The volcanic eruption on Tristan da Cunha in 1961

On 10th October 1961, after two months of localized earth tremors, a dome of lava was extruded, about 300 yards east of the island settlement, on the northern coast of Tristan da Cunha. The dome grew rapidly to about 300 feet above its surroundings, and its seaward margin slowly moved outwards. Between 20th and 27th October the seaward side of the dome collapsed or was carried away, and mobile lava was emitted. This formed two series of lobes, extending into the sea along a front of about 1300 yards. This activity had ceased by the end of January 1962. In February, a dome up to 130 yards wide and 320 yards long, was pushed up 100 feet above the lava field, extending along the line of the lava stream from the central vent. By mid-March, this activity had ceased, and the only subsequent activity has been the continuation of fumarolic vents in the lava field. The trachyandesitic lava was of almost identical composition at each stage of the eruption, and closely resembled that from a previous eruption about 300 years ago at the other end of the island. The island has many well-preserved volcanic cones and scoria mounds. Such features as explosion craters, lava levees and lava channels also occur. The rocks of the Tristan da Cunha group range from ankaramites through basalts and trachybasalts to trachyandesites and trachytes. Leucite-bearing rocks are frequent and all the rocks have relatively-high contents ofK, Ba, Sr andNb. The origin of such a group of potassic rocks in a mid-oceanic environment is of considerable petrological interest. The 1961 eruption and the general geology of the Tristan da Cunha group was studied by members of the Royal Society Expedition to Tristan da Cunha. Their findings will be published in the Philosophical Transactions of the Royal Society (P. E. Baker, I. G. Gass, P. G. Harris andR. W. Le Maitre).