Sint Eustatius and Saba Islands are the northernmost potentially active volcanic islands in the Lesser Antilles, the volcanic front that defines the subduction of the Atlantic portion of the North and South American Plates beneath the Caribbean Plate. This subduction created a long volcanic arc from near the US Virgin Islands in the north to Trinadad – Tobago in the south.
Political ownership has multiple nations involved, some European, some in the Western Hemisphere, and some independent nations. There are at least 8 sovereign states (independent nations), and 17 non-sovereign states and territories with political affiliations with 6 other nations. The geology is not the only complex attribute of these islands.
The islands are divided into three main groups: Leeward Islands, Windward Islands and the Leeward Antilles. The Leeward Islands are generally to the north and include the northern islands of the Lesser Antilles chain. The Windward Islands comprise the bulk of the islands along the arc southward to Trinidad – Tobago. The Leeward Antilles are generally west of Trinidad – Tobago along the northern Venezuelan coastline.
The islands of interest in this post are located north of Montserrat and are politically associated with the Netherlands. Sint Eustatius and Saba are two small islands in the inner northern Leeward Islands – part of the main Lesser Antilles Volcanic Arc. Politically, after separate referenda on each island, they were assigned the status of a special municipality of the Netherlands in 2010. Together with the island of Bonaire north of the Venezuelan coastline, these three now form the Caribbean Netherlands, shortened to the BES (Bonaire, Eustatius, Saba) islands.
Sint Eustatius (Dutch spelling and pronunciation for Saint Eustace) or Statia is a small, rocky island in the northern Leeward Islands. It has 3,138 people occupying 21 km2. It is unclear if the island was first populated by the Carib people before western discovery. It took a little more than a century to become a neutral free port, a central transportation hub for all manner of contraband including the slave trade. The island was quite rich through the end of the 18th Century before getting embroiled in multiple wars between European nations and a French occupation starting 1795 which sank its economic star completely.
Climate on Sint Eustatius is tropical monsoon, with tropical storms and hurricanes common. Beaches on the island are volcanic sand that are nesting sites for endangered sea turtles. There are multiple parks on and around the island. The local government is the largest employer on the island. An oil terminal is the largest private employer. Official language is Dutch, but English is common.
The American company NuStar Energy has 56 storage tanks on the island with a total capacity of 13 million barrels of petroleum (over 2 million m³). The loading and unloading facilities are located at Tumble-Down-Dick Bay; they can accommodate the world’s largest oil tankers (up to 520,000 DWT) and are available 24 hours a day. Partly due to the presence of the oil terminal, Sint (St.) Eustatius has been one of the most favorite ports for purchasing fuel in the region for many years. St. Eustatius is considered the cheapest bunkering location in the Caribbean.
Saba is the smallest special municipality of the Netherlands. It has a population of 1,933 on a land area of 13 km2. It may have been populated as long ago as 1100 BC by people who preceded the Caribs in the region. The island supported sugar, rum and trade during the 17th and 18th Centuries. The island coastline is forbidding and steep, rocky with cliffs of 100 m and no permanent beaches, making it a sanctuary for families of smugglers and pirates. Like neighboring St Eustatius, its climate is tropical monsoon with tropical storms and hurricanes common. Largest current piece of the local economy today is tourism, though agriculture and Saba lace also contribute.
Monitoring is done by the Royal Netherlands Meteorological Institute (KNMI). Its current Netherlands Antilles (NA) seismic network consists of 7 broadband seismometers: 3 on Saba (SABA, SABQ and SABW), 3 on St. Eustatius (SEUS, SEUG and SEUT) and 1 on Sint (St) Maarten (SMRT). Since 2006, the KNMI has been measuring ground movement on both islands using seismometers with the aim of detecting (volcanic) earthquakes. In order to be able to monitor the volcanoes even better, a KNMI team visited Saba and Statia in January 2018 to a) further improve the quality of the seismic data and b) install GPS sensors to measure the deformation of the volcanoes.
