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Geology of St Helena

Upon this rock…

It is impossible to approach and see this singular island for the first time, without wondering how the deuce it got there{c}

St Helena was created not just by one but by two volcanoes


St Helena is an Oceanic Island because it does not sit on a continental shelf. Indeed, strictly it is a an isolated intra-plate island. Like St Helena, most Oceanic Island are volcanic in origin. The few oceanic islands that are not volcanic are tectonic in origin and arise where plate movements have lifted up the ocean floor above the surface.


In geological teRMS St Helena is very young. Its origins lie in the Mid-Atlantic ridge and the outpouring of molten rock from between the diverging African and Latin American tectonic plates, as first postulated in 1836 by Charles Darwin.

The island is an isolated, broadly conical volcanic structure, rising more than 4,000 metres above the ocean floor with a base perimeter of 255Km, the island itself being just the top of the volcano with only a small amount (around 5%) of it is exposed above the sea. The island may be small but its supporting structure is immense: the base on the sea floor measures some 130km in diameter and the volume of the cone is estimated to be twenty times that of the largest European volcano, Mount Etna.

St Helena actually results from two separate volcanic eruptions. That in the north-eastern part of the island is the oldest. This erupted about 15 million years ago near what is now Flagstaff. A second series of eruptions, following a more complicated pattern, occurred to the south-west, around Sandy Bay, between 10 and 7 million years ago, which partially overlaid the old volcano.

Despite its volcanic origin, volcanic activity is no longer a threat to St Helena. Tectonic movement since the last eruption has carried the island some way from the active part of the ridge. St Helena is on the African tectonic plate (east of the Mid-Atlantic ridge), so our distance from Africa remains constant but we are moving away from South America at a rate of approximately 2cm per annum.

Resultant geology

The lavas of the island are of mantle origin; they tend to contain no quartz, unusually high concentrations of sodium and potassium, and have characteristic patterns of radioactive and trace element abundance. The chemistry of these lavas suggests that they are the result of selective partial melting of the most easily mobilised components of the original mantle.

St Helena has a great range of structural complexities, such that geologists are frequently at variance in determining the exact cause of a particular formation.

The island’s geology provides little in the way of mineral resources. There are fairly widespread occurrences of manganese and phosphate deposits, but there is insufficient tonnage for commercial extraction.

The largest area of level ground on the island is Prosperous Bay Plain, in the eastern arid area (this being the site chosen for St Helena Airport).

Despite St Helena being a small (by global standards) island, it is itself surrounded by many even smaller ones

Erosion: how we got the current island profile

The diagram below shows how the original volcanic spill eroded to give our island its current profile:

16,000 years ago

Sea floor profile
Sea floor profile

16,000 years ago the earth was in the grip of a major ice age. With so much of the earth’s water locked up in ice, sea levels were much lower - around 135m lower than today. Naturally this affected the shape of St Helena. If humans had been around 16,000 years ago and had discovered St Helena they would have found an island of approximately the same shape, but around twice the size. And Diana’s Peak would have been around 995m above sea-level. Most of our known islands would have been hills and, interestingly, there would also have been an additional island. Lying 27.5km due west of James Bay and currently submerged to a depth of around 70m, it would have been an island some 65m high and would probably have supported plant life and could easily have been a seabird colony, as Egg Island is today.

Due to erosion at the time, a distinct under-sea shelf can still be found at a depth of around 135m, as the diagram (right) shows.


There are many other volcanic prominences around St Helena, but none breaks the surface. Known as ‘Seamounts’ these areas seem to be good places for offshore fishing. A list of the nearest is below with a map beneath:

ObjectLocationDistance (Km)Max. Height(m)
St Helena15°55’24.3”S; 5°43’3.5”W{2}0818m
‍Tolstoy Seamount‍15°24’S 6°28’W65n/a
‍Bonaparte Seamount‍{3}15°36’S 7°6’W110-105m
‍Benjamine Seamount‍16°12’S 8°31’W175n/a
‍Kutzov Seamount‍15°8’S 8°21’W180n/a
‍Josephine Seamount‍16°23’S 9°0’W220n/a
‍Charlie Boar Seamount‍(see below)15°33’S 5°18’W57-1,205m

