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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
Lieutenant James Prior, on the frigate Nisus, 31st January 1812

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

This page is in indexes: Island History Saint Helena Island Info Geology of St HelenaIsland History, Island Nature Saint Helena Island Info Geology of St HelenaIsland Nature, Island Detail Saint Helena Island Info Geology of St HelenaIsland Detail

Saint Helena Island Info Geology of St Helena
{1}

Below: The MythFormationResultant geologyErosion: how we got the current island profileSeamountsEarthquakesGeology-relatedRead More

The Myth 

Golden Sand Saint Helena Island Info Geology of St Helena
Golden Sand

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…

By the way, it’s also a myth that St Helena has no golden sandy beaches!

Formation 

St Helena is classed as an isolated intraplate island. 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. It 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.

The island is actually the result of 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 east and it is now some way from the active part of the ridge, on the African side.

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).

Hills & Valleys Map Saint Helena Island Info Geology of St Helena
Hills & Valleys Map

Land Steepness Saint Helena Island Info Geology of St Helena
Land Steepness{2}{a}

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:

Island cross-section Saint Helena Island Info Geology of St Helena

16,000 years ago

Sea floor profile Saint Helena Island Info Geology of St Helena
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.

Seamounts 

Seamounts map 2016 Saint Helena Island Info Geology of St Helena{7}

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:

Object

Location

Distance (Km)

Max. Height(m)

St Helena

15°55’24.3”S; 5°43’3.5”W{3}

0

818m

Tolstoy Seamount

15°24’S 6°28’W

65

n/a

Bonaparte Seamount{4}

15°36’S 7°06’W

110

-105m

Benjamine Seamount

16°12’S 8°31’W

175

n/a

Kutzov Seamount

15°08’S 8°21’W

180

n/a

Josephine Seamount

16°23’S 9°00’W

220

n/a

Earthquakes 

Recorded Earthquakes

7th June 1756{b}

21st May 1763{b}

???? 1780{c}

26th January 1782{b}

21st September 1817{b}

12th August 1818{b}

15th July 1864{b}

2nd August 1917{d}

26th June 1920{d}

12th December 1923{d}

25th August 1925{d}

30th March 1930{d}

? September 1975{e}

1st December 1984{d}

3rd May 1987{d}

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.

Earthquakes are extremely rare on St Helena, but are not unknown. The most recent that was 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.{e}

The Records list various earthquakes that were felt here (table, right). Apparently the one in 1817 lasted 16-18 seconds, during which time it shook houses and rang church bells (story below). None has ever resulted in loss of life or even serious injury.

Writing in 1844, Lucia Elizabeth Abell (neé Balcombe A.K.A. ‘Betsy’, a family close to Napoleon), describes the 1817 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 waggons 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 conicalshaped mountain, called Diana’s Peak, and conducted by it into the sea{5}.

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

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.

Earthquake Years Saint Helena Island Info Geology of St Helena
Earthquake Years

You can read a paper (731.1Kb) on St Helena’s earthquake history{2}.

Coloured soils in Fisher’s Valley Saint Helena Island Info Geology of St Helena
Coloured soils in Fisher’s Valley

Geology-related 

Below: Rock ClimbingSoilMarine Reservoir Corrections: St HelenaPuzzling diagrams

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 Geology of St Helena page believed some of the rocks might be climbable, perticularly those in Sandy Bay. Phonolite was mentioned (see St Helena: A Physical, Historical and Topographical Description of the Island, including the Geology, Fauna, Flora and Meteorology, by John Melliss, published in 1875). At the time of writing we are waiting to see if they will come here and give it a try.

Soil

Soil Type

Proportion

Loam

7.9%

Clay

58.7%

Clay/Loam

3.2%

Loamy sand

4.8%

Sandy clay loam

1.6%

Sandy loam

4.8%

Silt clay loam

6.3%

Silty clay

12.7%

Contains:

Loam

Clay

Sand

Silt

 

28.6%

82.5%

11.2%

19.0%

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 Saint Helena Island Info Geology of St Helena

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 mollusk 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 Namibian 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 artifacts found in a midden, demonstrate association of the mollusk shells with a narrow historic period of AD 1815-1835.

