We think of oceans as eternal and stable; what could affect these endless fields of water? But everything changes, the plains, mountains, and the Earth itself. Throughout our planet’s geological history, oceans have been born and destroyed, and now we’re one step closer towards knowing how this happened.

The maps are showing different viewing options for the region under Southeast Asia. Illustration: Grace E. Shephard.

Seeing the past

Millions of years ago, the Earth was a different place. It’s not just the biodiversity and the climate, the oceans and the land looked different, too. That happens because the surface of the Earth is in constant motion. It doesn’t move by much (on the scale of centimeters per year), but give it enough time, and the change becomes noticeable. This happens due to the relative motion of the planet’s tectonic plates, pushed around by upwelling magma from the mantle.

The movement isn’t only lateral — it’s vertical as well. New crust is formed at mid-oceanic ridges, such as the Mid-Atlantic Ridge, and older crust is destroyed. Some of the crust goes up, the other sinks into the mantle, in the “geological graveyard.” This cycle also affects oceans. Sometimes, it opens up or spreads new oceans, and other times it closes them down. The Pacific, for instance, is currently expanding, while the older Atlantic is slowly shrinking. Now, researchers have used seismic tomography to have a “look” at this geological graveyard up to 2,800 km beneath the surface, identifying how the planet might have looked like 200 million years in the past.

This is not the first time something like this has been done. Several studies, using several different approaches, have retraced our planet’s evolution, sometimes ending up with different models. Now, Grace Shephard at the University of Oslo has found a simple, yet powerful way to combine images from alternative seismic tomography models.

“There are many different ways of creating such models, and lots of different data input can be used,” explains Grace Shephard, who has been a postdoctoral researcher at CEED since she took her Ph.D. at the University of Sydney four years ago.

“We wanted a quick and simple way to see which features are common across all of the models. By comparing up to 14 different models, for instance, we can visualize where they agree and thus identify what we call the most robust anomalies.”

The Arctic is a region where little is known about plate tectonics far back in time. That is one of the reasons why Australian Grace E. Shephard decided to join the CEED team of the University of Oslo. Photo: Dag Inge Danielsen/UiO.

A continental traffic jam

This tomography data gives more accurate and more easily available information about the movements of the oceans and continents, as well as the interaction between the Earth’s crust and the mantle. By seeing at what depth the former seafloor lies (the paleo-seafloor), and supposing that it sinks at a rate of 1 cm/year, researchers can calculate when the paleo-seafloor sank. Using this method, scientists found that there was a period around 100–140 million years ago that experienced more ocean destruction; it’s unclear why this happened. They also learned of an area of the mantle which is more ‘sticky’ — viscous — and exerts more opposition to sinking plates, resulting in a “traffic jam.”

The analysis raises new possibilities of understanding our planet’s history, but there’s still plenty of work to do. The models have to be constantly fine-tuned and improving using real-life observations.

“Studying these processes in new ways opens up new questions. That is something we welcome, because we need to find out what questions to ask and what to focus on in order to understand the development of the Earth. We always have to keep in mind what is an observation and what is a model. The models need to be tested against observations, to make way for new and improved models. It is an iterative procedure.”

Using the same approach, we can also derive information about the future. While it’s impossible to gauge how life will evolve over such a long timescale, geologically speaking, not that much will change.

“If you look at Earth from space, the distribution of continents and oceans will then look much the same, even though life, the climate and sea level may have dramatically changed. If we move even further ahead, say 10 or 100 million years, it is very hard to say how oceans may be opening and closing, but we have some clues. Some people think that the Atlantic will close, and others think the Arctic or Indian oceans will close. We can follow the rules of the past when we look to the future, but this task keeps geoscientists very busy.”

Journal Reference: On the consistency of seismically imaged lower mantle slabs, G. E. Shephard, K. J. Matthews, K. Hosseini & M. Domeier, Scientific Reports 7,