The lithosphere — the outermost layer of the Earth — has long been assumed to be composed largely of solid rock. But a recent Brown-affiliated study challenged this assumption by finding small amounts of partially melted rock at the lower boundary of the lithosphere in the Southwestern United States.
The presence of this partial melt at the boundary of the lithosphere and the asthenosphere — the weaker layer of the Earth under the lithosphere — could have meaningful implications for understanding and preparing for natural phenomena such as melting ice sheets and earthquakes, according to the study’s authors.
“Understanding these processes will help scientists figure out where geologic hazards are high, and where to focus on hazard mitigation,” Eva Golos, lead author of the study and an assistant professor at the University of Wisconsin, Madison, wrote in an email to The Herald.
When ice sheets melt, the mantle’s rocks tend to “rebound” to a higher elevation and sea levels rise, explained Karen Fischer, an author on the study and a professor of geological sciences. This process occurs when the immense weight of ice sheets depresses the Earth’s crust over thousands of years.
When the weight is removed, the underlying rock rises. The Earth’s reaction to this effect greatly depends on the strength of the rising rock, which depends on the presence of partial melt, Fischer explained.
The degree of partial melting and the resulting strength of the rocks at the lithosphere-asthenosphere boundary may also help researchers better assess earthquake risks across the region, explained Zachary Eilon, an author on the study and an associate professor at the University of California, Santa Barbara. Partial melt behaves more flexibly than cold, brittle rock during earthquakes, deforming rather than breaking.
Knowing where the lithosphere is warmer and where it is colder, therefore helps scientists determine how much of the upper crust is capable of hosting earthquakes in the first place, which in turn refines our understanding of where seismic risk is greatest, Eilon explained.
The study used seismic waves to measure temperature under the Earth’s surface. The researchers combined two complementary types of waves: surface waves, which propagate across the surface of the Earth, and converted waves, which change their direction of propagation when they collide with boundaries inside the Earth, Fischer explained. Surface waves are good for measuring overall seismic velocity, while the converted waves are good for identifying “sharp boundaries” between layers of the Earth, she explained.
If a rock is warmer, these waves travel through the rock slower and if the rock is cooler, the waves travel through it faster, Eilon explained. Mathematical models can then relate the speed of these waves to the temperature of the Earth at a given location and determine its physical state, whether the rock is fully solid or partially melted, Golos added.
The researchers ultimately found variation in the degree of partial melting across the Southwest.
The lithosphere beneath Wyoming is thick and cold because that region has experienced “relatively little tectonic activity for maybe a billion years,” Fischer explained. With so little disruption over such a large span of time, the lithosphere there has had the opportunity to cool and thicken, leaving it largely free of partial melt.
Conversely, the province under Nevada and Arizona — the Basin and Range — is much younger, Eilon said. Over the last 100 million years, a vast tectonic plate has been “subducting” — or slipping under — the Pacific plate, which has caused the continental plate above it to “deform,” according to Eilon.
In response to this disruption, the Basin and Range region has been “stretching,” making the lithosphere in that region thinner and hotter than in Wyoming, he added.
The abundance of hot rock and fluid in the crust across much of the Western United States also provides the possibility for the development of geothermal energy resources, Golos wrote.
“These types of data are really important to add to our ability to understand the physical evolution of the Earth with time,” said Vice President for Research Greg Hirth, who was not involved with the study and is a professor of Earth, environmental and planetary sciences.
Nishita Malhan is a senior staff writer covering science and research.