Our Newest Seamounts

Charlie Boar Seamount location

In 2018 a marine survey vessel The James Clark Ross discovered a previously unknown seamount. A competition in January/February 2019 assigned it a name: the Charlie Boar Seamount. Charlie Boar was the nickname of Mr Charles Henry, who died in 2018 - the year the seamount was discovered. He had been a seaman all of his life and was an acknowledged an expert on or local waters. He had attempted to join the 100 men in 1949 but was refused because his role as a fisherman made him too valuable to the island. He was described in the citation as our very own famous fisherman who outlived the rest of his generation. The name was officially accepted by the Sub-Committee on Undersea Feature Names (SCUFN) on 11th November 2019. Here are some images from the research data{d}:

New seamount, 2022

Then in December 2022 visiting survey ship Royal Research Ship (RRS) Discovery completed its survey of St Helena’s waters for the UK Government’s Blue Belt Programme, during which another new Seamount was discovered. Located on 25th November 2022 at 13°46’57.865”S 007°30’35.339”W and rising to 620m below the ocean surface and with a total area of about half the size of St Helena, in September 2023 it was proposed that it be named the ‘Captain Rodney Young Seamount’ after the late Rodney Young, Saint-born former Captain of RMS St Helena (1990-2018).


Below: AtlantisHas no Golden Sandy Beaches


In the past, the existence of St Helena, isolated as it is towards the middle of the Atlantic, was explained thus:

When the lost continent of Atlantis, which joined Africa to South America, sunk into the sea, St Helena, Ascension Island and Tristan da Cunha, being the highest peaks on the former continent, were the only places that remained above sea level.

Quaint though this story is, nowadays we know better…

St Helena has no Golden Sandy Beaches

Golden Sand
Golden Sand

Sandy Beach

Every tourist brochure says this but actually it is incorrect. St Helena does have golden sand in significant quantities, complete with sea shells. Sadly, none of it is at the seashore. It can be found on high ground, usually around 500m above sea level…

These deposits were first discovered in the 18th Century when looking for sources of lime to make lime mortar for building and repairing fortifications. As a result much of what was there has been ‘quarried out’, but a lot remains and you can see an example on the hills above the Sandy Bay (black sand) beach - just look up into the hills for a golden patch (second photo, below).

Geologists theorise that the elevation of these beaches in not caused by the sea level falling but by the land rising. These beaches were once, they believe, at sea level, but the land was forced upwards by volcanic activity, leaving them - literally - high and dry.

If you look at the picture below (of Sandy Bay from the Gates of Chaos) - you can clearly see some patches of golden sand on the valley slopes (highlighted):


Below: EarthquakesManganeseRock ClimbingSoilMarine Reservoir Corrections: St HelenaPuzzling diagrams


Recorded Earthquakes

7th June 1756{e}

21st May 1763{e}


26th January 1782{e}

21st September 1817{e}

12th August 1818{e}

15th July 1864{e}

2nd August 1917{g}

26th June 1920{g}

12th December 1923{g}

25th August 1925{g}

30th March 1930{g}

September 1975{h}

1st December 1984{g}

3rd May 1987{g}

10th June 2020

28th July 2022

Earthquakes are extremely rare on St Helena, but are not unknown. None has ever resulted in loss of life or even serious injury. The most recent that was widely noticed by the general population was in September 1975:

Various people reported an earth tremor on the island last Wednesday evening in Longwood, Sandy Bay and Jamestown. According to one report, at about 11:30pm the person concerned felt as if a giant hand took hold of his house and shook it from end to end. This tremor was said to last approximately 5 seconds.{h}

The Records list various earthquakes that were felt here (table, right). Apparently the one in September 1817 lasted 16-18 seconds, during which time it shook houses and rang church bells. Writing in 1844, Lucia Elizabeth Abell (née Balcombe a.k.a. ‘Betsy’, a family close to Napoleon), describes this earthquake in ‘Recollections of the Emperor Napoleon on the Island of St Helena’ thus:

