Structure and thermal characteristics of the Summit dome, March 1990 - March, 1991: Volcán Colima, Mexico

Abstract

A new exogcncous dome lobe erupted on the summit lava dome of Volcán de Colima, Mexico, early in March, 1991. Within the first day of cntptivc activity the new dome lobe grew to a volume of approximately 2,200 m3, was circular in shape, had a diameter of about 20m and a height of approximately 6.5 m. This dome lobe was surrounded by a shallow moat, from which degassing was intense. Radial fractures cxtend.:d from the dome lobe, indicating that principal horizontal compressional stresses were oriented radial to the dome at the time of extrusion. The most prominent faulting resulted in the formation of a small graben, approximately 20 111 in width and bounded by scarps with 3 m to 5 m of vertical displacement, extending NNW of the new dome lobe. Geophysical observations made on the summit lava dome throughout 1990 indicate that the initial extrusion of this dome lobe was controlled by a pre-existing structure. The dome lobe extruded approxiiJlately 30m south of the only high-temperature (350-575°C) fumarole field occurring on the summit dome during 1990. Four fumaroles monitored during March, May and December, 1990, degassed in a steady-stale from this field throughout 1990 at low mass Oows. Little change in mean fumarole temperatures occurred during this period. Ground magnetic observations collected in December, 1990, along a traverse across this ·high-temperature fumarole field reveal a short wavelength -2000nT magnetic anomaly, nearly centered on the fumarole field. Summit dome rocks have comparatively low magnetic susceptibilities (8.42xl0·6-8.06x10·5 mass-corrected Sl) and high remanent magnetizations (1.08 -5.57 A/m). Our model of the magnetic data, incorporating these rock magnetic observations, indicates that the source of the magnetic anomaly was a small, shallow body of rock which was heated by the ascending fumarole gases to above the NRM unblocking temperatures of summit dome rocks. This body of hot rock was elongated in a NNW direction and coincides exactly with the eastern scarp of the graben formed during the initial extrusion of the March, 1991 dome lobe. Application of a numerical heat and mass transfer model suggests that fumarole degassing in a steady-state and low mass flow from a shallow, volatilc-depleted magma, will create a thermal anomaly similar to that deduced from the magnetic model. Rock-aval:mching from high on the O:mks of the volcano is also most intense along the northern and southern extensions of the new graben. The coincidence of these structural and thermal features suggests that the NNW -trending graben formed during 1991 activity is the surface manifestation of a major structure transecting the upper edifice of the volcano, along which additional deformation may occur in the future.