There’s a lot going on volcanically in the world at the moment.
Slow-moving eruptions such as those at Bárðarbunga and Kilauea supply photogenic evidence of the dynamic Earth at work creating new land, while the sudden and tragic eruption of Japan’s Mount Ontake provided a reminder of how vulnerable human life can be to these acts of nature.
These aren’t the only volcanoes making their presence known – at the time of writing, the website Volcano Discovery listed 38 ongoing eruptions worldwide, from Bárðarbunga in the north Atlantic to Erebus in the Antarctic and from Kilauea in the centre of the Pacific to Nyamuragira in the heart of Africa.
This week’s recent and continuing eruption of Fogo, in the Cape Verde islands, provides an opportunity to look a little more closely at a particular volcanic setting — hot spot volcanism.
The Cape Verde Islands and the Current Eruption of Fogo
Fogo is a volcanic island which makes up what the Smithsonian Institution/National Museum of Natural History’s Global Volcanism Program website (GVP) calls “the most prominent of the Cape Verde Islands,” an island group in the Atlantic which lies around 500km off the coast of Senegal/Mauretania.
Three of the islands, Fogo itself, Brava and Sao Vincente, are listed in the GVP’s volcano database although all of the other islands are volcanic in origin and have probably been evolving for over 20 million years.
The inhabitants of Fogo, however, will be less concerned about the past 20 million years than they are about the past few days. Fogo has by no means been quiet in recent years (its last eruption was in 1995 and lasted for around six weeks) but its sudden awakening on 23 November has caused immense problems locally.
News reports such as Yahoo News speak of: “Molten rock from a thundering volcano threatens to engulf several villages in Cape Verde – days after its initial eruption.”
The slightly more measured assessment from the GVP is that: “The activity was characterised by explosions, lava fountains, and ash emissions…Lava flows had crossed a main road and taken down communication poles… By 25 November the lava flow was 4 km long.”
Hot Spot Volcanism
Most volcanoes are associated with the creation and destruction of the Earth’s crust at tectonic boundaries. In particular, major subduction zones such as the Aleutian islands (which has 41 volcanoes listed in the GVP database, one currently erupting and two in a state of unrest), Indonesia and the Andean margin of South America are notorious for their volcanic activity.
Volcanoes are also associated with constructive margins, where new crust is created along ocean ridges, and with potential rift zones (incipient plate margins), such as the volcanoes of the east African Rift Valley.
But there’s a third setting in which volcanism occurs. A tectonic map of the Earth’s plates and its known volcanoes (active or otherwise) shows a number of significant volcanoes, of which Fogo is one, at great distances from any margin. These are hotspot volcanoes and their source is rising plumes of molten rock from within the mantle. Where this magma reaches the surface, volcanic activity occurs.
Hot Spots on Land and in the Oceans
The Cape Verde Islands are just one of many island groups which overlie a mantle plume. In the Atlantic alone: Other examples include the Canary Islands and the isolated islands of Ascencion and Tristan da Cunha.
Probably the best-known example is in the middle of the Pacific where the Hawaiian Islands are also the product of a hotspot. Hawaii is particularly interesting because the movement of the Pacific plate over the hotspot has left a chain of islands and submarine rises (seamounts) which have evolved over a period of around 70 million years and which run from the newest (currently active Kilauea) to the oldest (the long-extinct Meiji seamount).
Such hotspots are not confined to the ocean. Probably the most dramatic example of a continental hotspot is that which underlies Yellowstone and is responsible for the ongoing geothermal activity and, in the past, for three notorious ‘supervolcano’ eruptions.
What Next for Fogo?
For those living on a volcanic island, an eruption has potentially serious consequences, especially when that island is relatively small. In 1961, residents of the South Atlantic island of Tristan da Cunha were forced not just from their homes but from the island by an eruption. Whether the same fate will befall the present-day residents of Fogo remains to be seen.
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