The day had been one of the most sultry ever experienced within the recollection of the oldest inhabitant of St Helena. Suddenly we heard a lumbering heavy noise, as if loaded wagons were rumbling over the ground immediately under us. Those seated near the billiard-room sprang up aghast, thinking the house was falling about their ears. Dr. O’Meara and Major Blakeney, who was appointed captain of the guard at Longwood, rushed immediately from their rooms, expecting to find the ladies half dead with fear. All the household, some of whom were in bed, ran out in the greatest alarm; some were gazing up at the sky, others looking stupefied with wonder and amazement as to what had caused such a commotion. Little Tristram Montholon, who had some time previously retired to rest, came screaming to his mother, declaring that somebody had been trying to throw him out of bed. The cause of our terror proved to be an earthquake, the only one remembered to have occurred at St Helena for nearly a century.

Napoleon had retired to bed, and it was not till the next morning that we saw him. He asked us if we had been frightened by the ‘tremblement de terre’ on the previous evening, observing that I looked pale and quiet. He mentioned to General Bertrand that he at first thought the ‘Conqueror’, a 74 lying in the harbour, had blown up, and that the great powder magazine had exploded, but on feeling the third shock he perceived it to be an earthquake. It lasted from 16 to 18 seconds.

Many people fancied the rumbling noise they at first heard to be thunder, but when it was remembered that such a phenomenon as thunder was never heard, nor had lightning ever been seen since the discovery of St Helena, that idea was abandoned. Thunder and lightning have never been known to disturb the harmony of the climate. To account for this, it is said that the electric fluid is attracted by a high and conical-shaped mountain, called Diana’s Peak, and conducted by it into the sea{4}.

I was too much alarmed after the occurrence of the earthquake to go to bed for many nights.

You can read a paper on St Helena’s earthquake history{1}. You can also read a report of a minor earthquake that was recorded near St Helena on 10th June 2020{i}.

A very small earthquake (4.9 on the Richter Scale) occured 143Km NNE of Jamestown on 28th July 2022 and was felt by a few people on the island. No damage occurred.

However, an item in the London Chronicle of 21st April 1770, stating: Advice has been received of a dreadful earthquake at St Helena, which had entirely sunk the same in the sea was a hoax - common at the time.

By the way, if you’re here and think you are encountering an earthquake, it’s very possible that actually it’s just somebody driving past with their sub-woofer volume set to ‘stun’

As can be seen from the table and diagram below, earthquakes come in groups, as would be expected. Earthquakes are more common on Ascension Island, it being closer to the Mid-Atlantic ridge.


Deposits located, 1951
Deposits located, 1951
Contrast emphasising valleys
Contrast emphasising valleys

As discussed in the article below, St Helena does have significant deposits of Manganese Ore.

Sadly, a survey conducted in 1951 concluded:

As an economic proposition, the deposits must be rejected, not on grounds of quality or difficulty of separation from the matrix, but because of insufficient tonnage…the concentrations could not yield sufficient ore to justify underground mining, even in the form of adits.{j}

Rock Climbing

Surrounded by sea cliffs you might think rock climbing would be a major sport on St Helena. Sadly, however, most of our cliffs are not stable enough for climbing. In 2018 Saint Helena Island Info was contacted by some climbers who, after examining our page Geology of St Helena, believed some of the rocks might be climbable, particularly those in Sandy Bay. Phonolite was mentioned (see ‘St Helena: A Physical, Historical and Topographical Description of the Island{5}’).


Soil TypeProportion
Loamy sand4.8%
Sandy clay loam1.6%
Sandy loam4.8%
Silt clay loam6.3%
Silty clay12.7%

According to the 2018 study ‘Mapping St Helena’s Biodiversity and Natural Environment’ around 60% of the island’s soil is pure clay. Soil containing clay amounts to a total of 83%, whereas loam and loam mixes is only just under 30%. The detailed table is as here:

We think this provides an additional explanation as to why agriculture has never been successful on St Helena.

The same report concludes that only 0.4% of our land area is in use for arable farming and only 8.8% is pasture (though the 1.5% that is ‘rural gardens’ clearly contributes some). Other interesting facts revealed are that 1.3% of our land is urban; Flax covers only 2.48% of our total land - it really grows only in the high areas; and Surfaced roads account for 0.51% (0.63% if you include unsurfaced tracks).

