The island appears to overlie a NE-SW fault zone as indicated by recent seismic activity and the distribution of hot springs along the longest axis of the island.
The oldest dated rocks on Saba are around 400,000 years. Because there are no flank deposits rich in carbonised wood, pits were dug on the island to locate the youngest pyroclastic deposits. In a pit dug within the island capital of The Bottom, the youngest pyroclastic deposit was found to be a thin ash surge with accretionary lapilli (dated at 280 years B.P.) that overlies Amerindian remains but underlies deposits containing European debris following European settlement on the island in AD1640. European settlers may have been particularly attracted to the island by the presence of meadows of grassland instead of tropical rain forests. These meadows resulted from pyroclastic flow eruptions shortly before European arrival and before the rainforest had re-established itself.
Between 1995 and 1997 an increase in the number of local earthquakes was associated with a 7 to 12 degree Centigrade rise in temperature of the hot springs . The seismic activity that was felt on Saba is thought to represent a mild volcano-seismic crisis, with the increased heat flow resulting from either deeper circulation of ground water or possibly renewed magma movement.
The volcanoes of the northern Lesser Antilles are built upon broad platforms of submarine banks where Mesozoic volcanic and sedimentary rocks are capped by Tertiary limestone layers. This platform is well known in the Saba Bank (13.5km southwest of Saba) standing around 1000m above the sea floor, measuring 70 km east-west and 40 km north-south and with a minimum water depth of only 20 m in the south and 40m in the north. It has been drilled for oil so that its stratigraphy which includes Mesozoic rocks is known. The presence of the underlying bank is indicated on Saba island by the presence of a rare and exotic suite of crustal inclusions contained in the volcanic deposits. These include Tertiary fossiliferous limestones, sheared plagiogranite, a biotite-hornblende-clinopyroxene adamellite and hornfels.
The presence of the rounded hypabyssal nodules belonging to the same magma series as the volcanic rocks suggest the presence of composite dikes beneath the island. Also the shape of the island, the distribution of hot springs and the locations of recent seismic epicenters all suggest that the island is built on top of an ENE fault that cuts the underlying submarine banks. Magma is stored in this fault zone in the form of vertical sheets or dikes in which several magma compositions coexist.
For a more detailed description of the volcanology of Saba, the reader is referred to: Roobol, M.J. and Smith, A.L., 2004, Volcanology of Saba and St. Eustastius, Northern Lesser Antilles: Royal Netherlands Academy of Arts and Sciences (Koninklijke Nederlandse Akademie Van Wetenschappen), Amsterdam , the Netherlands , 320pp. This is available at website: www.knaw.nl/edita/antilles
For an up to date evaluation of the volcanic hazard the reader is referred to the soon to be published: Lindsay. J., Robertson, R., Ali, S., and Shepherd, J. (Eds), in press, Volcanic Hazard Atlas of the Lesser Antilles: Seismic Research Unit, University of the West Indies, St. Augustine , Trinidad , W.I.