Alan L. Smith

A working terminology of pyroc lstic deposits

Abstract A nomenclature for pyroclastic deposits is proposed which it is hoped will provide a working terminology. Three basic types of deposit may be distinguished: fall, flow and surge, and descriptions are given of their dominant characteristics. No unique classification for pyroclsstic rocks can be made and at 1,, ast two systems are required: (1) a genetic classification to interpret the genesis of a deposit, and (2) a lithological classification which may be solely descriptive, but also which may be used to discriminate on a lithological basis the mechanisms which, produced a particular pyroclastic deposit. Genetic classification scheme are presented for various types of fall, flow and surge deposits. A lithological classification is given based on grain size limits and distribution, constituent fragments and degree and type of welding. A glossary of some other terms in use to describe pyroclastic deposits is also givijn.

GPS measurement of surface deformation around Soufriere Hills volcano, Montserrat from October 1995 to July 1996

Global Positioning System geodesy was used to measure surface deformation on Soufriere Hills volcano, Montserrat from October 6, 1995 to July 1, 1997 during initial dome growth and gravitational collapse. Our data from this period show non-axially symmetric horizontal displacements, and decreasing subsidence as a function of radial distance from the former topographic high of the volcanic edifice. Forward modeling suggests that surface deformation is caused by a shallow vertical dike (< 3 km), which expanded approximately 1 m, coupled with a deflating Mogi source at about 6 km depth. These inferred source parameters are in good agreement with independent observations of regional dike widths and preemption magma storage depth.

Destruction of St. Pierre, Martinique, by ash-cloud surges, May 8 and 20, 1902

St. Pierre, Martinique, was destroyed by ash-cloud surges on May 8 and 20, 1902, that developed from the tops of block and ash flows derived from Mount Pelee. The dome was small during these eruptions, and eyewitness accounts indicate that the eruptive sequence started with vertical eruption columns followed by pyroclastic flows. The flows were topographically controlled and moved down the valley of the Riviere Blanche. Their associated ash clouds expanded to several times the width of the associated flows, and it was these expanding ash clouds that overwhelmed St. Pierre on May 8 and 20. It is our conclusion that the early nuees ardentes of the 1902 eruption of Mount Pelee were formed by column collapse into the crater and not, as commonly stated, by dome collapse or directed blast. The resultant mass poured through a cleft in the southwest side of Pelee, thereby heading the flows in that direction. It was only the later nuees ardentes that appear to have been directly affected by the presence of a dome.

Pyroclastic stratigraphy of the Soufriere Hills Volcano, Montserrat ‐ Implications for the present eruption

The stratigraphy of the Soufriere Hills volcano is dominated by dense-andesite block and ash flow deposits. Forty-one radiocarbon ages permit a reconstruction of the eruptive history over the past 31,000 years. A single stratigraphic member yielding radiocarbon ages of 22,000 to 20,000 years B.P. is composed of block and ash flow deposits with semi-vesiculated clasts concentrated in the upper third of each deposit. The present eruption is also producing block and ash flow deposits with both dense and semi-vesiculated clasts. It marks a return to a more explosive driven type of activity as occurred 22,000 to 20,000 years ago. Deposits of this member have a broad sheet-like form and volumes in excess of any produced so far by the present eruption.

Calderas or gravity-slide structures in the Lesser Antilles island arc?

Abstract Large arcuate depressions (up to about 10 km in diameter) on the back-arc side of St. Lucia, Dominica and St. Vincent in the Lesser Antilles arc are discussed. The Qualibou depression on St. Lucia has been previously described as a caldera, partly because it was believed to be the source of ignimbrites on the island. Remapping and stratigraphic studies of these ignimbrites show they were not erupted from the depression, but from central vents in the highland rainforests and are much younger. Dominica is geologically similar and has two scarped depressions at the southern end of the island. Soufriere volcano, St. Vincent has a prominent arcuate scarp opening to the west, the southern half of which has been buried under the youngest volcanic products of the volcano. We interpret these structures as the root zones of gravity slides, perhaps eruption-related and triggered by magmatic inflation. Post-slide activity (lava domes and fumaroles) has occurred within them due to reduction in lithostatic load and pressure release on the flanks of these island volcanic complexes. The presence of these structures only on the western coasts is correlated with the steeper 9° slopes into the Grenada Basin, in contrast to the 1° submarine slopes of the Atlantic side.

