E.S. Calder1, P.D. Cole2 and S.R. Young3
1Department of Geology, University of Bristol, Bristol, UK
2Department of Geology, University of Luton, Bedfordshire, UK
3British Geological Survey, Keyworth, Nottingham, UK
Prehistoric pyroclastic flow and debris flow deposits dominate the geology of the flanks of Soufriere Hills Volcano. Exposures occur in river cuttings and in sea cliffs on both the west and eastern sides of the island. These deposits have been described in some detail by Rea (1975) and briefly by Baker (1985) and Wadge and Isaacs (1988). Recent studies and work in progress by Smith et al.. (pers. comm.) concentrate on the broad stratigraphy of the Soufriere Hills units. A large number of radio-carbon dates for these deposits have now been obtained ranging from 350 to 23,500 a BP (Wadge and Isaacs, 1988 and Smith et al., 1996). Comparison of the textural and lithological variations in the prehistoric and the 1996 deposits have been used to determine whether previous flows were associated with explosive activity or were solely a result of gravitational instability.
Prehistoric deposits in Fort Ghaut on the west expose a sequence of at least three possibly four pyroclastic flow units. Individual units are often separated by thin ash or surge horizons. Flow units have undulating boundaries and are up to 7 m thick although units as thin as 50 cm are present. Maximum clast size is the order of 1 m. These deposits are composed dominantly of dense clasts although the fines-poor basal layers contain an appreciable quantity of vesicular material. Similar units are exposed on the east coast around Spanish point and Whites Bottom Ghaut. Here the main unit is at least 5 m thick for much of its extent and comprises layers, lenses and channel-form structures rich in either pumice or dense clasts. The lenses or subunits of either pumice-rich or dense clast-rich material are typically <2 m thick, inverse graded and can be traced for several tens or even hundreds of metres. Bread-crust inflated bombs with vesicular centres and 1-2 cm glassy rim comprise 5% of >10 cm blocks. Radially jointed or prismatically jointed blocks (a product of rapid chilling) are abundant particularly in the deposits comprising essentially dense material. Most prehistoric pyroclastic flow deposits have at least a small component of moderately vesicular to vesicular juvenile material. The pumice is characteristically crystal rich and dense. Bread-crust bombs are typical in deposits which possess vesicular material although are more common in the east coast deposits than Fort Ghaut. A few small deposits, both on the east and west coast are composed predominately of pumice such that they can be termed pumice flow deposits, some of which were identified by Wadge and Isaacs (1988).
Pyroclastic deposits produced during the 1996 activity by gravitational collapse of the dome are confined to the Tar River valley on the east. Dense basal avalanches giving rise to coarse block-rich material in the valley bottom and upper turbulent ash cloud surges depositing thinner layers of finer ash-rich material over a broader less topographically constrained area. The 1996 deposits are nearly entirely composed of dense material with <3% vesicular or friable clasts. In early August an increased proportion of vesicular material was seen in the deposits in the Tar River valley and this was associated with an increase in extrusion rate (6 m3s-1). Bread crusted, vesicular blocks and pumiceous tephra fall material were produced during the explosive eruption of the 17th September, but preliminary studies of the pyroclastic flow deposits produced during this event contain only minor amounts of pumice and no bread crust textured clasts. Radially jointed blocks with joints on the scale of 2-10 cm and well defined dark red-brown alteration margins 3-7 cm deep were found on the delta margins where the hot blocks had been rapidly chilled by the water.
Although no lapilli fall layers have been observed in association with prehistoric flow deposits on the east or west coast, fine ash layers are often present at the base of these sequences. These ashes typically rest on a palaeosoil and are up to 20 cm thick although there is considerable thickness variation. Carbonised leaves and twigs have been observed in these ashes and stratification is probably related to multiple depositional events. These are thought to be formed by precursory, minor explosive activity before the onset of major pyroclastic flow forming eruptions. Preliminary electron microscope investigations of one of the Fort Ghaut ash layers indicates that it is composed of non-vesicular, blocky grains typically generated by lava domes, whether these were generated explosively is yet to be established.
Deposits on the east coast from Farm Bay through Spanish Point to beyond Whites Bottom Ghaut are laterally extensive and can be traced for over 2 km. They can also be traced at least a few km up Whites Bottom Ghaut and thus define a broad relatively continuous fan or sheet to the northeast of the Soufriere Hills Volcano. Although volumes are poorly constrained average thickness of 2 m for these type of fan deposits implies volumes larger than those produced during the current activity. Flow unit geometries of the Fort Ghaut deposits suggest that they are valley fill type deposits, although the general topography of the Amersham area clearly represents a deposit fan. Volumes of the Fort Ghaut deposits are hard to ascertain, however the thickness of layer 1 subunits found in Fort Ghaut (20 cm) are larger than those found in the 1996 deposits (5 cm) implying a larger volume of material involved in the flows. The 1996 gravitational collapse flows are topographically constrained and have so far been largely confined to the Tar River valley. Much of the volume of the recent deposits is now concentrated in the delta fan which by end October had reached a volume of 11 x 106m3.
Collapse of a large volume of material from a dome is, in itself, likely to initiate explosive decompression. Flows generated with an explosive component are less likely to be topographically confined and small volume, gravity-driven flows are less likely to be preserved. Remnants of the five old domes that make up the Soufriere Hills Volcano are all substantially larger than the volume of the present dome (24x106m3 mid July). We propose that most of the prehistoric deposits are the products of explosive events associated with dome growth although probably larger than that which occurred on the 17th September 1996. The widespread occurrence of vesicular material in the prehistoric deposits implies a more gas-rich magma and/or rapid decompression both of which are associated with explosive activity. The absence of lapilli fall deposits in the prehistoric sequence indicates that explosive eruptions were short lived and that sustained convective columns were not produced. The significant proportion of bread-crusted, vesicular bombs, and the large columns of material involved in these collapses are all consistent with an explosive history of the Montserrat domes.