If you know what that means and would like to read more you can download the document (3.4Mb).

Puzzling diagrams

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

Puzzling geology diagram 1 Saint Helena Island Info Geology of St Helena
Puzzling geology diagram 1

Puzzling geology diagram 2 Saint Helena Island Info Geology of St Helena
Puzzling geology diagram 2

 

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: St Helena’s Geology Could Solve ‘Controversial’ MysteryArticle: It’s not Gold, but it has a value

More stories on our page Read articles about St Helena.
For a chronological summary of our island’s history please see our A Brief History page.

Article: St Helena’s Geology Could Solve ‘Controversial’ Mystery

Published on the sixmonthsasaint.com 2nd January 2018{2}

Sandy Bay one of St Helena’s many dramatic scenes and one of the researchers’ planned sampling sites. Saint Helena Island Info Geology of St Helena
Sandy Bay, one of St Helena’s many dramatic scenes and one of the researchers’ planned sampling sites.{f}

Right now, St Helena is trying to promote its unique, remote island to tourists. And to say St Helena has a unique magnetism, wouldn’t be a lie.

Andy Biggin and Yael Engbers of the University of Liverpool will arrive on St Helena 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 paleomagnetism - 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 paleomagnetism 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 1967, and everything written since then has been read and discussed with the authors.

Q: Could you explain what paleomagnetic sampling is, and how it is 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.

Read the full article. More about Paleomagnetism on the Wikipedia. The team left St Helena on 20th January 2018.

Yael Engbers’ Report

One of the team, Yael Engbers, published the following on www.geomagnetism.org, 15th March 2018, under the title ‘The excluded island of Saint Helena’{2}

geomangnetism.org 20180315 1 Saint Helena Island Info Geology of St Helena
geomangnetism.org 20180315 2 Saint Helena Island Info Geology of St Helena

This January, Andy and I went to Saint Helena to collect rocks for my PhD. Just organising the trip there was already somewhat of an adventure. The airport just opened this October, and there had been some flights that had to return back to Africa because the wind was to strong to land on the cliff of the island called the Barn. Bringing our equipment on the plane is always a risk, but this time it was definitely not an option. The little plane that were to take us there would in no circumstance take our heavy drills and other tools. So we had to arrange a shipment and had to hurry with that too, since it would take over a month for the parcel to arrive on Saint Helena.

When the day finally came we flew to Johannesburg first. The plane to Saint Helena only goes once every week, so to avoid a delay of 7 days we decided to take 24 hours between the arrival in Joburg and the departure to Saint Helena. We had a nice hotel with pool, where Andy was working and I was enjoying the Sun (which we wouldn’t see much of in Saint Helena so I was happy I at least had some vitamin take-in). After a visit to the Apartheid museum and a nice meal in town, we were ready to get on the scary little plane the next day. After a stop in Windhoek we arrived on the magical island of Saint Helena. Literally in the middle of nowhere. Andy almost had a hart attack before we landed because he was sure we would fly into the cliff instead of land on it, but luckily everything went very smooth and soon we were in our little bungalow on top of the Ladder Hill. Ladder Hill Road is a road that takes you all the way down to town, and where if you are there at the wrong time and you want to go down, you will be stuck for at least 30 minutes, since you have to give way to the traffic driving uphill.

The first few days we were mainly exploring the island and trying to get permission to drill wherever we wanted. They had warned us that drilling on Ladder Hill would probably not be permitted because of the rockfall and avalanche risk. Luckily the Rock Guards not only gave us permission within seconds of meeting us, but they also decided to stay with us the entire trip, which gave us a lot more possibilities of drilling far away from where the car could get, since they could carry A LOT of weight. Besides these lovely islanders (one of which had never been off of Saint Helena), we also had help from our field assistant Dave, who called himself the Bugman (everyone on Saint Helena has a nickname which even gets passed over through generations) because of his insane knowledge of every insect. A true walking encyclopedia. Dave was the person who helped us find our way on the island and even helped us with his Geology knowledge from a degree from years ago (which not only says something about Dave’s memory, but also about mine and Andy’s knowledge in geology).

