Hawaiian Seamount Chain & Iceland
During his recent visits to Hawaii and Iceland Alan was able to observe first hand evidence of the origins of both Hawaii and Iceland. This talk addressed the current controversy in the geology community about the origins of seamount chains and drew up a list of the pros and cons of the various hypotheses.
He started with a review of the models of the Earth’s interior making the point that no actual materials have ever been recovered from depths greater than 100kms. Given that the Earth’s radius is ~6400 km this means that most of the Earth’s internal structure must necessarily be modelled using indirect methods such as seismology. Consequently there is still uncertainty about the models and in particular the structure and fluid dynamics of the mantle and asthenosphere.
The theory of Plate Tectonics only started in the mid 20th century and now is generally accepted as a accurate description of the movement of the Earth’s 13 plates. New oceanic crust is formed by up welling molten mantle material along mid ocean ridges and subsequently subducted underneath lighter continental crust. This is believed to be a cycle where the denser subducted oceanic crust sinks back into mantle and is, over geological time, returned to mid ocean ridges in a massive convection cell.
However in the Hawaiian island chain is more or less in the middle of an oceanic plate where, according to conventional Plate Tectonics, there should be no volcanism. Clearly there has to be a different mechanism and in 1971 the ‘Hot Spot’ hypothesis was conceived. In this there are so called ‘Hot Spots’ in the mantle caused by mantle ‘Plumes’ rising from deep within the mantle or even from the mantle/outer core boundary at ~3000km (D’’ layer). The hot spot hypothesis became a bit of a ‘band wagon’ and many other volcanic features on land and undersea was attributed to mantle plumes e.g. Yellowstone, the Deccan Traps and Iceland.
One important assumption about mantle plumes is that they need to be fixed relative to the motion of the Tectonic Plates. So far no tenable theory has been put forward.
The types of lava produced at the various hot spot candidate sites also varies from ‘sticky’ silica rich to ‘runny’ silica poor material. This indicates that a common mechanism is unlikely.
Hawaii is small in comparison to Iceland being essentially one big volcano. It is also only about 1my old. The volcanoes are large and high but also very rounded and dome like and filled with lave tubes. All this results from very runny, low silica lava. The lava tubes are formed as the surface lava streams cool and solidify on top leaving the lava running underneath.
The main Hawaiian island is currently active on its eastern side and inactive on its western side. This indicates that the pacific plate appears to be moving over what may be a mantle plume. This is also borne out by the chain of inactive islands to the west.
In comparison Iceland is large and lies right on the Atlantic mid ocean ridge. Normally mid ocean ridges are deep in the ocean with the erupting lava cooled rapidly near the ocean floor. It is also much older that Hawaii at ~13 my and consists of a large number of volcanoes. This suggests that whatever is causing it is relatively stationary with respect to the mid Atlantic ridge both East West and North South. As one would expect the oldest rocks are on the east and west coasts with the youngest (10 ky) in the middle, close to the mid Atlantic ridge.
The Mantle Plume hypothesis has it that plumes are produced by convection cells rising deep from the mantle core boundary with a head and tail. Eventually the head of the plume ‘ponds’ under the lithosphere and locally pushes the lithosphere up into a dome. This dome eventually cracks and produces flood basalt. Once started the tail of the plume continues to supply basalt but in spurts resulting in island chains.
However to do this the plume has to remain stationary with respect to the plate above it and as mentioned previously no mechanism has so far been proposed to explain how this can happen.
There have been attempts to image plumes using seismic tomography and images have been produced. However the differences in velocity measured are less that 1% and may very well be experimental error.
The Hawaiian chain seems to have changed direction in the distant past. This has been attributed to a change in the motion of the Pacific plate. However recent magnetic anomaly measurements do not support this change. Other evidence supporting no change in direction are the ages of volcanoes in the chain and other Pacific island chains having different directions.
More recently a rival hypothesis has been proposed which does not need the presence of mantle plumes. This hypothesis relies on recent measurements that the temperature of the asthenosphere/upper mantle is higher than previously believed and may even be hotter that the lower levels of the mantle. The additional heat comes from the frictional shear forces caused by Plate Tectonics i.e. the movement of the oceanic and continental plates relative to the mantle. This additional heat causes local melting and a non-homogeneous asthenosphere. These local hot spots can break through the crust in a similar way to plumes. However such hot spots would still have to be stationary with respect to plate motion to explain island chains.
Alan summarised by attempting to answer the following questions:
Igneous Province?
Alan concluded that even with some evidence supporting both hypotheses it was sill not possible to say which is correct. It may be that both mechanisms are at work or there may yet be another mechanism waiting to be discovered!
Dick Harris
December 2014