Structure and Thermal characteristics of the Summit Dome, March 1990 - March, 1991: Colima Volcano, Mexico

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Dome, temperature, fumaroles, magnetism, graben


A new lava lobe was extruded on the dome of the Volcán de Colima at the beginning of March, 1991. On the first day of eruptive activity, the new lobe of the dome grew to an approximate volume of 2,200 m 3 of circular shape with a diameter around of 20 m and height of approximately 6.5 m. A shallow depression with intense degassing formed around the lobe of the dome. Radial fractures to the dome were observed during extrusion. The most prominent faulting resulted in the formation of a small graben, approximately 20 m wide and bordered by escarpments with 3 to 5 m of vertical displacement extending towards the NNW of the new lobe of the dome.

Geophysical observations made on the dome in 1990 indicate that the initial extrusion of the new lobe was controlled by a preexisting structure. The lobe of the dome extruded approximately 30 m south of the only high temperature fumarole (350-375 °) that was over the dome in 1990. Four fumaroles monitored during March, May and December 1990, were degassed in steady state 1990 with low mass flows. There was little change in the average temperatures of the fumaroles. Magnetic observations of short wavelength, 2000nT, centered on the field of fumaroles. The rocks of the dome show comparatively low magnetic susceptibilities (8.42x10 -6 -8.06x10 -5corrected for SI mass) and high remaining magnetizations (1.08-5.57 A / m). Our model of the magnetic data, incorporating these observations of the magnetism of the rocks, indicates that the source of the magnetic anomaly was a small and shallow body of rock that was heated by fumarolic gases above the NRM blocking temperatures of the rocks of the dome. This body of hot rock was elongated in the NNW direction and coincides exactly with the eastern exhaust of the graben formed during the initial extrusion of the lava lobe of March, 1991. The application of a numerical model of heat transfer and mass suggests that the fumaroles that are degassed in a constant state and with low mass flow in a shallow magma depleted in volatiles will create a thermal anomaly similar to that which is deduced in the magnetic module. Rock spills on the upper parts of the volcano are more intense on the northern and southern extensions of the new graben. The coincidence of structural and thermal features suggests that the graben with NNW orientation that was formed during the 1991 activity is a superficial manifestation of a larger structure that cuts off the upper part of the Volcano and on which additional deformation may occur in the future.

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Citation / Publisher Attribution

Geofosoca Internacional, v. 32, issue 4, p. 643-657