The Quill is almost entirely composed of varied pyroclastic deposits representing five styles of pyroclastic activity: Pelean-, St. Vincent- ; pumiceous Plinian-style activity, semi-vesicular andesite Asama- style and phreatomagmatic activity. The deposits of the Plinian style activity are very striking and best seen in the seacliffs below Orangestand on the west coast and in Bargine Baai on the east coast, as white pumiceous lapilli airfall and ignimbrite (pumiceous pumice flow) deposits. Fine grained and thin bedded pumiceous ash fall and surge deposits from the collapse of Plinian eruption columns (up to 50km high) have been exposed where an oil storage facility has been constructed on Pisga Hill in the Northern Centers. These ashes contain powdery carbon and the remains of hermit crabs in seashells (latter swept up and transported by the surges). The ashes indicate that during Plinian activity the entire island can be swept by ash cloud surges resulting from the collapse of the Plinian eruption column. Radicarbon dating of the pyroclastic deposits of the Quill suggests that the Quill began forming around 50,000 years ago.
The excellent pyroclastic exposures of the Quill have permitted an unusually high level to stratigraphic correlation around the island and seven stratigraphic divisions have been recognized. From youngest to oldest these are:
The last erupted bedset of the Quill ( Division 7) is particularly well exposed and the five pyroclastic layers have been mapped individually. They have been named marker Units K (oldest) to Unit O (youngest). These deposits are dominantly pyroclastic flows and show a systematic change in chemistry from andesite at 58% SiO2 up the stratigraphy to basalt at 51% SiO2. Early formed mixed-magma deposits (Marker Unit K airfall and flow units) show both dark colored basaltic andesite and light colored andesite flow banded together within single clasts, suggesting that the eruption was triggered by the introduction of basalt into an andesite chamber followed by mixing and hybridization with subsequent emptying of the chamber. Our interpretation of the radiocarbon ages (including Indian settlement ages) suggests that the last erupted bedset probably formed between 1755 and 1635 years B.P. Six recently-drilled water wells in the flanks of the Quill have revealed heated groundwater suggesting that the Quill is dormant.
Petrographically the Northern Centers comprise a single group of homogeneous andesite lavas with a silica range of 59.7% to 60.4%. In contrast volcanic components of the White Wall range change from basaltic andesite (56.1% SiO2) near the base up the stratigraphy through andesite to dacite (63.7%SiO2). The overlying Sugar Loaf succession shows a more restricted range from dacite (64.2% SiO2) at the base of the succession to rhyolite (71.2 % SiO2). Petrographically, the Quill shows the greatest silica range of all Lesser Antillean volcanoes ranging from basalts (50.4% SiO2), through basaltic andesite, andesite, dacite to high silica soda- rich rhyolite (72.3% SiO2).
The three volcanic components of St. Eustatius (Northern Centers, pyroclastic deposits of the White Wall-Sugar Loaf ridge and The Quill) show a varied geochemistry. The volcanic rocks of the Northern Centers - White Wall-Sugar Loaf form a distinct chemical trend that ranges from basaltic andesite and andesite of the low-K calc alkaline suite to dacite and rhyolite of the medium-K calc alkaline trend. That is the single chemical trend crosses field boundary used in classifying calc-alkaline magmas. In contrast the Quill situated only 35km from Saba and coeruptive with that island over the past 50,000 years, differs from Saba by erupting a low-K calc alkaline magma series. St. Eustatius is unique in the Lesser Antilles in that rhyolite evolved twice in its history, first in the Sugar Loaf and then later in the Quill pyroclastic succession. An estimate of the time required for a basaltic magma chamber to fractionate to produce such rhyolites is around 30,000 years.
Like Saba the pyroclastic deposits of the Quill contain a suite of ejected blocks derived from the underlying submarine bank which includes Tertiary fossiliferous limestone and green metavolcanic rocks. However unlike Saba the deposits of the Quill lack the rounded hypabyssal nodules. Instead, and again unlike Saba , there are coarse-grained plutonic blocks of igneous cumulates of the type found on the floors of magma chambers. From these differences it is concluded that the Quill is underlain by a magma chamber in which crystal fractionation occurs to produce a wide range of lava compositions. In contrast Saba is believed to be underlain by a system of parallel dikes emplaced into a fault zone. This conclusion of the contrasted crustal plumbing systems of the Saba and the Quill (represented by hypbysssal nodules and plutonic blocks respectively) has now been applied successful to other volcanoes of the Lesser Antilles where for example the Soufriere Hills volcano at present erupting on Montserrat is similar to Saba whereas Mt. Pelee, Martinique that erupted in 1902 and 1929 is similar to the Quill.
For a more detailed description of the volcanology of St. Eustatius the reader is referred to: Roobol, M.J. and Smith, A.L., 2004, Volcanology of Saba and St. Eustatius, Northern Lesser Antilles, 2004, Royal Netherlands Academy of Arts and Sciences (Koninklijke Nederlandse Akademie Van Wetenschappen), Amsterdam , the Netherlands , 320pp. The latter is available at website: www.knaw.nl/edita/antilles.
For an up to date evaluation of the volcanic hazards, 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.