattern .
Eruption of Costa Rica 's Poas volcano in April and May was followed by 10 years of increasing gravity , thought to result from the intrusion of subsurface magma into the volcanic lake . After Poas 's last eruption in 1953 , lake depth stayed the same until 1983 when it began decreasing as heating magma caused water to evaporate . Erupting water geysers in June 1987 brought levels down faster and water was gone by January 1989 when Poas erupted in a mile - high cloud of ash . Eruptions subsided when rains refilled the lake . Nicaragua 's Pacayo volcano , which erupted 10 years ago , has a different gravity change pattern .