“The 7th June anno 1656. Att evening wee arrived att Ascention and anchored on the NW side of the iland. On our rightt hand was a faire sandy bay and on our left were a multitude of rarreg [ragged], craggy, sharpe pointed hard rocks for many miles along the shoare, and up toward the land, appearing white with the dung of sea foule, of which were innumerable of several kinds. The most desolate, barren [land] (and like a land that God has cursed) that ever my eies beeheld (worse than Kerne Ky, etts. in Cornewall).
I conceave the whole world affoards not such another peece of ground: most part of the collour of burnt bricke, reddish, the substance of stones, somewhat like pumice stones, the rest like cinders and burnt earth. The hills, of which there are many, were meere heapes of the same. It may bee supposed thatt the fire in former ages hath consumed the substance therof, hath made it incapable of producing any vegetalls.”
This delightful piece of historic description by Peter Mundy, British merchant and traveller, represents how about all visitors at the time must have perceived this uninviting little island. Ascension Island, in the South Atlantic, of only 88 km², is really hard to find on the maps – you need to zoom in before you actually know where to. It lies in the middle of the ocean, halfway between Angola in western Africa and the State of Paraiba in eastern Brazil and is part of the British Overseas Territory of St Helena, Ascension and Tristan da Cunha. Its triangular shape is about 12 x 14 km in size and rises, at its highest point (The Peak, or Green Mountain), to 859 m a.s.l..
Off the east coast lies the small and uninhabited nature reserve Boatswain Bird Island, originally surely of some dark rock colour, but now entirely “white-washed” with dung of the countless sea birds nesting there.
Visitors to Ascension Island at all times had two major complaints: One – there was no water. Two – walking on the loose and razor-sharp pumice and scoria that are all over the place was always an effort, and going uphill was like one step forward and two back. One wrong step could mean a fall, bad cuts to the skin or even a broken limb. All moaned about the soles of their shoes getting shredded in no time. – The problem of water has been solved with desalination plants, but, as for the shoes, you’d better bring two or three pairs…
Image below: In this amazingly detailed image from the Ikonos satellite on February 24, 2003, marbled-looking lava flows can be seen dominating the north-west coast, with smaller flows visible on the south-west coast below the island’s Wideawake Airfield, as well as on the south-east coast. North-east of the airstrip, a large cinder cone is visible, its dark brown center fading to tan is a series of pale rings. This feature is called Devil’s Riding School. The rugged island is barren in many places, and has no indigenous human population. Instead the residents of the island are there because of Ascension’s main industry: communications. The island has a long history as a communications hub for telephone and radio communications and as a base for satellite tracking stations, including a NASA station built in the 1960s that no longer operates. The European Space Agency operated a tracking station for its Ariane spacecrafts on the eastern side of the island. (Text shortened)
Ascension I. sits 90 km west of the Mid-Atlantic Ridge (MAR) and 50 km south of the Ascension fracture zone. As with other volcanic islands that are near ocean spreading ridges, the origin of the Ascension stratovolcano is not quite clear yet: It may be a result of eruptions from a diverted shallow mantle plume. Though, more recent data from radiogenic isotopes indicate that the magma was produced by the MAR and then carried westward by plate motion. Any way, volcanism is thought to have commenced here 6-7 million years ago.
From the sea bottom to the top of The Peak the entire edifice rises over ∼3 km, and covers 2000 km² of ocean floor. The subaerial part of the volcano, i.e. Ascension Island itself, accounts for only ∼1% of its total volume.
Large parts of the island are old lava fields. There are 44 volcanic craters and more than 100 youthful parasitic cones and lava domes, many aligned along two main fissures. Faults mapped on the island appear to be related to regional structures that can be traced on the bathymetry of the submarine parts of the volcano.
A BURST OF COLOURS – THE GEOLOGICAL TAPESTRY
The variety of rocks on Ascension I. is quite unusual for an ocean island: Compared to most other volcanic islands that present only the endmembers of mafic (basalt) and silicic (rhyolite) rock types, Ascension proudly displays the entire range in SiO2 content. There’s everything from mafic to the most silicic rocks, including basalt, hawaiite, mugearite, benmoreite, trachyte and rhyolite lavas – which also represent a wide range of eruptive styles.