Marine Reservoir Corrections: St Helena

Marine Reservoir Corrections Figure 2

This document seems to be a scientific study of rocks on St Helena, and is (probably) something to do with carbon dating. The document Abstract reads:

We present the first marine reservoir age and ΔR determination for the island of St Helena using marine mollusc radiocarbon dates obtained from an historical context of known age. This represents the first marine reservoir age and ΔR determination in the southern Atlantic Ocean within thousands of Km of the island.

The depletion of 14C in the shells indicates a rather larger reservoir age for that portion of the surface Atlantic than models indicate. The implication is that upwelling old water along the Namibia coast is transported for a considerable distance, although it is likely to be variable on a decadal timescale.

An artilleryman’s button, together with other artefacts found in a midden, demonstrate association of the mollusc shells with a narrow historic period of 1815-1835.

If you know what that means and would like to learn more you can download the document.

Puzzling diagrams

The following diagrams relate to the geology of St Helena (probably). Maybe they mean more to you than they do to us…

The answer may be here: www.geokem.com/‌OIB-volcanic-atlantic.html, and if you can interpret it for us (in plain English!) please contact us.

Read More

Below: Article: Measuring Global Magnetism on St HelenaArticle: St Helena’s Geology Could Solve Scientific MysteryArticle: It’s not Gold, but it has a value

Article: Measuring Global Magnetism on St Helena

By David Pryce, published in The St Helena Connection #26, June 2019{1}

Sampling on Ladder Hill
Sampling on Ladder Hill

St Helena geology has recently become of great interest, specifically in relation to its palaeomagnetism - the study of the history of the earth’s magnetic field. Approximately every 600,000 years the magnetic field flips, so that south is at the top and north at the bottom as we usually look at the earth.

The ‘inclination’ of the magnetic field is a measure of the angle of an imaginary line drawn from a point through the earth to the magnetic north or south pole. There is one large anomaly present, encompassing southern Africa, St Helena and Tristan. The inclination to the north pole on St Helena should be about 22° but the current value is 60°. This anomaly has been growing in intensity over the last 3-400 years and is the subject of much scientific debate. As the last recorded reversal was 700,000 years ago, are we witnessing the start of a new reversal?

Sampling on Mundens
Sampling on Mundens

St Helena is particularly interesting as it is the only place within the area of the anomaly where long series of lavas, which record the orientation of the magnetic field as they cool, are exposed and relatively easy to sample. In February 2018 two researchers from Liverpool University, Andy Biggin and Yael Engbers visited for two weeks to see if the anomaly had been present in the past and if they could find any reversals recorded in the rocks. The author acted as their field assistant during the investigation.

A series of lava flows were sampled along Ladder Hill Road and in Bank’s Valley and Prosperous Bay Plain. Sampling involved taking 25mm cores 25-100mm in length with four to nine samples per flow. The orientation of the cores was carefully measured using a sun compass so that the orientation of the magnetic field, at the time the flow cooled, could be determined later in the lab. The work required carrying two petrol powered diamond core drills, other sampling equipment and lots of cooling water to each site. It was hard work but our efforts were rewarded by finding a reversal either side of a lava bed on Ladder Hill Road. The end of a second reversal was also found in the Prosperous Bay Plain samples. This prompted three researchers, from Mountain Universität Leoben in Austria, to visit in April to conduct further investigations. Dr. Elisabeth Schnepp led the team with husband Stefan Sperling and a researcher Patrick Arneitz; I was the field guide for this group.

The work was mainly centred on Mundens Hill, where we sampled from the battery to the hill top (about 19 lava flows) and re-sampled and extended the Prosperous Bay Plain sequence (increased from five to about 12 flows). Initial indications are that the work has been a success but much further analysis will be required in the lab to confirm results and it will be several years before the results are fully published. From the Mundens Hill work we will also hopefully learn the rough frequency that eruptions took place on the island.

Article: St Helena’s Geology Could Solve Scientific Mystery

By Emma Weaver, SAMS, published in The Sentinel 4th January 2018{k}

St Helena’s Geology Could Solve Scientific Mystery

Right now, we’re trying to promote our unique, remote island to tourists. And if we said St Helena has a unique magnetism, we wouldn’t be lying.

