New explanation for the Hawaiian-Emperor bend
International team of researchers finds evidence of large-scale tectonic changes in the Pacific region 50 million years ago.
A chain of extinct volcanoes that mostly lie under water stretches across the Pacific from Hawaii to Kamchatka. This chain, known as the Hawaiian-Emperor Chain, is the trail left by a volcanic hotspot. But why does it change direction in the middle? An international team of researchers, including members of GEOMAR Helmholtz Centre for Ocean Research Kiel, the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, and FAU have found an explanation in large-scale tectonic changes that took place around 50 million years ago. Their study has recently been published in the international journal Nature Geoscience.*
The model is very simple in principle. In certain places in the earth’s core, especially hot material rises towards the earth’s crust in what is known as a plume. It melts through the tectonic plates like a blowtorch, depositing magma on the surface. A volcano is formed. However, over time the plates move away from the volcanic hotspot, taking the volcano with them. As the plume continues to expel hot material, a second volcano forms next to the first which is now extinct. In this way, over millions of years a chain of volcanoes is formed. One of the most famous examples of this is in Hawaii, where the chain starts with today’s islands and extends in an almost perfectly straight line towards the north-west. The Hawaiian Islands mark the location of the active hotspot, while the islands and under-water mountains become older and older the further you go to the north-west.
Yet as is so often the case in nature, the reality is more complex than the model. Around 3500 kilometres north-west of Hawaii the chain of volcanoes suddenly changes direction and goes north. From here onwards the submerged mountains are known as the Emperor Chain. ‘Until now there have been several theories as to the cause of the Hawaiian-Emperor bend, but no really reliable explanations. We have found evidence that the Pacific Plate underwent large-scale deformation between 47 and 53 million years ago and that the bend in the Hawaiian-Emperor Chain occurred at the same time. We have come to the conclusion that large-scale changes in plate tectonics and convection currents in the mantle that happened around 50 million years ago could be the cause,’ explains Prof. Dr. Kaj Hoernle from GEOMAR Helmholtz Centre for Ocean Research Kiel. He and nine other researchers from Germany, the Netherlands, Australia, the United Kingdom and the USA have now published their findings in the international journal Nature Geoscience.
The researchers were helped by volcanoes with some interesting names. North of the Hawaiian Chain is a range of underwater mountains called the Musicians Seamounts which are named after composers such as Beethoven, Bach or Donizetti. For a long time it was thought that these seamounts were also created by a hotspot. In their study the researchers dated and examined many samples from the Musicians Seamounts using geochemical methods for the first time. They discovered that these underwater mountains do not show a progression from young to old as would be expected for volcanoes formed by a hotspot. ‘The samples that we analysed were mostly between 47 and 53 million years old,’ explains lead author Dr. John O’Connor, a researcher at GeoZentrum Nordbayern at FAU who collaborates with colleagues from Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research and the University of Amsterdam in his research on the dynamics of the earth’s crust.
‘The geochemical analyses also had a surprise in store for us. The composition of the samples from the Musicians Seamounts is more similar to volcanoes that form in mid-ocean ridges that those that grow over a hotspot,’ explains Dr. Folkmar Hauff from GEOMAR, co-author of the study. It is possible that around 50 million years ago large cracks appeared in the Pacific Plate, leading to the formation of these volcanoes. We know from previous studies that this was also the time when the plates in the north and west of the Pacific Ocean began to overlap. The Izu-Bonin-Mariana Arc and the Aleutian Arc are impressive testaments to these processes. ‘Our analyses of the Musicians Seamounts show that all of these events could be related, and that these large-scale changes probably also caused the bend in the Hawaiian-Emperor Chain,’ says Professor Hoernle.
Dr. John O’Connor
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