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Geology of St Helena

Upon this rock…

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We are, all of us, growing volcanoes that approach the hour of their eruption; but how near or distant that is, nobody knows
Friedrich Nietzsche

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

This page is in indexes: Island History, Island Detail

{1} [Saint Helena Island Info:Geology of St Helena]
{1}

Go to: FormationResultant geologyErosion: how we got the current island profileSeamountsEarthquakesPuzzling diagramsRead More

Formation

In geological terms St Helena is a very young island. Its origins lie in the Mid-Atlantic ridge and the outpouring of molten rock from between the diverging African and Latin American tectonic plates. 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 our new Airport).

Hills & Valleys Map [Saint Helena Island Info:Geology of St Helena]
Hills & Valleys Map

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]{5}

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°58’S 5°44’W

0

818m

Tolstoy Seamount

15°24’S 6°28’W

65

n/a

Bonaparte Seamount

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

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.{a}

According to the Records{3}, earthquakes were felt here in 1756, 1780, 1817 and 1864. Apparently the one in 1817 lasted 10 seconds, during which time it shook houses and rang church bells. None has ever resulted in loss of life or even serious injury.

Earthquakes are more common on Ascension Island, it being closer to the mid-Atlantic Ridge.

.

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 2 [Saint Helena Island Info:Geology of St Helena]

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

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{4} 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.

More stories [Saint Helena Island Info:Geology of St Helena]

More stories on our page Read articles about St Helena.

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Laugh at funny geology humour - LOL [Saint Helena Island Info:Geology of St Helena]


Credits:

{a} St Helena News, July 1975{2}



Footnotes:

{1} See definitions of: Trachyte and Breccia

{2} Reproduced for educational non-commercial use only; all copyrights are acknowledged

{3} The St Helena Records is a collection of documents dating back to the earliest days of St Helena, held in the Government of St Helena Archives. The Archives can be accessed in person or via email - see our Family And Friends page for more. You can search our events database, extracted from the Records and other sources, on our Chronology page.

{4} 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

{5} Map by maps.ngdc.noaa.gov/viewers/bathymetry{2}.



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