The silicic rocks form two eruptive centers that may have erupted in the same period. They include the oldest rocks exposed on the surface which are rhyolites with K-Ar ages of ~1 million years.
The central and highest sector of the island is the silicic complex of Green Mountain and Middleton Ridge. Green Mountain is primarily composed of pyroclastic deposits, both scoria and pumice. However, it has a mafic scoria cone high on its flank, the source for much of the mafic ash that covers its SW slope. Middleton Ridge is made up of trachytic flows and domes and a small volume of rhyolite and basalt, with an age of some 829 to 652 ka. Here the deposits of multiple pyroclastic density currents (PDC’s) can be seen, containing volcanic breccia and pumice lapilli.
Towards the Letterbox Peninsula, the eastern silicic complex is made up of numerous trachytic to rhyolitic domes and flows including those of Devil’s Cauldron and White Hill. Silicic pyroclastic deposits are composed of a mixture of pumice, ash, and crystals and represent the more explosive periods in Ascension’s volcanic history.
The northern, southern and western parts consist of a maze of mafic lava flows and scoria cones that have flown and grown across and over one another. The youngest deposits on Ascension I., based on stratigraphy and appearance (such as surface morphology, no soil, no vegetation) are from high volume eruptions mainly around Sisters Peak in the NW. These flows followed the cinder eruptions that once had formed Sisters Peak. Most mafic lava flows (basalt to benmoreite) on Ascension have been erupted from breached scoria cones. The mafic flows typically are a few metres thick, scoriaceous, vesicular and have very blocky surfaces. In the immediate vicinity of vents, some ropey lavas can be found. In addition, ponding of lava in the vent areas was common.
Another feature in the south is Devil’s Riding School, where an intrusion of trachyte has warped up the overlying mafic flows, forming a so-called crypto-dome. Further deformation, ash-filling and then erosion has produced a notable concentric outcrop pattern that gave the feature its name. (Note, this is not a ‘crater’ even though it had been described as such in older papers and articles).
Modern Ascension beaches on the east and south coasts are mostly fine-to-medium grained sand derived from the disintegration of volcanic rock. They consist predominantly of feldspar and pyroxene. Some density sorting has been done by the waves, so there are zones consisting almost entirely of either of the two minerals: e.g. feldspar at Coconut Bay and pyroxene at Crystal Bay.
HOW OLD ARE THE ‘YOUNGEST’ PARTS OF ASCENSION VOLCANO?
Short answer: “Younger than you thought”…
Ascension Island has been first sighted in 1501. Since that time, it has frequently been used as a stopover point for sailing vessels to take on provisions of turtle and goat meat. Even so, no reports of eruptions were ever passed on by the sailors. The only reference to possible volcanic activities concerned residual fumarolic activity and also the obviously quite fresh appearance of some volcanic deposits.
On an inhabited island it is always an essential question weather or not the local volcano might erupt again. To get a foothold, a few years ago Katie Preece et al. began looking into the probable age of the more recent lavas. The lava flows South Sisters, Comfortless Cove and Davidson were targeted to determine the age of the most recent eruptions on Ascension, using Ar^40/Ar^39 technology. While the South Sisters and Davidson flows erupted from the Sisters Peak scoria cone complex, the Comfortless Cove flow issued from a vent near the NW coast of the island. These are ‘a‘ā lava flows that travelled distances of ~1.7–3.0 km from their source. The associated eruptions would have been of mild explosive activity with scoria production.
Preece et al. published the results of this research in their 2018 paper:
the ages found were
510 ± 180 years for South Sisters flow (ca. 1508),
550 ± 120 years for Comfortless Cove flow (ca. 1468), and
1640 ± 370 years for the Davidson flow (ca. 378).
These data provide the first definitive evidence that Ascension Island has erupted within the Holocene (last 10k years), and it is therefore classed an ‘active’ volcanic system.
Geologist M.S. Rosenbaum writes in his 1992 paper: “Ascension is geologically active and the last on-shore eruption probably occurred just 400 to 600 years ago, […]. An eruption just offshore took place in 1838.” While he was certainly dead right with his estimate of the last onshore eruption, I made an effort – but could not find a single second reference to an Ascension offshore event in 1838.
