EXPLOSIVE BASALTIC VOLCANISM FROM CERRO-NEGRO VOLCANO - INFLUENCE OF VOLATILES ON ERUPTIVE STYLE

The 1992 and 1995 basaltic eruptions oi Cerro Negro volcano, Nicaragua , had contrasting eruptive styles. Although they were nearly identical in composition, the 1992 eruption was explosive, producing a 7 kilome ter-high sustained ash column, whereas the 1995 eruption was essential ly effusive. The differences in water and carbon dioxide contents of m elt inclusions from the two eruptions define minimum saturation pressu res and show how decompression of initially similar magmas influences eruptive style. Before eruption, the explosive 1992 magma retained wat er and carbon dioxide while ascending to a moderate crustal level (abo ut 6 kilometers), whereas the nonexplosive 1995 magma lost all carbon dioxide by degassing during ascent to shallow crustal levels (about 1 to 2 kilometers).