29 Apr 2003
The rocks of the Cape Verde Islands

So why write a piece on the rocks of the Cape Verde? Well, for my sins I am a geologist and so am keen to see and understand the geology of places I visit. The landscapes there were very dramatic and despite being so far from home, had much in common with familiar scenes, in County Antrim. I also believe the wonders of geology should be preached at every opportunity.
Geology is everywhere – you cannot escape it. Clues to understanding the development of land are there if you know where to look and how to interpret them. This is often a slow process. Not so on the Cape Verdes. Firstly the almost complete absence of the geologists number one enemy – vegetation – meant the rocks were easily seen. Secondly, the rocks and landforms both pointed towards the same story – elements of the same tale. And finally, the geological development of these islands is not just a history lesson – the formative processes that gave them birth are still ongoing.

The plates go wandering First some background. To understand how the islands have formed we need a quick refresher in plate tectonics. It is known that the earth, including the ocean floor, is divided into a number of plates each moving independently. The movement of each plate is probably caused by enormous convection currents, flowing slowly within the earth’s mantle, part of the semi-molten interior.

Where plates are moving apart as is the case across the Atlantic Ocean (Eurasia is moving further from North America and Africa is moving further from South America), new ocean crust is created in the mid-Atlantic Ocean. In such spreading situations new islands can develop, literally growing from the sea-floor, as with Iceland and the Azores.

Yikes, it’s hot down there But the Cape Verde Islands are nowhere near any of these spreading areas. Geologists believe that a huge column of upwelling lava, known as a ‘plume’ or ‘hot spot’ lies at a fixed position under this part of the Atlantic Ocean, part of the African Plate. As the ocean floor moves eastwards over this ‘hot spot’, the upwelling lava creates a steady succession of new volcanoes that migrate along with the plate.

But unusually in the Cape Verdes, the ‘hot spot’ appears to have two forks. The consequence of this is that, as ocean crust passes over them, two island chains have developed. As might be predicted, the eastern islands are older than the western ones, having formed when ‘their’ bit of ocean crust was over the ‘hot spots’ but now lying further to the east as the conveyor moves relentlessly on.

The southern ‘branch’ of the hot spot is currently only active under Fogo. As the plate continues to move eastwards, if the ‘hot spots’ remain active, new islands will continue to be formed to the west. This same process, involving a single ‘hot spot’, also explains the volcanic origins of the Canary Islands.

One striking feature of the islands is that the eastern ones have no land over 500m, and precious little over 200m, while the western ones have many peaks over 1000m, while Fogo towers to 2900m. This may reflect their respective ages, erosion having had more time to reduce the eastern islands to their rather flat, often monotonous state.

And so to the rocks

No wonder everything looked familiar. The main rock present here, and in Antrim, is basalt. Erupting from volcanoes, or elongated cracks known as fissures, these lava flows have built up, layer upon layer, to produce great basalt plateaux. Standing above these are the volcanoes themselves, enormous layers of ash and other debris, telling of past explosive activity. There are also larger numbers of volcanic cones everywhere, built up from ash and basalt layers but from short-lived activity.

The collapsing tops of some of the large volcanoes have produced caldera’s, often attractive to islanders due to their flat and fertile nature. On Fogo, the enormous Cha de Caldeira is home to 3 villages together with their vineyards, coffee plantations and other agricultural enterprises - this despite having suffered an eruption as recently as 1995.

Fresh lava flows still retain their original textures – the sharp and angular ‘aa’ flows with blocks over 5m high pushed aside, and the distinctive rope-like flows called ‘pahoehoe’. Now you know as much Hawaiian as I do.

It was surprising that in such a barren, arid landscape, evidence of river activity was widespread. And this was more than just the widespread small wadis and channels which dissected many of the islands. Great piles of sandstone were not uncommon, showing where quite substantial rivers had eroded volcanic rocks in the mountains, redepositing water-transported fragments on lower ground.

Finn McCool was here

But the most obvious geological link to Ireland, showing that similar geological environments had existed in both places – albeit 60 million years ago in Ireland and rather more recently in Cape Verdes – was the presence of columnar basalts.

And there is a lot more down there

One fact that I had to constantly remind myself of was that, even on the flat eastern islands, there was one heck of a lot of volcanic rock under the sea. Water depths, away from the islands, are in excess of 6000m. That means, for the islands to begin to get their heads above the sea, underwater volcanic activity had to lay down at least 6000m of volcanic rocks. And then there was the actual island mass on top of that making the main peak on Fogo more than 8500m above the ocean floor.

Ian Enlander