As far as you can go – The moving volcano

St Helena was formed by a volcano sitting over the mid Atlantic Ridge about 15 million years ago.  Sitting in the deep ocean, it must have taken millennia for any rock to break the surface, and the little island you can see today must sit on an immense pile of submarine mountain.  But any geologist will inform you that the material being spewed out from the mid Atlantic ridge is steadily moving apart.  Ascension Island was formed by a similar process and is moving westwards as it sits on the South American Plate, but St Helena was just on the other side of the divide and moved eastwards towards Africa.  More volcanic eruptions meant a second blob of land appeared to the west of the first and gives the slightly dog’s head look to its shape.  The second volcano partly smothered the deposits of the first and made one single island, with about 40 tiny islets around its perimeter.  In fact these little islands are more likely to have been loosely joined to the mainland for years and gradually eroded away.  About 7 million years ago eruptions in St Helena stopped, the island had drifted so far eastwards that it now was away from the ridge itself where magma from the earth’s interior is being spewed out, pushing the continental plates apart and forming new surfaces.  So now the mountains of St Helena are probably a lot smaller than they were.  Years of weathering have eroded the softer rocks into jagged peaks, or rounded off the massifs, and heavily incised the mountains to form the deep valleys with streams at the bottom.  Some are permanent streams, others are more often dry river valleys called guts.  I knew guts of old as they were a common name for similar temporary drainage features in the Caribbean, although there they were sometimes spelt Ghut.

Ascension Island had been formed in a similar way but much more recently, and still being close to the ridge it was still technically an active volcano zone although it has been several hundred years since the last eruption.  St Helena has had more time to attract the plants and animals on to the land.  As in all rocky regions the first to arrive are the lichens and mosses, the so called Bryophytes.  Ferns were probably next, the tiny spores carried hundreds, even thousands of miles.  Then flowering plants have managed to arrive.  Animals would not have been far behind.  Some insects and spiders had found their way, and many other invertebrate species, and sea birds had found an amazing secret place to colonise.  By some freak a land bird arrived, the wirebird, and somehow started to breed.

And that was probably it for millions of years, apart from one thing.  These animals, remote from their original gene pool, started to evolve separately till they became an endemic species, that is unique to one habitat location and unable to breed with their ancestral line.  The natural species found their niches at different elevations, or in the wetter or drier part of the islands, or mutated over many generations to better cope with the conditions they found.

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Life on Mars – The Yellow (and Purple) Peril

Back in the Land Rover we drove over a ridge and dropped into Breakneck Valley.  The vegetation increases here and the geology softens into the background for the first time.  But it does so with a struggle – a number of caves in a black ash cliff strike you right in the eyeballs as you go over the ridge.  I wanted a closer look so we shut off the engine and took a walk across the valley.  Edsel asked if we were going to do this everywhere.  I retorted  – why not – the end of the road is barely 5 miles away.  It won’t take us long.

The caves varied in size and shape and it would be easier to describe them as ash that had fallen down over lava flows and frozen in situ, leaving fangs of rock hanging down over the spaces they had consumed. So many times Ascension looks like a geological text book.  As much as the biodiversity, the volcanic features should be preserved.  We did document in our mapping what others have noted as key geological features, things like the fumaroles that are long tubes where lava flows hollowed out previous deposits, or the most likely areas to find the hard black pieces of obsidian after which the hotel was named.  And the colour of the rock changes frequently , greys, oranges , reds , blacks and whites to make a geological tapestry that rarely gets covered by anything else.

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The Weird and Wonderful Geology

 

Passing the island’s motorcycle scrambling track where kids and machines get bright red every weekend, you rise up out of Breakneck Valley and for the first time feel the full force of the south easterly winds on the south coast.  I noticed that along the road there were a series of yellow emergency telephone stands – I wondered how often there had been the need to report a break down or report that the road had been blocked by a landslide when the NASA station had been in operation.

