Hotspot volcanism has been considered to be related to mantle plumes originating below the stirring upper mantle. The idea suggests that ocean island basalts provide information about chemical characteristics of the deep mantle. Previous geochemical studies reveal heterogenous lower mantle that involves four or five end member components – including HIMU, which has very high Lead Isotope ratios. Limited data on HIMU is available because it appears only in St. Helena Island and three islands in French Polynesia This last fact was completely new to us, and quite exciting to consider. We may not have precious metals, but perhaps the island.

Around 40 people attended the monthly talk hosted by the St. Helena National Trust on Tuesday 20th January. These talks are becoming a regular and popular event in the monthly calendar, and although this clashed with another Public Meeting at PAS, we were delighted to have such good support. The talk on the Geology of St. Helena – and other things – was given by Dr. Ian Baker, who has a long lasting connection with the island. Ian came here in 1964 to study the geology for his Ph.D., returning in 1965, to finalise his thesis. He told the meeting that, at age 22, he thought he knew it all, and had the geology sorted. He even managed to write a pantomime at the same time and had it performed in the Paramount cinema in Jamestown!

In recent years, since 1995, Ian has made 3 more visits, in part to try to understand some of the things he had missed earlier! The audience caught some of his enthusiasm for the wonders of the island’s geology, which still holds many surprises for him now that he is older.

We learnt that St Helena, like Ascension and Tristan da Cunha, is a volcanic island associated with the Mid-Atlantic Ridge. St Helena is the top of a huge volcano that goes 3 miles down to the ocean floor and is 80 miles across at the base. Only half of one percent is above sea level! The Island would have emerged out of the Atlantic about 15million years ago. There are two main volcanic centres, the older Northeast volcano building up a third of the Island leaving such features as Flagstaff and the Barn. The other larger volcano that built up the greater land mass was in Sandy Bay. Volcanic activity ceased approximately 7 and a half million years ago.

Ian explained that the lava did not come through a single vent, as it did in Tristan, but came through large cracks or fissures building up much broader shield volcanoes. Volcanic activity was prolonged and variable, with lava flows and cinder cones gradually building up the Island. Many of the lava flows were fed from linear fissures preserved as dyke swarms in Knotty Ridge in the North East and Sandy Bay in the South West. A complete range of oceanic rock types occurs on the Island from basalts, through intermediates, to highly alkaline trachytes and phonolites. In the dying stages of the volcano there were large-scale underground intrusions that today are seen as the irregular walls of dykes, which include the Asses Ears and Lot’s Wife. Ian in his recent visits has at last come up with an explanation for a feature that had always been puzzling him - the tilting of the Barn and the horizontal lava flows in the adjacent eastern area towards Prosperous Bay. Lava flows usually dip away from a volcanic centre at angles of up to ten degrees. He explained that volcanic material is unstable, with say a heavy, huge volume of lava rocks sitting on softer ash. During the period of volcanic activity there was a major intrusion that raised and tilted the Barn and Flagstaff at the same time making a third of the land built up by the NE volcano slip towards the sea, creating a large depression that was later filled up by a succession of lava flows.

St Helena is special in that, unlike anywhere else that Ian knows, the building up by volcanic activity of a land mass is so strikingly visible. There is so much to see and, as he is still finding out, so much to discover.

The SHNT would like to thank Ian warmly for his time and effort, and wish him well in further discoveries. We hope to continue these monthly meetings on the third Tuesday of each month, drawing on the knowledge of the islands from visitors and local residents.

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