The geology of Saint Helena, Ascension Island and Tristan da Cunha James Cresswell (UK) he British Overseas Territory of responsible for the Cameroon Line, Saint Helena, Ascension Island which includes seamounts and the Tand Tristan da Cunha is made up islands of Pagalu, Sao Tome, Principe of some of remotest islands on earth. and Bioko. However, Ascension Island is It spans a huge distance of 3,642km different and has no chain of seamounts and runs along the Mid Atlantic Ridge or islands. In fact, Ascension lies on the in the South Atlantic Ocean. All the South American plate, while the hotspot Fig 4. The geographic subdivisions of Tristan islands are volcanic, but are formed by is on the other side of the Mid Atlantic da Cunha. Taken from Hicks (2012), who hotspots rather than being part of the Ridge under the African plate, and the adapted it from Google Earth. ridge itself. (Fig. 1 shows the location of magma flows along the ridge to reach (Fig. 2) and is only approachable by the territory.) Ascension Island. The chemistry of the ship, because there is no airfield (Fig. 3). Hotspots are places where anomalously lava on each island is different, showing Tristan’s nearest neighbour is St Helena, hot mantle rises in a plume and burns each hotspot derives from a different four days by ship to the north, and the a hole in the overlying crust, creating source. The Tristan lavas contain the nearest airport is a week away by ship an active volcano. The most famous chemical signature of sediments, which in Cape Town, South Africa. examples are the Hawaiian islands, perhaps adds weight to the subducted The main island of Tristan da Cunha is which have left behind a series of plate idea. The Saint Helena lavas do a large oceanic strato volcano, meaning islands and seamounts, as the Pacific not contain this signature and the it is a volcano rising from the floor of the Plate has moved over the hotspot. Their Ascension lavas show they have been ocean, made up of alternating layers of cause is not fully understood, with heavily mixed by lavas from the Mid volcanic ash and lava flows. If measured some scientists arguing it may be due Atlantic Ridge (Weaver’s website). from the sea floor, the volcano is 5,500m to convection within the mantle and Tristan da Cunha high, with only the top 40% rising above others that the plume could be caused sea level. The island has a maximum by the remnants of subducted plates Tristan da Cunha is the remotest part diameter of 12km. Geographically, the finally melting near the mantle/core of the territory. Gough Island, lying island can be arranged into three main boundary. at a latitude of 40° 19′ S, is the most zones: (i) the base and peak; (ii) the southerly of the islands and is thought main cliffs; and (iii) the coastal strip to be formed by its own Gough Hotspot. (Fig. 4). The other three islands - Nightingale, The base and peak. The boundary Inaccessible and Tristan da Cunha between the base and peak is not well - all lie close together, further to the defined, but the base is the flatter area, north, and were formed by the Tristan while the peak is the steeper area. These hotspot. Apart from six scientists living areas are crossed by many drainage on Gough, all the inhabitants of Tristan channels called ‘gulches’ (Fig. 5). It is da Cunha live in the capital ‘Edinburgh heavily vegetated, especially on the base. of the Seven Seas’, on the main island The rock surface of the base is largely of Tristan da Cunha, which has a made of lava flows, while the peak is population of 382. The island is the made up of pyroclastic material, which world’s most isolated inhabited island is material that has flown through the air after it was ejected from the main vent of the volcano. Also, on the peak and base are numerous scoria cones. Fig 1. Location map of the territory, showing These are produced by fire fountains the submarine ridges and chain of islands, which were created by the hotspots. Taken from Weaver (2002). The hotspot that created Tristan da Cunha was probably involved in the spitting up of the super continent of Fig 2. The welcome sign to Tristan da Cunha - Gondwanaland, some 135mya. Flood the world’s most remotely inhabited island. basalts exist in modern day Parana, Brazil, Etendeka and Nambia, and were erupted when the two places were connected, but began to split apart. As the south Atlantic has spread apart, a series of ancient Tristan da Cunha’s were formed, their eroded stumps are now the shallower areas of the Walvis Fig 5. Original 1939 map by Allan Crawford, Ridge