Region
Saba and Sint Eustatius are just to the NW of St Kitts & Nevis islands. Saba is 142 km E of the tip of the US Virgin Islands and 316 km ESE from San Juan, Puerto Rico. There are multiple larger islands within 50 km of Saba including Anguilla, Gustavia and St Kitts. Barbuda and Antigua are another 100 km toward the E. Sint Eustatius is 31 km SE of Saba. St Kitts & Nevis are 13 km SE of St Eustatius. T currently active Soufriere Hills volcano on Montserrat is 150 km SE of Saba.
St Kitts (Saint Christopher) Island is a recently active volcanic island, with the newest volcanic center at Mount Liamuiga on the NW end of the island. Smithsonian GVP has a pretty good writeup on the volcano, from which I will take the following thumbnail sketch. The most recent major eruption took place around 160 AD. It was a VEI 4 that produced pyroclastic flows and mudflows. There are unconfirmed reports of two other eruptions in 1692 and 1843. There are three lava domes on the W and SW flanks of the volcano. It is the youngest of three volcanic centers on the island. The volcano has active fumaroles and recent earthquake swarms in 1988 and 1999.
Neighboring Nevis Island is also a volcanic island, though this one not nearly so recently active. It has a single volcanic center that is the high point of the island. The crater is breached to the west. There was a small lava dome within the inner crater that produced pyroclastic flows and mudflows. There are an additional four domes on the flanks. There are no historic eruptions known, though there are active fumaroles and hot springs. Multiple seismic swarms occurred during the last century.
Sint Eustatius (Stadia) Island
The Island of Sint Eustatius was constructed out of coalescing volcanic centers, the older one in the north and the younger one centered around The Quill to the south. Pyroclastics and ashfall from The Quill joined the two islands.
In the north of the roughly pear-shaped island, the older ‘Northern Centres volcanic complex’ is formed by five adjacent volcanic centers. Here are the oldest volcanics found on Statia – 2 Ma. These are basaltic-andesite and andesite eruptive products. This part of the current island was originally an independent volcanic island. The island is made up of tuffs, solidified pyroclastic block and ash deposits, lava flows and domes. The Little Mountain-Pisga hill center contains an 800 m circular crater with an interior lava dome called Bergje which seems to be the last eruptive part of the original island. This was followed by an extended period of erosion which removed the softer parts of the edifice.
In the south lies Statia’s main landmark, the dominating 601 m high The Quill or Mount Mazinga. The Quill is the youngest volcanic structure on the combined island, a smallish active stratovolcano. Quill derives from Dutch kuil, meaning ‘pit’ – referring to its 320 m deep crater. The Quill shares many characteristics with Mt. Pelée and is a typical composite arc volcano. Lava flows at the Quill are rare, but its pyroclastic deposits resembles that of Mt. Pelée.
The interaction of rhyolitic magma from early Quill eruptions with sea water produced pyroclastic deposits that joined the two islands. A cross section of these deposits can best be seen where the oil storage facility has been constructed on Pisga Hill in the Northern Centers. The ash layers contain powdery carbon and the remains of hermit crabs in seashells (the latter swept up and transported by the surges). This made radiocarbon dating possible.
The Quill volcano
The Quill is a nearly perfect cone, truncated by a 750m-wide crater. The crater is about 320 m deep and has a prominent gap on its western side. This debris avalanche was a hot eruption and the source of a voluminous pyroclastic flow fan towards the west coast. The main settlement of Orangestad is built on this deposit. A few lava dome remnants are exposed in the inner walls of the crater. Otherwise, the cone consists almost entirely of pyroclastic materials, and the coastal cliffs cut into the flank deposits reveal some of the best pyroclastic sections in the Lesser Antilles.
Among the various styles of The Quill’s eruptions were Plinian and phreato-magmatic events. The evidence of the Plinian style activity can be seen in the sea cliffs as impressive white pumice lapilli airfall and ignimbrite deposits. These layers nicely document the eruptive history of a volcano. The five pyroclastic layers of the last erupted sequence are individually mapped. They are dominantly pyroclastic flows and show the evolution of rock chemistry from andesite (58% SiO2) up the stratigraphy to basalt (51% SiO2).
Some deposits show both dark colored basaltic andesite and light-colored andesite flows banded together within single clasts. This would indicate that an eruption was triggered by the intrusion of basalt into a pre-existing reservoir of andesite. Subsequently, the chamber was emptied by eruption of the mixed magma.
