Breaking new ground – the rise of plate tectonics
Exactly 100 years ago, German geophysicist Alfred Wegener presented his theory of continental drift – the idea that the continents of Earth are gradually drifting apart. And now we have some compelling new information.
Wegener’s theory was party based on the observation that the large landmasses of Earth fit nicely together like a puzzle but it was also supported by evidence of the same fossilised plants on opposites sides of oceans.
My colleagues and I have published a paper in Nature that could further extend our understanding of the motion of our planet’s surface.
Wegener argued that all the earth’s land masses were once assembled into one large coherent continent, a supercontinent later named Pangea.
And while Wegener’s drift theory seemed to be accurate, he had no good explanation for it and his theory was generally rejected. It was not until roughly 50 years later, with the discovery of seafloor spreading, that his theory gained widespread acceptance.
(Quite simply, seafloor spreading is the process by which the ocean floor is extended when two tectonic plates separate.)
Wegener’s theory has now been replaced by the theory of plate tectonics – an extension of continental drift theory that suggests continents migrate due to the movement of the tectonic plates on which they lie.
Today, the theory of plate tectonics is the foundation for understanding geodynamics – the movement of the earth’s surface – at the most basic level. Plate tectonics can help explain why Hawaii is “followed” by a chain of islands and how mountain chains form by the collision of continental plates.
The basic idea is that the convection cells (regions of different densities) in the mantle flow up beneath spreading zones (where plates are separating) and moves down beneath subduction zones (where plates are coming together).
At the mid-ocean spreading zones, the mantle partially melts during decompression, and the melted rocks are pushed out to form the oceanic crust – the seafloor.
At subduction zones the oceanic crust has cooled down by reacting with ocean water and it is then transported back it into the mantle. During the subduction process the mantle is flushed by fluids from the down-going oceanic crust. This leads to a second melting event where a more permanent crust is formed.
Such a process led to the formation of the islands – and continental arcs, where present day examples are Japan and the Andean mountain belt.
So where does our new Nature paper come in? Well, we now think we have an idea of when the process of plate tectonics might have started on Earth: 3.2 billion years ago.
If that’s right, it means plate tectonics has been underway for roughly two-thirds of the planet’s history. This transition into the modern-day tectonic regime is an important point in Earth’s history.
From this period onwards the earth has operated as a relatively well understood system.
Our findings come from a study of hafnium isotopes in 2.8 to 3.9 billion-year-old rocks from Greenland – some of the best preserved and oldest rocks on Earth.
Our isotope measurements were done in zircon, a very robust mineral that can preserve its isotopic composition even when its host rock is melted.
We found that rocks of more than 3.2 billion years old had a different isotopic fingerprint to younger rocks. This suggests that a change happened around that time, in the way crust forming processes operated and that rocks older than 3.2 billion years were not formed by plate tectonic processes.
So what do we know of how a pre-plate tectonics system would have worked? And what effect would a pre-plate tectonics system have had on ocean and atmospheric chemistry, for instance?
It’s likely that changes in geodynamics of the earth’s history reflect changing mantle temperatures. And changes in mantle temperatures basically reflect the decrease in radiogenic heat production from radioactive elements (such as U, Th and K) and in response, the earth’s mantle has cooled through time. The initiation of plate tectonic processes is likely a consequence of a cooler mantle.
The answer to such issues is: we are not entirely sure. But it is well documented that life on Earth emerged before 3.2 billion years ago, meaning the environment that gave rise to early life was governed by unknown geological processes.
And while we don’t know exactly how pre-plate-tectonics geodynamics might have worked, we do know crust-forming processes of some kind have been happening for almost as long as Earth has existed – 4.57 billion years.
We know this thanks to research that investigated the oldest known zircon grains on Earth – grains that are up to 4.3 billion years old.
Not only were crust-formation processes underway before plate tectonics began, a number of studies suggested as much as 50-70% of the planet’s crust was formed during the first 2 billion years of Earth’s history.
Furthermore, it is only from around 2.5 billion years ago that we see a pronounced change in the chemical composition of our planet’s upper crust. This seems to suggest there are several transitions or modifications to the processes involved in stabilising and growing the continental crust.
To understand these processes we have to invoke a major change in geodynamics. We need a theory that can explain how the earth’s surface was changing prior to 3.2 billion years ago. And to get there we need to a lot more research.
A century ago, Wegener laid out the path to understanding the geodynamic processes behind the development of the earth. But we now have to acknowledge that when we look back in time, there have been considerable changes in the geodynamics of the earth.
Understanding these changes is essential if we want to learn more about the early development of Earth and the conditions affecting the climate and environments under which life on Earth began.