Andy Biggin and Yael Engbers of the University of Liverpool will arrive on St Helena on Saturday, Jan. 6th. The Earth Scientist and his PhD student will be collecting samples to measure the magnetic properties of the island’s rocks.

Andy and Yael will be the first to ever look into the magnetic properties - or palaeomagnetism - on St Helena. It is believed that St Helena is located in a unique chink in the Earth’s magnetic field, which Andy called a controversial topic in his field of research. Therefore, the findings could be Earth-shattering - or at least, Earth-Science shattering.

The island has never been studied for palaeomagnetism before, which makes it a very exciting prospect, Andy said. When I heard about the new airport being built, I tried my hardest to get funding for a project and finally succeeded last year.

And PhD student Yael, too, was eager to venture to the island. She hopes to include their findings from St Helena in scientific journal publications as well as her PhD thesis.

And the pair had some note-worthy help preparing for the journey.

Dr. Ian Baker (friend of many people on the island and very regular research visitor in the past) has worked alongside of us to prepare this fieldwork, Yael said. He will also be helping in the period after the fieldwork. [He] talked me through his PhD thesis that he wrote about the island in 67, and everything written since then has been read and discussed with the authors.

Q&A with Earth Scientist Andy Biggin

What’s your background in this field of research?

I have a BSc in geophysics from Uni. Liverpool, a PhD in palaeomagnetism from Kingston Uni, and 17 years of postdoctoral research experience in 4 countries. I returned to Liverpool in 2009 and have been professor of palaeomagnetism here since 2016.

Could you explain what palaeomagnetic sampling is, and how its conducted?

Basically collecting specimens of rock to measure the magnetic properties of in our lab. The samples need to be oriented in the field so we can rotate the direction of magnetisation we measure in the lab back into geological coordinates. For many igneous rocks like those on the island, we would expect the direction of magnetisation to be parallel to the direction of Earth’s magnetic field at the time and place the rock formed, approx. 10 million years ago for St Helena. The most efficient way to orient rock specimens in the field is to drill small (1 inch wide) core samples using a portable petrol powered drill.

What locations will you aim to sample, and why, while on-island?

Saint Helena formed as a series of lava flows erupted between 8 and 11 million years ago and we are aiming to get as much coverage as possible from this time range. However we also need to go to where rocks are well-exposed and unweathered. Precise sampling locations will depend on permissions, accessibility, the condition of rocks in the field etc but potentially include: Sandy Bay, Sugar Loaf, Ladder Hill, Lemon’s Valley and Fisher’s Valley.

How did you choose St Helena as a destination?

Because of its unique location close to a patch of anomalous (especially weak) magnetic field whose source is a controversial topic in my research field. Also, the island has never been studied for palaeomagnetism before, which makes it a very exciting prospect. When I heard about the new airport being built, I tried my hardest to get funding for a project and finally succeeded last year.

How has the booking process been?

Booking the flights was fairly straightforward (though rather expensive). People at the tourist office were very helpful in booking us accommodation and a vehicle and the Office of the Environment and Natural Resources Directorate are trying hard to get us a field assistant guide.

Anything else you’d like to mention?

The Earth’s magnetic field acts as a shield against solar wind radiation but it has a chink located in the south Atlantic. This project will test a hypothesis that this patch of weakness has appeared repeatedly in geological time because of unusual conditions in the planet’s core and mantle 2900km below the surface here. This project could therefore help to solve a major research question for the Earth Sciences and any help the inhabitants could give us in allowing us access to land and permission to take samples would be very gratefully received.

