A model of degassing and seismicity at Arenal Volcano, Costa Rica

Arenal volcano is the most active volcano in Costa Rica and has emitted at least 1.3 Mt Of SO2 since its reactivation in July 1968. Gas emissions from the volcano have been both by passive degassing and explosive eruptions, w ith passive degassing being dominant. Based on correlation spectrometry (CO SPEC) measurements made during 1982, 1995 and 1996, the minimum average dai ly output is 130 +/- 60 t d(-1) SO2 emitted from Arenal. Arenal is extremel y active, with tremor fluctuations showing a distinct correlation with Eart h tides; decreased explosive activity and increased tremor appear to coinci de with the maximum rate of change in Earth tides. This suggests that a sys tem such as Arenal may be extremely sensitive to small changes in the confi ning pressure or stress regime of the conduit. The cyclic nature of explosi ve activity also may be caused by corresponding fluctuations in the extrusi on rate of lava. At high extrusion rates, lava from the non-explosive condu it may overflow into the explosive conduit, temporarily blocking it with a resulting pressurisation of the system. Arenal is likely tapping a deep to mid crustal magma chamber and, unlike many volcanoes, there is a comparativ ely small difference between petrological and COSPEC SO2 estimates (0.41 vs . 1.3 Mt, respectively, since 1968), suggesting that Arenal is being contin uously supplied by fresh magma. (C) 2001 Elsevier Science BN. All rights re served.