Geochemical Variations Indicating Changing Hydrothermal Activity At Nisyros Island (greece)
Abstract
Nisyros Island belongs to the Aegean active volcanic arc and was affected by several hydrothermal eruptions in historical times (last ones in 1871-73 and 1887). Based on chronicles, it seems likely that seismic shocks played a fundamental role in trigger- ing at least the 1873 hydrothermal events. Soil CO2 diffuse degassing was studied, by means of 2900 measurements, in an area of ~2 km2, encompassing the hydrothermal craters area, where a fumarolic field is present today. The total CO2 output from the surveyed area was estimated to be ~100 t/d and it is accompanied by a heat flux of ~70 MW. A clear correlation exists between high CO2 fluxes and the structural setting of the area. One of the high CO2 flux alignments corresponds to a buried fault, which probably acted as a pathway for uprising deep fluids during past hydrothermal erup- tions. Significant changes were detected in the chemistry of fumarolic gases between 1997 and 2001, probably in response to the strong seismic crisis of 1996-97, although no hydrothermal eruption occurred so far. The observed increase in the H2S/CO2 ratio and decrease in the CH4/CO2 ratio suggest a higher involvement of magmatic fluids in the recharge of the hydrothermal system. In addition, several fumaroles showed con- current increases in H2 and CO contents, probably reflecting higher temperatures and pressures in the hydrothermal system, decreasing transfer times of gases from the deep reservoir and/or heating up of the upflow channels. In the Stephanos crater, diffuse CO2 soil and heat fluxes were regularly monitored during the last 5 years. After steady total fluxes observed from 1997 until May 2001, September 2001 was characterised by a significant increase in total diffuse soil CO2 and heat fluxes. Altogether, these changes suggest an increase in the P, T conditions of the hydrothermal reservoir(s) and therefore an increased hazard of hydrothermal eruptions. The observed geochem- ical variations anticipated the opening of a 350 m - long fracture, which occurred in the beginning of December 2001, on the caldera floor close to the hydrothermal craters area.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27.6068C