Petrochemistry of the Peléan-Type Volcanoes of Martinique

The northern part of the island of Martinique in the Lesser Antilles consists of three overlapping volcanic cones: Mont Pelee, Pitons du Carbet, and Morne Jacob. The southern half of the island consists of a complex of older, smaller, more deeply dissected and weathered centers. The deposits of the island are largely volcaniclastic, originating as nuee ardente pyroclastic flows, air-fall material, explosion breccia, mudflow, and lahars. Many of the deposits cannot be grouped as above because of extensive reworking shortly after their eruption and deposition in an extremely friable and uncompacted state. Lava flows, domes, spines, and pitons compose only a small part of the exposed volume of the island. Chemically, the majority of rocks are close to 60 percent ±3 percent SiO 2 , but distinctly different abundances of alkali elements are present in the different centers. The older Morne Jacob lavas have more than double the K, Rb, Ba, and Th of the Mont Pelee rocks, whereas the latter have higher Na and Sr. Other element abundances are similar. Mont Pelee is thus one of the low-K andesite series typical of island arcs, whereas Morne Jacob tends toward the Andean type in alkali content, although still composed of calcic andesite.

The lesser antilles — A discussion of the Island arc magmatism

The active island arc of Lesser Antilles marks the junction between the Atlantic and Carribbean lithospheric plates. With the exception of the alkali basalts of Grenada, the volcanics of the arc can be regarded as belonging to the low-K, island arc, calc-alkaline suite. Although compositions ranging from basalt to rhyolite have been described, porphyritic andesite appears to be the dominant rock type on most volcanoes (intermediate centers). Variable amounts of basalt and basaltic andesite occur and rarely predominate over andesite (latter are basic centers), whereas the more silicic members are only occasionally found. The calc-alkaline suite is characterized by relatively high Al2O3 and CaO and low K2O, Rb and Ni. Variations, especially in the alkali elements, occur both with space and time. A characteristic feature of many of the volcanoes is the occurrence in the basalt and basaltic andesite volcanics of plutonic blocks, often showing cumulate textures. The blocks which ware composed of plagioclase — amphibole — olivine — clinopyroxene — magnetite are thought to be the products of fractionation. The differences between basic and intermediate centers is probably due to the frequency that the magma ascended to the surface or remained in high level chambers where fractionation occurred.

A comparison of the recent eruptions of Mt. Pelée, Martinique and Soufrière, St. Vincent

The simultaneous eruption of Mt. Pelée, Martinique and Soufrière, St. Vincent are regarded as the first recognized examples of Pelean-type and St. Vincent-type pyroclastic eruptions. Both produced nuées ardentes, the former usually laterally directed because of the presence of a dome and the latter vertically directed from an open crater. Both volcanoes have subsequently erupted for a second time this century. The 1902–05 and 1929–32 eruptions of Mt. Pelée produced andesite lava of almost identical composition and mineralogy. Both contain two generations of plagioclase, orthopyroxene, Fe-Ti oxide, corroded brown amphibole and olivine rimmed by pyroxene. In contrast, the Soufrière material is more basic in composition varying from basaltic andesite to basalt in 1902–03 and basaltic andesite in 1971–72. The Soufrière material contains two generations of plagioclase (with those of 1971–72 having additional zones of labradorite), clinopyroxene, orthopyroxene, olivine and Fe-Ti oxide. The pyroclastic deposits are strikingly different, those from the Pelean-type eruption are termed «block and ash deposits» being characterised by poorly vesicular lava blocks up to 7 m in diameter, while the St. Vincent-type eruption produced «scoria and ash deposits» containing vesicular ropey blocks or bombs no larger than 1 m in diameter. The differences in styles of eruption are attributed to differences in viscosity and mechanism of eruption of the magmas. Stratigraphic studies of Mt. Pelée reveal that the volcano has produced basaltic andesite scoria and ash deposits from St. Vincent-type eruptions. It is concluded that the recent eruptions of Pelée tapped a deep level magma during both eruptions releasing magma of similar composition, while the 1971 Soufrière magma is thought to be a remnant of the 1903 basaltic magma which remained at a high level within the volcano where it underwent enrichment in plagioclase and loss of olivine and oxide.