In 2 weeks you can do a lot more sampling than I would have expected before hand. Things ran smoothly and although we had some problems with the drills, we managed to spend our time very efficiently. That also lead to us being absolutely exhausted after the first week and a half. In the start we were so ambitious, drilling from 8.30 or 9 to 19.00 everyday, that we manage to drill 26 sites at Ladder Hill in 4 days, and 10 sites in Bank’s Valley (an hour walk with all our equipment each day) in another 2 days. We continued drilling the beautiful dykes at Sandy Bay in the south of the island, where we did a beautiful hike to a non-existing set of flows. After this first week I saw the number of samplebags on the cupboard increasing and increasing, and slowly my fieldbook started to get full (and extremely valuable). The fear for having to measure these samples for the rest of my life started to increase each day, but we did not want to waste ANY time. So we kept going and ended up with 300 sample cores and 62 hand samples from 52 different sites. There were definitely some life threatening situations both in cars as in the field, but we made it in the end. And both Andy and me were quite happy with the results.

As a reward for working this hard, we treated ourselves to a nice dive/snorkeling session where Andy had another scare for his life when he got eye to eye with a huge devil ray. He was seriously debating trying to get to shore (with high waves splashing on high rocks) when he saw the diving group swim up to take pictures and realised it was probably fine. Happy like a little kid I came back, and was desperate to go again the next morning. Andy almost kissed the ground when we came back, but he had a great time too.

After this amazing (or amazingly scary) experience we still had to pack all our rocks in the box that had to be send back to the U.K. I say all, but technically we should have probably not packed ALL the samples, seen as they are still not here and I have been back over a month. However we were too tired to realise this and very disciplined checked off every sample bag and handsample when packing it in the big box with the equipment.

The only thing left to do was get this big box in our car and down to the shipping containers. This, however simple it sounds, was definitely a difficult job. But we made it and the relaxing could finally begin. But who wants to relax when you can do a hike to the highest point on the island? Dave took us on a beautiful walk through the green areas of the middle of the island. Something completely different than the views that we had seen so far. The cliffs on the shore of the island combined with the immensely green hills land inwards gave such a beautiful and dramatic view that we almost didn’t want to go down. Luckily we did, or we would have missed a smash goodbye party with the rockguards and a lovely meal with loads of breadrolls.

We had an amazing fieldwork thanks to the help of the Saint Helena Government (specifically Sam and Isabel) and our help in the field from Dave the Bugman and the Rock guards. And of course thanks to Andy!

Hopefully the rocks will be here soon, and I can finally start some measurements.

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

Published in the St Helena Independent 25th April 2014{2}

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 Saint Helena Island Info Geology of St Helena
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, including the Geology, Fauna, Flora and Meteorology, by John Melliss, published in 1875 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.

Closing Humour Saint Helena Island Info Geology of St Helena

Laugh at funny Geology of St Helena humour LOL Saint Helena Island Info


Credits:

{a} Mapping St Helena’s Biodiversity and Natural Environment{b} the Records{c} From St Helena: A Physical, Historical and Topographical Description of the Island, including the Geology, Fauna, Flora and Meteorology, by John Melliss, published in 1875{d} British Geological Survey World Seismicity Database{e} St Helena News, July 1975{2}{f} Six Months A Saint



Footnotes:

{1} See definitions of: Trachyte and Breccia.{2} Reproduced for educational non-commercial use only; all copyrights are acknowledged.{3} Location of Jamestown according to latest GPS data.{4} Discovered by Captain Tom Whatley in 1965.{5} 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.{6} In St Helena: A Physical, Historical and Topographical Description of the Island, including the Geology, Fauna, Flora and Meteorology, by John Melliss, published in 1875.{7} Map by maps.ngdc.noaa.gov/viewers/bathymetry{2}.



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