Hold the front page!
Two updates from the journal ‘Science’
1. Forget the Anthropocene, for now, we are still in the Holocene.
In early March 2024 the journal Science confirmed that a panel of over 20 geologists voted against a proposal to end the current span of geological time Holocene, which began 11,700 years ago at the end of the last ice age, and inaugurate a new geological era, the Anthropocene.
Some scientists, and geographers, have stated that from the dawn of the industrial age, a new geological time had emerged when the influence of humans on the planet, mainly through climate change, was overwhelming. However, the International Commission on Stratigraphy (ICS), the body that governs the geological timescale, has said otherwise.
Opponents of the proposal to identify the Anthropocene do not doubt the enormous impact that human influence is having on the planet. However, some felt the proposed marker of the new era - the 10cm of sediment from Crawford Lake in Canada that has captured the global surge in fossil fuel burning, fertilizer use, and atomic bomb fallout that began in the 1950s, was not definitive enough.
Others have questioned whether it is even possible to assign one date to the start of humanity’s planetary influence. Others could include the rise of agriculture, or the significant changes that followed European encroachment on the New World. Why not choose those?
Those who support the concept of the Anthropocene will now have to wait for a decade before the proposal can be considered again. The ICS has instituted such a ‘cooling-off period’, because of the furious debates there have been in the recent past. For example, there was debate in the past over the boundary between the Pliocene and Pleistocene, and whether the Quaternary era - our current geologic period - should exist at all.
No doubt many academics and others will continue to express the view that the Anthropocene should be recognised in geological timescales. They argue that there is no going back to where the planet was 100 years ago - the Earth system changes that indicate the Anthropocene are irreversible.
2. The formation of the Himalayas
In January 2024, Science also highlighted new thoughts on the creation of the Himalayan Mountain chain.
The consensus of how the Himalayas were formed is that they are result of the Indian plate moving north and colliding with the more static Eurasian plate. This process began between 60 and 55 million years ago. As the two plates collided, sedimentary materials between them, laid down in the so-called Sea of Tethys, were folded upwards to form the mountains. The reason why the Indian plate moved was initially attributed to convection currents moving from south to north, but many now state it was due to the process known as slab pull whereby the older rocks of the descending plate ‘pulls’ the newer rocks behind it – rather like pulling a tablecloth off a table (as against trying to push the cloth). The term subduction is given to this downward movement. It has also been suggested in the case of this region that the original descending slab has broken off from the main descending plate. But this then begs the question – what is causing the current level of movement? Enter the journal Science….
Continental tectonic plates, like the Indian plate, are relatively buoyant, so they do not easily sink, or subduct, into the mantle during collisions. Some scientists believe the Indian Plate resists plunging into the mantle and continues to slide horizontally under Tibet. Others suggest the most buoyant part of the Indian Plate crumples like a rug along the front edge of the collision zone, making it easier for the lower half of the plate to sink, or subduct.
A new analysis of seismic waves travelling beneath Tibet and gases, such as helium-3, rising to the surface points to another possibility - one that in effect splits the difference between these two scenarios. Part of the Indian Plate appears to be ‘delaminating’ as it slides under the Eurasian Plate, with the dense lower part peeling, or tearing away from the top, as if the upper and lower parts were being separated as in the undoing of a zip (see diagram).
It is likely that the difference in subduction speeds along the Indian/Eurasian ‘front’ has tugged the Indian Plate in multiple directions. Scientists have proposed many possible tears, both horizontally and vertically, in recent years. A recent analysis based on a set of earthquake waves points to a tear on the western edge of the delaminated slab. West of the proposed break, the bottom of the Indian Plate appears to be some 200km deep, suggesting it is still intact; to the east, where the slab splits in two, mantle rock is flowing into the ‘gap’ at a depth of approximately 100km.
The key message here for both topics, is that nothing is set in stone, pardon the pun.