SO₂ fluxes from Galeras Volcano, Colombia, 1989-1995: Progressive degassing and conduit obstruction of a Decade Volcano

Galeras volcano has emitted a minimum of 1.9 Tg SO₂ during the past seven-year period of re-activation from 1989 to 1995. Emissions from the volcano are almost exclusively by passive degassing and represent at least 1.4% of the annual volcanic output of SO₂. The volcano was characterized by high SO₂ flux during 1989-1990, frequently exceeding 2000 metric tons/day. Fluxes decreased to less than 1700 tons/day in October 1990. A further decrease to less than 1000 tons/day occurred in late 1991 after emplacement of a lava dome. Currently, SO₂ fluxes are less than 500 tons/day. The general decrease of the flux with time is interpreted as: (1) progressive degassing of a single batch of magma; and (2) obstruction of the conduit by emplacement of the dome. During 1992-1993, six explosive eruptions occurred which destroyed the dome and excavated the crater. Three of the largest eruptions were immediately followed by intense long-period seismicity (up to 600 events/day) and by comparatively large SO₂ fluxes. This high seismicity and SO₂ flux are interpreted as rapid degassing of partially solidified magma that had been unroofed by the eruptions. Although the volcano is currently in a comparatively degassed state, the six recent eruptions demonstrate that there is still residual magmatic gas which is unable to be released freely. This can lead to pressurization, resulting in short, periodic eruptions or gas emissions that rapidly dissipate the pressure buildup.