The Quill shows the greatest silica range of all Lesser Antillean volcanoes. Quill rocks range from basalts (50.4% SiO2), through basaltic andesite, andesite, dacite, to high silica rhyolite (72.3% SiO2). The dacitic and rhyolitic samples came from the Sugar Loaf/White Wall pumices (pre-Quill deposits). St. Eustatius is unique in the Lesser Antilles in that rhyolite evolved twice in its history, first in the early Sugar Loaf period and then later in The Quill pyroclastic succession. The time required for the basaltic magma reservoir to fractionate and evolve into rhyolite here is estimated to be around 30,000 years.
The oldest carbon date reported for The Quill’s volcanic products is around 22,000 years, although volcanic activity likely extends back to 40-50,000 years. An eruption some 6000 years ago was classed as VEI 4. The last eruption took place about the year 250-400. Six recently drilled water wells in the flanks of the Quill produced heated groundwater. So, the Quill is still an active, if presently dormant volcano.
There is an uptilted wall and ridge of shallow marine limestone that makes a conspicuous white mark on the flank of The Quill. It runs for 1.2 km E-W and is known as the Sugar Loaf and White Wall sequence. This is thought to be part of the limestone cap of the underlying submarine platform. It has clearly been uplifted quite recently, perhaps with the ascent of a crypto-dome intrusion into the edifice. Embedded within the marine limestones are abundant pumiceous pyroclastic deposits. These pumices are dated much older than The Quill volcano but not as old as the products found at the Northern Centres. This means that there was a previously unknown pre-Quill volcanic episode in between the two centers. As far as is now known, the quiet period must still have been quite long with about 1.7 Ma.
Saba and Mount Scenery
Saba is a mostly circular 13 km² island. It measures 4.5 km in diameter and is bordered by cliffs. There are no natural beaches. The island is entirely of volcanic origin and consists largely of the Mt Scenery stratovolcano. The highest peak of Saba’s four volcanic domes rises 887 meters, making it the highest point in all of the Kingdom of the Netherlands. Measured from the sea floor the volcano reaches 1500 m in total.
Visually, Saba island may appear as a simple stratovolcano, but this is deceptive: it has been built up of a large number of Pelean style domes and their aprons of coarse pyroclastic deposits. These form a distinctive shoulder on the island at about 450 to 500 m a.s.l. The Mt. Scenery cone is perched eccentrically on this foundation of domes and pyroclastics. Mt. Scenery itself is of Holocene origin.
Deposits of pyroclastics, ashfalls and lahars, as well as several craters, are witness to Saba’s long history of violent eruptions. The system has numerous hydrothermal springs and sulfur deposits, suggesting that the volcano has been active quite recently. It is thought that its last eruption took place on its SW flank in or around the year 1640, just prior to European settlement. The new settlers may have been attracted to the island because of the presence of inviting grassland instead of the usual thick tropical rainforest. Had they known this was the result of a recent volcanic devastation they might have thought twice…
There are multiple andesitic lava domes on its flanks, some of which produced short thick lava flows that are steeply inclined on the island slopes. Erupted rocks have tended from basalt to basaltic andesites to andesites over time, with very little basalt left exposed. There are basaltic andesite lavas and pyroclastic deposits. A prominent basaltic andesite lava flow, with clearly pronounced levees, covers much of the NE quarter of the island. On its edges the flow made the only portion of land level enough to build a (very short) landing strip on, the Flat Point Peninsula, thus creating Saba International Airport.
From the center towards the SW, a prominent sector collapse scar with an elongated horseshoe shape open to the SW dominates the landscape but is now mostly filled up with products from Mt. Scenery. Chronologically, the scar divides Saba’s history in two parts: The older parts of the island predate the sector collapse and the younger deposits, including Mt Scenery, postdate the scar.
Offshore Saba to the west 1.3 km, a single parasitic submarine Pelean dome (called The Pinnacles) rises from depths of 300 m to only 23 m below sea level. The platform’s top, roughly the size of a football field, represents one of the most famous and spectacular dive sites around the island. About 250 m north of Saba is the uninhabited, rocky island of Green Island, which is overgrown with tropical plants.