Pumice eruptions of the lesser Antilles

Stratigraphic studies on the active and potentially active volcanoes of the Lesser Antilles have revealed two main types of andesitic pyroclastic deposit. One with dense clasts in a poorly vesicular ash represents nuée ardente eruptions of Pelean type and the other group of vesicular pumice and ash represent both Plinian airfall and ash-pumice flow eruptions. The pumiceous deposits can be divided into airfall lapilli, airfall ash, crystal-pumice surge, ashpumice flow and ash hurricane types. No pumice eruptions have been witnessed in the Lesser Antilles during the period of written history although the stratigraphy of archaeological sites shows they occurred in pre-Columbian times. Detailed stratigraphic studies of Mt. Pelée, Martinique, and the Quill, St. Eustatius, show that, throughout their history, pumice eruptions have alternated with nuée ardente eruptions with approximately equal frequency. The widespread occurrence of pumiceous deposits on many of the West Indian volcanoes and the frequent alternations in the stratigraphic sections suggest the high probability that they will be witnessed in the future. On Martinique, some on the late prehistoric pumiceous pyroclastic flow deposits (the ash hurricanes) have been traced 20 km from the central vent to the out-skirts of Fort de France, indicating that they are the major hazard in the Lesser Antilles. Measured stratigraphic sections show that the Pelean type nuée ardente deposits are separated from the pumiceous pyroclastic deposits by others of intermediate vesicularity and appearance. The presence of such deposits of intermediate vesicularity could provide a future warning of impending change in pyroclastic style. As no such deposits formed on Mt. Pelée this century the present «safer» episode of nuée ardente (Pelean type) activity is expected to continue.

Radiocarbon redating of contaminated samples from a tropical volcano: the Mansion ‘Series’ of St Kitts, West Indies

Heavy rainfall and dense vegetation on tropical volcanoes produce abundant carbonized wood in pyroclastic deposits, in addition to easy contamination of this wood by root systems and soluble humic material. Because the physical nature of the charcoal varies, some samples are more prone to contamination. Two independent studies of the same volcano, Mt Liamuiga on St Kitts in the Lesser Antilles, sometimes using samples from the same carbonized tree, yielded a systematic difference in radiocarbon ages. An exchange of samples and a re-investigation of three physically distinct types of charcoal yielded the following results. Rare, hard, dense charcoal, lacking contamination, which had yielded a spurious age of 2860 years bp, was redated at 1845±58 years bp. Common soft, friable charcoal with good cellular structure proved to be susceptible to contamination. A field decontamination technique utilized by one group seems significant as it yields older ages than when only routine laboratory pre-treatment was used, indicating that the latter technique only partly removes the dried and hard residue produced by the decomposition of modern plant rootlets. A previous date of 24870 years bp obtained from powdery charcoal in a horizon beneath the Mansion ‘Series’ contradicted ages older than 41000 years bp from common friable charcoal in the lower Mansion ‘Series’. The soft powdery charcoal was re-investigated using a sample collected a few centimeters from the original, although field decontamination of this sample was not possible, more extensive laboratory treatment yielded an age of ca. 43000 years bp, again proving that routine laboratory pretreatments are inadequate. A revised geochronology for the Mansion ‘Series’ is described and a cautionary discussion is presented for the benefit of investigators using radiocarbon ages to date volcanic deposits.