It seems that we have here another case of “the thing will certainly erupt again – the question is just when“. Nobody on Ascension Island really expects their volcano to ever become active again. Currently (or at least in 2018), no specific volcanic risk management plans are in place, despite the island housing important military and communications-related infrastructure. With fresh-looking lava flows but no historical accounts of eruptions, and former research results giving Pleistocene ages, the activity status of the island and, hence, the volcanic risk is presently unclear, to say the least.
AN ISLAND AS ODD AS ANY
The known history of human settlement on Ascension Island is just over 200 years old, yet, in so short a time, it has become one of the oddest places on earth.
😲 Odd is that, of all the lush greenery that covers the center of the island, almost none are native plants. This was due to Charles Darwin. He visited Ascension Island in 1836 and was so excited about it that, along with British biologist and botanist J. D. Hooker, he began developing a plan to revive this barren island. A conundrum of different plants from all over the world were sent in, were planted and took root. Some of them didn’t make it, others thrived and created the Garden of Eden of Darwin’s dreams. By the end of 1870, a rich flora of eucalyptus, pines, bamboo and banana trees had developed around Green Mountain. Within a short time, a fully functioning ecosystem was created. Today researchers call this experiment by Darwin and Hooker the first and successful terraforming experiment; others call it an environmental catastrophe (or even crime) that will take hundreds of years to get corrected again.
😲 Ascension was not inhabited before the British settled in 1815. Oddly, in 1726, visiting sailors discovered a tent and a diary. They turned out to belong to L. Hasenbosch, a Dutch man who had been cast ashore as punishment for homosexuality. The notes in the diary told the terrible story of Hasenbosch’s increasingly desperate search for water and supplies. In the end he had to fall back on drinking the blood of turtles and seabirds and, eventually, his own urine! 🤢😲 Offshore there is this small island called Boatswain Bird Island, a bird sanctuary. Unavoidably, at one time, rats were introduced to the island who did much damage to the nests. One clever mind had then the idea to release some cats to fight the rats. However, the cats quickly discovered that birds taste much better than rats – and again decimated the birds. Since 1995, the emphasis is on hunting down the cats – eventually the population of birds is recovering and is meticulously monitored. The program to eradicate cats on main Ascension I. and monitor seabird populations cost taxpayers nearly $1.3 million – and “caused public consternation” as 94 of the estimated 661 cats killed were residents’ beloved pets… — I didn’t find any statement about the well-being of rats at present…
😲 Another oddity is that, officially, nobody is from Ascension Island. It has no indigenous or permanent population. The inhabitants comprise the employees and families of the organisations working there. The UK government denies the right of abode, turning Ascension’s 800 or so British citizens into temporary visitors – some of them having lived and worked on the island for decades, or even generations! To enter, you must get the written permission of the Queen’s representative. Even if, by chance, you are born there, when you grow up and don’t find a job on the island – you have to leave!
😲 And the last oddity for now: While huge sea turtles paddle a few thousand kilometres from Brazil to Ascension Island just to lay their eggs on land here – the local land crab population does a life-threatening journey over razor-sharp lava fields every year, from the highest peak down to the beach just to spawn their brood into the sea… what the heck?
Notwithstanding all oddities, this little island with limited infrastructure, few touristic amenities, and alien plants, stands in highest regards of its visitors – all reports tell of friendly and helpful local people, as well as beautiful landscape and unspoiled nature. And all are loath to leave for home again, having little chance of ever coming back.
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
SOURCES & FURTHER READING
– GVP, Ascension
– Ascension Island Volcanology (Sci Blog)
– Geology of Ascension Island (1996, paywalled)
– The geology of Ascension Island (1992, paywalled)
– Bridging the gap: 40Ar/39Ar dating of volcanic eruptions […] (2018)
– Ascension Island (Official website)
– 40Ar/39Ar Geochronology of Subaerial Ascension Island […] (2013)
– Ascension Island: Pristine Seas Heads to the Mountains (2017)
– Origin and evolution of silicic magmas […] Ascension I. (2016)
– The Geology of Ascension Island (Sci Blog)
– Life on Mars (Blog !)
– Ascension: The island where nothing makes sense (BBC mag)
– Excursion to Green Mountain on A. I. (2015)