A long grey scree ran off the side of Green Mountain coated in low growing ginger and guava plants, and also the healthiest lichens I have ever seen.  Their growths were almost the size of the flowering vegetation and must have represented hundreds of years of growth in this pristine air.  The air is moister this side of the island and as we progressed eastwards the vegetation started to grow larger, the odd tree here or there.  But our way was impeded by, horror of horror, ferocious yellow and purple warriors.

We were in a Land Rover several hundred times larger than our foe, but these sentries set up along the road were fearless and waved their weaponry purposefully at our tyres.  Just in case, we made sure we swerved and avoided them.  We stopped and got out; still they poked their armoury at us in an aggressive fashion, as if they were saying “Come on, I’ll ‘ave you”.  They were giant yellow land crabs.  That is their official common names although, as I say, there were also purple versions of supposedly the same species.  Their bodies were 8-10 inches across, and their legs extended a similar distance out.  For all the famous animals on the island, these have been the most  understudied until recently.  One lone scientist had done some work on them, logging their presence along the NASA and NE Bay Roads which we had been able to map as a sort of imprecise transect of sightings, but it did not really tell the whole picture.  More recent work had allowed tags to be implanted and the crabs monitored more extensively.  They seemed not to travel much further than from Green Mountain down to the sea in the south and east sides of the island.  They obviously preferred the richer vegetation over here.  I assumed either the pickings were too slim down the western and northern sides, or else they did not much relish too much of an insectivorous or mammalian diet.

I’ve watched them at many locations but find land crabs a bit incongruous.  I am used to crabs swimming about in the sea, but to be at such elevations juxtaposed in amongst twigs and leaf litter or climbing trees, and to be these vivid colours, was just ostentatious.

  Over the years more studies have been made and progress has discovered that they have a few key routes off the mountain, the NE Bay Ghut being the main one, to reach the sea for the inevitable release of eggs and spawning that needs sea water to be a success.  It is odd to see that turtles spend their life at sea and come ashore once to lay eggs, while the crabs do almost the complete reverse.

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Life on Mars – The Devil’s Riding School

The road reaches a T junction and most would turn right here down to the US Base, but if you turned left you were on the NASA Road (or A4 as I called it). The road passes by the Cable and Wireless Ground Station, and bends round the base of a hill.  Almost immediately there is a place to stop.  A walk up the hill to the right leads you to the Devil’s Riding School. Edsel and I first went up here in the mist.  The land is still volcanic but the south eastern quadrant of the island, right up to Green Mountain and beyond, tends to be of older origin. This hillside did not have the “fresh” black clinker of the lava fields, the rocks were consolidated ash, white and easy to turn to powder.  They had solidified into layers and when eroded formed plates.  As you walked over them too you felt there was crockery beneath your feet, they tinkled and rang like a nice piece of porcelain.

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The approach to the Devil’s Riding School

As you rose up the sides you realised much of these flat plates were a scree coming off heavily eroded hard rock.  It appeared that the outsides of the deposits hardened more than the subsurface due to weathering (the exposure to the rain and heat chemically reacting with the rock and forming a crust).  When the wind erosion etched through this crust it could suck out the softer material beneath much more quickly leaving very curious shapes and forms.  The pathway heads through a narrow gully between all these rocks and in the mist, the effect was eerie and chilling.  Shapes came out of the fog which looked like ogres or trolls, dragons and demons.  Was this why it was called Devil’s Riding School – did the Devil ride a horse through this hell hole?

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Platelets of rock

Eventually we reached the top, a plateau which at first sight appeared like a contorted limestone pavement of clints and grykes, but this was the ash version of such a geomorphological feature.  The mist was clearing and we could see over the airhead and US Base, some wind turbines whizzing around on the next ridge.

Orange lichens softened the greyness of the rocks, and the odd fern was finding a niche in a damp hollow here and there, but this was still a hostile environment.  Looking up to the east you could see where the vegetation on Green Mountain was reaching down into the valleys.  And then I looked down and finally realised why this hill had such a strange name.

We were at the rim of an old volcanic crater, mostly filled in with soft ash, and round the edge there was a ring of orangish ash, with curious lines tracing concentric circles round the crater.  It was like  all those sandy tracks laid out at horse riding schools with jodhpurful teachers barking out instructions from the centre.

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