When on Earth did modern plate tectonics begin?

Earth is a dynamic planet, for the simple reason that it is still cooling down from the heat of accretion and subsequent decay of radioactive elements. The main mechanism by which it loses heat is plate tectonics, a theory that has been widely accepted since the 1970s. The Earth is formed of a dense metallic core surrounded by a partially molten silicate mantle which itself is capped by a buoyant crust, either continental or oceanic. We live on the continental crust which largely exists above sea level. The ocean crust forms the floors of oceans and is only rarely exposed. The ocean crust forms by mantle melting at mid ocean ridges, such as the mid Atlantic ridge upon which sits the volcanic island of Iceland. New crust is constantly formed, forcing the older crust to spread outwards and oceans to grow larger. As the ocean crust spreads away from the ridge, it cools and becomes denser. Eventually it interacts with a continent, made of less dense material. The ocean crust is driven beneath the continent back into the mantle, a process known as subduction. Volcanoes form along the continental margin above the subduction zone and at least some of this activity results in addition of new continental crust. This may have been the main process responsible for initial formation and subsequent evolution of our continents. It can be observed now around the margin of the Pacific Ocean, where widespread volcanism is known as the "Ring of Fire". However, not all oceans can continue to grow! The Atlantic Ocean has stopped getting bigger as a response to the continued growth of the Pacific. Eventually, an ocean will close completely and the surrounding continents will collide, resulting in a linear mountain chain. A good example is the Himalaya, where India has collided with Asia. This whole process known as plate tectonics has a profound affect on our planet, providing us with land on which to live, seas in which to fish, freshwater to drink and our complex weather patterns. It is also a regulator of our climate since weathering of continental rocks results in drawdown of CO2 to the deep sea where it is stored. Understanding plate tectonics is central to Earth and Environmental Scientists. There are still important details that we know little about, such as how and when it began. This proposal seeks to investigate this by a novel study of critical rocks that characterise plate tectonics, in particular those that result from subduction. When ocean crust is subducted, increasing pressure and temperature change it into denser rock. As the Earth has evolved, the exact pressure and temperature conditions of this "metamorphism" have also changed. We propose to study this by using minerals that form within ocean crust during subduction. The rocks themselves are often destroyed by erosion, but tiny crystals of a robust mineral called rutile (titanium dioxide) can survive to be found in sediments derived from them. By dating these and using their chemical composition as a fingerprint, we can work out the pressure and temperature within the eroded subduction zone. Similarly, the volcanic rocks that form during subduction have changed through time. These are also often destroyed by erosion so that the exposed record may not be representative. Another robust mineral known as zircon (zirconium silicate) often survives the weathering and ends up alongside rutile in the younger sediments. Using similar methods with zircon we can also investigate changing styles of magmatism throughout Earth's history. . Currently the magmatic record implies that modern subduction began around 2500 million years ago, yet the metamorphic record implies a later start of around 700 million years ago. Our novel approach will test this. We will be able to say whether the younger date is correct and the older marks a different kind of plate tectonics, or whether the older date does indeed represent the onset of modern plate tectonics, and the exposed rock record is biased.

Who will benefit from this research:

The wider academic community will benefit significantly from the outcomes of this research. At one end of the range are Earth system scientists and climate researchers dealing with extreme deep time and at the other are analytical geochemists requiring in-situ or small-sample analysis. Associated with the latter, analytical companies developing and marketing the instruments we use will benefit from enhanced analytical protocols and the publicity that their instruments will receive. In addition, the oil and mineral exploration industries are well-established end users of accessory mineral geochemical studies.

Beyond the academic and commercial benefits, museums and other public educators will be able to drive increasing awareness of how the dynamic Earth works and has worked in the distant past. The detail that we will add to the understanding of plate tectonics will inform knowledge of natural disasters, and ultimately this will benefit aid agencies working in affected areas.

How will they benefit?

Climate researchers and Earth systems scientists will learn when plate tectonics started to operate as it does today and so began long-term regulation of climate and planetary systems. We will also highlight differences, for example when there may have been more or fewer mountain chains, leading to varied continent exposure and changing ocean and atmospheric circulation . Consequences for precipitation, erosion and therefore drawdown of CO2 are profound and will need to be addressed. Geochemists will be able to apply the new analytical tools to a range of Earth science problems. For example, what we know of early crustal growth is gleaned almost entirely from detrital zircons from western Australia, the oldest record of such material. Apatite inclusions will offer a new understanding of such records and our study will lead to renewed international research. The new approaches will be of significant benefit to manufacturers of laser ablation and MC-ICP-MS equipment. Particularly important will be the upgrade of the Neptune at Southampton to Neptune Plus, with an anticipated increase in sensitivity. This has huge, unexploited potential for the type of work proposed here.

The oil and mineral exploration industries will have new tools, which is clearly critical as natural resources run out. Detrital minerals are widely used to understand sedimentology and reservoir characteristics and the results of this study will significantly reduce uncertainty in their use. For mineral exploration, rutile often forms during hydrothermal activity that leads to ore deposits and our large database of rutile mineral chemistry will be a significant resource. Likewise, the potential clarification of ancient subduction-related mineral deposits will inform exploration strategies in relatively unexplored cratons.

Natural History museums offer a critical route to public understanding, and will benefit by posing and answering the question "when on Earth did it all begin?". This will develop interest in the long-term development of Earth's crustal reservoirs on which we all depend, potentially leading to re-invigorated exhibitions and educational events.

Benefits to the interested public will then flow. In the context of this project, a better understanding will be fostered of how lithospheric plates interact to trigger natural disasters such as earthquakes, tsunamis and volcanoes. Further and of particular topicality, an appreciation will be developed of long-term controls on climate variability through weathering of continental crust and CO2 drawdown to the oceans. Such educational work will transfer usefully to aid agencies working in developing countries that have suffered from natural disasters as a direct consequence of plate tectonics. These will then be better able to explain the underlying reasons, thus reducing panic and blame.