From the capital, named The Bottom, 900 steps carved into the steep rock lead down to Ladder Bay. Until well into the second half of the 20th century, the only way to enter or leave the island was the arduous ascent or descent via these steps, and all imported or exported goods had to be transported this way. The 350-hectare Mount Scenery National Park covers large parts of the north and north-west of the island and provides insights into the biodiversity of Saba’s flora and fauna. Here, another path leads through the rainforest up to the summit of Mount Scenery (1064 stone steps according to the sign), and on a northern trail … to old sulfur mines.
Several hot springs, with a maximum temperature of 82°C are located along an SW-NE line. Only the spring opposite Green Island is currently visible at the surface and monitored. The temperature has been a stable 82 °C.
In contrast to the magma chamber of The Quill on Statia, Saba is believed to be underlain by a system of parallel dikes emplaced into a fault zone. With that conclusion, activity on Saba would be similar to the Soufriere Hills volcano whereas The Quill would rather resemble Mount Pelee on Martinique.
Tectonics
The Lesser Antilles Volcanic Arc, together with its subduction zone, forms the eastern boundary of the Caribbean Plate. Current rate of subduction is 2-4 cm/yr, a slow rate that means magma production is generally low. The region is tectonically active with multiple earthquakes over time. However, megathrust earthquakes along the 900 km subduction zone are rare, with only 19 greater than M 6 over the last 50 years. The largest of these was a M 7.2 near Antigua in 1972.
Volcanism due to this collision began some 40 Ma. The volcanic front is divided in two parts, with the eastern one now extinct and its volcanic platforms covered by thick layers of carbonate. The western chain is currently active and has been for at least the last 20 Ma with about 20 active volcanoes. The two arcs combine south of Dominica and have a 2,900 m back arc basin (Grenada Basin) to their west. There have been landslides from the volcanic islands in places.
Despite of the Caribbean plate’s relative movement being to the east, the inner volcanic arc has migrated some 50 km westwards. This has been attributed to either a) the subduction of rigid ridges on the sea floor, or b) flattening of the subducting slab or c) to changes in the relative plate motions between the North American, South American and Caribbean plates.
Volcanic styles on the Leeward Island vary between multiple domes similar to the Souifrere Hills volcano on Monserrat and single stratovolcanoes like Mt Pelee on Martinique. Eruptions tend to be explosive in nature with long repose times between eruptions and well evolved magmas. While there are older basalts and lava flows, these tend to be more the exception rather than the rule.
Landing on Saba’s airport, the 400 m long runway built on a lava flow is the shortest commercial runway in the world. Length is comparable with that of aircraft carriers, though without either the steam catapult or arresting cables. Video courtesy Otts World https://www.ottsworld.com/blogs/saba-island-caribbean/
Conclusions
At first glance, it would appear that volcanic activity and eruptible magmas have migrated toward the south from this region. On the other hand, the highly active Soufriere Hills system on Montserrat is only 150 km SE. Neighboring St Kitts and Nevis periodically show both earthquake swarms and currently active hydrothermal systems. These are a mere 30 km from Sint Eustatius. Saba had an eruption in the last 500 years. Both have active hydrothermal systems with the occasional earthquake swarm. The most recent one at Saba was in 1992. Given the active hydrothermal systems, earthquake swarms on all four of these neighboring islands, writing these two volcanic systems off as soon to be inactive would be a mistake.
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
Additional information
Trip Savvy – Visiting the Lesser Antilles Island, R Curley, Jun 2019
Landscapes and landforms of the Lesser Antilles, CD Allen, Jul 2017
Volcanology of Saba and St Eustatius, Northern Lesser Antilles, Vol 1, Roobol & Smith, 2004
The geology, petrology, and petrogenesis of Saba Island, Lesser Antilles, Defant, et al, Jun 2001
Volcanoes in the Dutch Caribbean, KNMI for the Caribbean Netherlands
KNMI volcano surveillance of Saba and Sint Eustatius (Dutch language)
Spica
Granyia
volcanohotspot.wordpress.com 
