Source: Journal of Geophysical Research: Solid Earth
Topography, the contours of the Earth’s surface, is controlled by a range of geological processes, from climate and erosion to the slow shifting of tectonic plates. Many scientists argue that the vertical movement of plates from mantle circulation deep in the Earth can also create relief, but this remains hotly contested. Holdt et al. bring new data to the table that they believe demonstrates that mantle motion can affect crustal topography on a global scale.
In their model, heat within the mantle, correlated with less dense mantle material, causes the crust to rise and form elevated regions, while cooler, denser material sinks to form depressions. Some scientists do not agreesaying that the hot material of the mantle incapable support of topographic highs, because it is weaker. But the authors of this new study believe their observational data set offers more compelling evidence than modeling work alone that movement in the mantle is linked to vertical changes in topography.
“After all, you can’t just calculate the Earth. You should watch the Earth,” he said Nicky Whitegeophysicist at the University of Cambridge and co-author of the study.
To test how mantle motion contributes to surface topography, the authors assembled the largest collection of marine and terrestrial seismic data to date. Seismic data show how thick different layers of sediment and crust are, and allow scientists to build a detailed global model of topographic features that cannot be explained by cooling and isostasy alone. The ups and downs suggested by this model, called residual terrain, can be more than a thousand kilometers wide. But because the vertical motion is slow, only about 0.1 millimeter of change per year, satellites cannot yet accurately detect it.
The researchers used the thickness of the sediment layers, along with the age of the crust and the typical cooling rates of the slab material, to calculate the slight elevation differences that result from the slow vertical movement. Using this information, the researchers estimated what the height of the crust would be without any influence from the movement of the mantle. They then compared this estimate to the actual height of the Earth’s crust.
The resulting map of anomalous global highs and lows revealed clear patterns for the oceans and continents. The elevation of the Earth’s crust may be hundreds of meters higher or lower than predicted from cooling and age alone, suggesting that mantle processes affect the topography.
Some places, including Iceland and Hawaii, which are known hot spots with active volcanism, are more than a kilometer higher than expected. However, in areas where the mantle is considered cool, such as southern Australia and off the coast of Argentina, there are depressions that are kilometers deep. The study attributes these topographic features primarily to mantle convective processes.
Next, the authors will try to incorporate these observational data into computer simulations and examine how this process contributes to paleogeographic changes.
“That should keep us busy for the next few decades,” White said. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1029/2022JB0243912022)
— Rebecca Jombak, science writer