Source: Journal of Geophysical Research: Solid Earth
Topography, the contours of Earth’s surface, is controlled by a suite of geologic processes, from climate and erosion to the slow shifting of tectonic plates. Many scientists have argued that the vertical motion of plates from mantle circulation deep in the Earth can create topography, too, but it remains a hotly debated idea. Holdt et al. bring new data to the table that they argue demonstrate that mantle motion can affect the crust’s topography on a global scale.
In their model, heat within the mantle—which correlates with less dense mantle material—causes the crust to rise up and form elevated regions while cooler, denser material sinks and forms depressions. Some scientists disagree, saying that hot mantle material is incapable of supporting topographic highs because it is weaker. But the authors of this new study suggest their observational data set offers stronger evidence than model-only works that movement in the mantle is tied to vertical changes in topography.
“In the end, you can’t just compute the Earth. You have to observe the Earth,” said Nicky White, a geophysicist at the University of Cambridge and coauthor of this study.
To test how mantle movement contributes to surficial topography, the authors compiled the largest collection of offshore and onshore seismic profile data to date. The 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 can’t be explained by cooling and isostasy alone. The highs and lows proposed by this model, called residual topography, 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 can’t yet accurately detect it.
The researchers used the thicknesses of sediment layers along with the crust’s age and typical cooling rates for plate material to calculate subtle elevation differences that result from slow vertical motion. Using this information, the researchers estimated what the elevation of the crust would be without any influence from mantle movement. They then compared that estimate to the crust’s actual elevation.
On the resulting map of anomalous global highs and lows, clear patterns emerged for the oceans and continents. The crust’s elevation can be hundreds of meters higher or lower than predicted by cooling and age alone, suggesting a mantle-based process is affecting 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 thought to be cool such as south of Australia and off the coast of Argentina, there are kilometer-deep depressions. The study attributes these topographic features primarily to mantle convective processes.
The authors will next look to incorporate these observational data into computational simulations and will consider how this process contributes to paleogeographic changes.
“That should keep us busy for the next few decades,” said White. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1029/2022JB024391, 2022)
—Rebecca Dzombak, Science Writer