September Editorial      

One layer or two?

      The Earth's mantle is the large area between the surface crust and the outer part of the molten core. It constitutes approximately 85% of the Earth's volume. The mantle consists of hard rocks which are heated into a thick semi-liquid substance by heat from the Earth's core and decaying radioactive material within the mantle itself. The movement of the mantle is responsible for earthquakes and the movement of tectonic plates. It has been shown that during tectonic movement, for example, in the kind of movements which create tsunamis, the tectonic plates dip a long way down to the mantle. A question which has been unresolved for over 30 years is whether the mantle consists of one solid layer or whether there exists a second deeper layer which has never been exposed to the earth's surface Scientists estimate that if this second layer exists, this primordial layer would occupy the inner 1/3 of the mantle.

      So why is the question so difficult to solve? Well from one side studies of the chemical composition of rocks thrown out in eruptions on volcanic islands suggest the existence of a primordial layer within the mantle. These results are based on Helium. Helium is a very volatile gas which disperses into the atmosphere as soon as it reaches the earth's surface. This is particularly true of ³He ( the light helium isotope). So the high ratio of ³He/⁴He found in the erupting magma, (for example on ocean islands like Hawaii), has been interpreted as proof of a primordial layer deep within the mantle.

      On the other hand, recent seismological studies strongly suggest that tectonic plates sink deep in the mantle as far as the border between the mantle and the outer core. That in turn would suggest that the whole section is one layer. So which view is correct, and why is this important? The answer to the is important for scientists who study the Earth's cooling. The major way by which our planet cools down is by volcanic eruptions which release hot magma to the surface. Estimating the rate of the Earth's cooling depends on whether all the mantle is melted or one part is hard un-melted primordial rock.

      In the March article of the journal Science Dr Louise Kellogg and her collaborators proposed a model which would reconcile both theories. They argue that they are two layers in the mantle. In their model, the tectonic plates sink deep into the mantle but reach a geological barrier at the border between the two layers, which deflects the plates. Their model also assumes the existence of hot spots where the hot plumes are carried with the tectonic plates and pull some of the primordial material along with them to the surface. This would explain the presence of ³He in the magma during volcanic eruptions.

      However, in the August issue of Nature Drs Class and Goldstein not only showed evidence supporting the one layer theory, but did so by showing that the original helium theory was incorrect. The degassing of Earth through volcanic eruptions is irreversible. Therefore the high ratio of ³He/⁴He observed in oceanic basalts has been always considered as an evidence for a deep primordial reservoir. However, this latest study showed that these ocean basalts are very similar in chemical and isotopic composition to mid-ocean-ridge basalts which have been re-melted. To put it simply, both continent and ocean crust formation has a common melting history which means that there is no no primordial under-gassed reservoir of helium in the mantle. Will this finding solve the 30 year dispute? Knowing scientists, this is not very likely.

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Basalt