Article: It’s not Gold, but it has a value

Published in The Independent 25th April 2014{1}

St Helena has always been said to lack natural resources but after the airport development started and large scale excavation is in progress, some small finds of minerals have been discovered, or rather re-discovered. We have known, not least from discoloured drinking water, that there are sources of iron and manganese on the Island. Iron is relatively cheap and would not draw much interest but the presence of manganese would, at least in theory, be worthwhile taking a closer look. It attracts a reasonable value. We already knew about the presence of manganese oxide but the quantities on the Island and on the sea bed around us is still unknown. Already in 1869, a Captain J.R. Oliver wrote that on the north eastern side of the Island:

Beds of claystone are highly interesting because in them, and nowhere else, are found a vast number of veins of black oxide of manganese, varying from a quarter of an inch to nearly a foot in thickness. These veins seldom proceed far without swelling out into nodules, and it frequently happens that a number of them are interlaced in such a manner as to form considerable masses - never however quite free from the claystone, portions of which may always be noticed mixed up with the manganese and sometimes enclosed in it. At the surface the ore often stands out in a remarkable manner from the claystone having been washed away from around it. It has just the shape that we might suppose would be assumed by a quantity of melted lead forced into a mass of wet clay.

Were this manganese to be found in England it would be very valuable. At present, owing to the high price of labour and freight and the difficulties of land transport, it would not pay to work it. The claystone becomes hard at a short distance from the surface and the veins are very elusive, changing suddenly from large masses into mere streaks, so that a great deal of the rock has to be blasted away to obtain the ore in any quantity. Besides this, four-fifths of the manganese is of inferior quality and would never pay to export. These veins may be traced at the outcrop of the claystone beds at intervals the whole way from Flagstaff to Stonetop, and the actual quantity buried beneath the surface is probably enormous, since they doubtless underlie the whole district.

A rock from Horse Point believed to contain manganese oxide
A rock from Horse Point believed to contain manganese oxide
(Not scientifically verified)

The locally-born engineer John Melliss wrote in his ‘St Helena: A Physical, Historical and Topographical Description of the Island{5}’ that:

The Island cannot be said to be rich in minerals, nothing having yet been discovered of much commercial value. Pyrolusite, or black oxide of manganese, has been exported to Europe and obtained a fair market price, but it is an ore of a hard description, and difficult to separate from the clay-beds in which it occurs. This, combined with the heavy expense of transport, prevents it from being worked.

This was 140 years ago and the mining methods on the Island would not have been far more advanced than shovel and wheelbarrow but John Melliss actually says{6} that manganese oxide, which needs to be refined, actually has been exported in the past. This, together with Captain Oliver’s statement that the actual quantity (of manganese) buried beneath the surface is probably enormous may make manganese mining a possibility.

It is also likely that the ocean floor around St Helena and Ascension Island harbours large quantities of nodules containing minerals, among them manganese.

As late as 2009 UK applied to extend its rights to the continental shelf around its islands around the world. This has been said to protect the possible assets on the seabed. Only in recent years, to St Helena Independent’s knowledge (not necessarily known by SHG), two prospectors, one from Britain and one from Canada have shown interest in ocean floor mining around St Helena and Ascension Island.

Today’s value of manganese is about £1,400 per tonne. It is mainly used together with iron to make stainless steel.

Obviously, mining on the Island or on the ocean floor, if feasible and economically viable, are highly controversial issues, which will lead to a huge debate here and overseas. This is not dealt with in this article.

St Helena has also been mining for other valuable stones and minerals on a less intrusive scale.


{a} Mapping St Helena’s Biodiversity and Natural Environment{b} St Helena Travel (group){c} Lieutenant James Prior, onboard the frigate Nisus, 31st January 1812{d} Marine Science{e} the Records{f} From ‘St Helena: A Physical, Historical and Topographical Description of the Island{5}{g} British Geological Survey World Seismicity Database{h} St Helena News, July 1975{1}{i} Earthquakes Today{j} Hirst, T. ‘Observations on the Geology and Mineral Resources of St Helena’. Colonial Geology and Mineral Resources: The Quarterly Bulletin of the Colonial Geological Surveys 2, no. 2, 1951{1}{k} Copyright © South Atlantic Media Services Ltd. (SAMS), used with permission.


{1} @@RepDis@@{2} Location of Jamestown according to latest GPS data.{3} Discovered by Captain Tom Whatley in 1965.{4} Lucia is wrong about the occurrence of thunder and lightning on St Helena, and her explanation of its absence is quaint but probably current at the time.{5} …including the Geology, Fauna, Flora and Meteorology, by John Melliss, published in 1875.{6} In ‘St Helena: A Physical, Historical and Topographical Description of the Island{5}’.


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