Many people have seen aerial photographs of Hawaii – that exotic island group extending across the middle of the Pacific Ocean like a string of pearls. The Hawaiian Islands were formed due to the presence of a kind of weak spot located far below the sea floor. Deep beneath the surface, at the boundary between the Earth’s crust and mantle, a plume of extremely hot and light magma constantly surges upward. The magma is repeatedly forced upward to the surface where it spills onto the seabed or erupts as small volcanic eruptions on land. These constantly smoldering sites in the oceans where magma masses are able to well upward are called hotspots. Over the course of millions of years, oceanic plates move across the hotspots, resulting in the recurrent creation of new islands. This process forms the typical island chains. A frequent topic of recent debate has been the possible presence of an active hotspot in the Atlantic Ocean today. A team led by geoscientists Wolfram Geißler of the Alfred Wegener Institute (AWI) and Paul Wintersteller of MARUM - Center for Marine Environmental Sciences at the University of Bremen, has now presented decisive findings obtained through sea-floor surveys. Near Tristan da Cunha, an island the size of Sylt located in the middle of the South Atlantic, there is a demonstrable hotspot that has been active for 130 million years.
During a ship expedition, the scientists surveyed the sea floor with sophisticated instruments. One of these was a multibeam echosounder, which records a depth profile of the sea floor, and another was a sub-bottom profiler, which can penetrate up to 200 meters into the sediment. “To the west of Tristan da Cunha, we discovered several previously unknown submarine volcanic cones,” says Wolfram Geißler. “The structures on the sea floor are typical for those found at hotspots” – an initial indication of volcanic activity.
Data from the sub-bottom profiler helped the scientists to determine whether there has been magmatic activity in recent geologic history – i.e., over the past several thousands of years. The data indicate the presence of solidified magma masses lying on top of the sediments. The magma must have flowed up from the subsurface relatively recently and been deposited upon the older sediments. The findings of Wolfram Geißler and Paul Wintersteller thus indicate that Tristan da Cunha is indeed an active hotspot. Until now the evidence for this has been limited and uncertain. Several years ago, German and Japanese researchers investigated the sea floor around Tristan da Cunha using seismic and electromagnetic instruments to penetrate deep into the substratum. The measurement data they produced indicated that there could be a weak area there similar to that observed at the Hawaiian hotspot. But these indications were too tenuous to draw a definite conclusion.
With their detailed new data from the sea floor, the AWI and MARUM scientists have now provided evidence. “To be completely certain we still have to collect rock samples from the seabed,” says Wolfram Geißler. “However, we are presently going on the assumption that this hotspot is still active today.”
Wolfram H. Geissler, Paul Wintersteller, Marcia Maia, Anne Strack, Janina Kammann, Graeme Eagles, Marion Jegen, Antje Schloemer, Wilfried Jokat: Seafloor evidence for pre-shield volcanism above the Tristan da Cunha mantle plume. Nature Communications 2020. DOI: 10.1038/s41467-020-18361-4