and Rio Grande Rise. Similarly, showing the gulches and place names of the Saint Helena hotspot could be Fig 3. Arriving at Tristan da Cunha by ship. Tristan da Cunha. Deposits Magazine - Issue 45 (2016) - Page 16 Deposits Magazine - Issue 45 (2016) - Page 17 erupting out of small side vents and, as called sector collapse (Hicks, 2012). the material falls to the ground, these The loss of the part of the island then cones are built up. Some of the cones resulted in further eruptions, which also have their own small lava flows produced the coastal strips. Edinburgh flowing from them (Fig. 6). is situated on the coastal strip (Fig. 12) The top of the peak is named Queen and it is the only place on the island Mary’s Peak. At 2,062m, it is the where permanent habitation and crop highest point in the territory. There is a cultivation is possible. The lava that 300m-wide crater at the summit, which forms the coastal strip erupted from the contains a lake that is frozen in winter. It ‘Hillpiece’ (Figs. 13 and 14). This is now is common for the summit to be covered partly eroded away by the sea, showing in snow at this time, but snowfall can the flat area of the coastal strip would occur almost anytime. Fig. 7 shows once have been larger. The coastal strip fresh snowfalls that occurred in April also hosts the potato patches. Potatoes 2014, during Tristan’s autumn. are the only crops grown on the island Fig 6. A simplified geological map of Tristan The main cliffs. The main cliffs ring and are said by the islanders to be da Cunha. Taken from Barry Weaver’s the entire island and are 600m high. the best in the world. Potatoes were website, originally adapted from Baker et al. They have been formed by marine once used as the currency on the island (1964). erosion, though near to Edinburgh, they and the earliest stamps were issues in were created by a portion of the island ‘potatoes’ rather than pennies. (Fig. collapsing and breaking off. (Fig. 8 15 shows the potato patches, and Fig. shows the main cliffs behind Edinburgh 16 shows potatoes being loaded onto and Fig. 9 shows the main cliffs on the our ship, so we could transport them southeast side of the island.) They are to Saint Helena.) Near the patches are made up of alternating layers of lava a series of mounds called hornitos. flows and pryroclastic material, locally These were formed when lava flowed interspersed with parasitic centres. In over ponded water, producing phreatic Fig. 10, the red colour shows where eruptions, which are eruptions caused localised eruptions previous occurred. by steam. (Fig. 17 shows horintos and The coastal strip. If you look at Fig. the bus stop at the potato patches and 11 (a photograph from NASA), you will Figs. 18 to 22 show other general scenes notice the island has an unusual shape. around the settlement of Edinburgh.) Fig 7. Fresh snowfalls on Queen Mary’s Peak. This is because, between 34 and 25 The coastal strip of the settlement is not thousand years ago, the northwest the only coastal strip on Tristan. There part of the island fell off, in a process are two other smaller areas to the south the island, around Hackel Hill and Stony Hill (Fig. 5). In both places, lava has erupted from these hills, creating the flat coastal areas. The eruption at Stony Hill occurred just 200 to 300 years ago. The 1961 eruption. Tristan has been Fig 8. The main cliffs behind Edinburgh. settled for just 200 years and the most dramatic moment in the island’s history Fig 11. A NASA image of Tristan. happened in 1961, when an eruption occurred right next to the settlement, forcing the entire population to be evacuated to the UK. Two months before the eruption, there was gradually increasing seismic activity, with numerous rockfalls behind the Fig 9. The main cliffs on the southeast side of settlement. Surface deformation Tristan. Fig 12. A photograph of ‘Edinburgh of the occurred, causing cracks in buildings Seven Seas’, taken from the top of the 1961 and buckling pipes, doors and window lava flow. frames. On 9 October 1961, a mound began to form and the eruption started the next day. The lava was extremely viscous, more like toothpaste than treacle and its chemistry constituted a tephri-phonolite. This lava extruded from the dome in a blocky flow, which grew the dome to 147m, and blocks and cinder were seen rolling from it. A Fig 13. The Hillpiece, the eruptive centre of cone then formed on the seaward side Fig 10. The red colour in the cliffs marks the the settlement coastal strip, seen from the location of a former parasitic cone.