Sulfur deposits on the N rim of the Gran Cratere, September 1995. Much of this area has not been visible due to dense steam clouds in the past years; the diminished fumarolic activity of 1995 makes the fumarolic area more accessible.
Since the end of the 1888-1890 eruption, there have been two major episodes of increased fumarolic and seismic activity, one in the mid-1920's and another one starting in 1985 that seems to have delined in 1995. Montalto (1996) speculates that the recent increased activity could have been triggered by a magmatic intrusion to shallow depth. Much of the following is summarized from Montalto (1996); more recent (post 1991) data are from various sources.
The first notable event that affected Vulcano after the 1920s' increased fumarolic activity was a magnitude 5.5 earthquake on 15 April 1978 (causing about 8 deaths in northern Sicily). Its hypocentral area lay about 5 km south of Vulcano. Starting in 1985, there was an increase in the fumarole temperature from about 200 to about 300° C and in the overall gas output from the fumaroles at the Gran Cratere. This was accompanied by changes in the chemical composition of the fumarolic gases. New fractures and vents opened on the N rim of Gran Cratere, and notable ground deformation was observed (Montalto 1996).
Increased shallow seismic activity (a swarm of high-frequency events) began on 25 April 1985 and lasted until 5 June, then was followed by another one from 8 July until 25 August 1985. This was accompanied by an increased steam output from fumaroles at Gran Cratere and other hydrothermal phenomena. Both seismic swarms were accompanied by inflation.
From late 1986 to early 1987, fumarole temperatures at Gran Cratere. rose gradually from about 300 to >400° C, and new fumarole fissures and vents became active on the N crater rim and on the N flank of the Fossa cone. A part of the NE flank of Fossa cone collapsed on 20 April 1988, during a period of high regional seismic activity (lasting from March until June 1988). Early speculations that this landslide was triggered by a small phreatic explosion in that area proved incorrect.
A new episode of increased local seismicity began on 9 August 1988, affecting mostly the S part of Vulcano island. This activity levelled off on 14 August but was immediately followed by shallow events immediately below the Gran Cratere, lasting until October 1988. Thereafter, a marked increase in the fluid output of the fumaroles was observed, together with an increase in the soil CO2 output.
The period December 1989-May 1990 was characterized by areal contraction which is interpreted by Montalto (1996) as a possible sign of cooling and crystallization of a shallow magma body emplaced sometime during the increased activity observed before. Another similar event took place before February 1991.
The most significant episode of unrest seems to have been that of July 1994 when seismic activity reached alarming levels. Info about this episode is still sketchy but it seems that for a brief time, the risk of an immediately imminent eruption was considered very high.
Since then, the visible activity has shown a progressive decrease (see the comparison images below). When visited on 18 April 1995 by Giada and myself, fumarolic activity at the N rim of Gran Cratere had notably diminished but become more vigorous in the deep central pit of the crater. A further notable decrease of the visible fumarolic activity on Gran Cratere's N rim was evident during a visit on 13 September 1995. There was continuing fumarolic activity in the upper Forgia Vecchia crater where strong hydrothermal alteration is continually undermining the steep and unstable slope immediately above the village of Vulcano Porto.
20 September 1989
30 March 1990
6 November 1990
18 March 1992
18 April 1995
13 September 1995
Comparative views of the N part of the Gran Cratere, from September 1989 until September 1995, showing variations in fumarolic activity. All images were taken from the highest point of the crater rim on its SE side. Note village of Vulcano Porto in the background.