Brown researchers recently published a study analyzing the Fennoscandian Shield, the exposed ancient bedrock underlying Norway, Sweden and Finland, to better understand cratons, the old, stable cores of continents that extend hundreds of kilometers into the Earth.
Although the Fennoscandian craton is less studied than other regions, researchers chose the region because it has a dense network of seismic stations, instruments that record ground motion.
Yiran Huang, first author and postdoctoral scholar at Northern Arizona University, used data from these seismic stations to examine waves generated by earth movement and better understand the structure of the craton.
While most studies of seismic waves analyze either body or surface waves, Huang and the other researchers used a newer approach to examine the two in tandem.
Researchers sought to understand how cratons remain stable in the long term, according to Colleen Dalton, an author on the study and professor of earth, environmental and planetary studies.
According to Dalton, surface waves measure absolute speed, whereas body waves help researchers identify properties such as changes in that speed.
“By combining them we were kind of exploiting their complementary advantages,” Dalton explained. Using the two different data sources, researchers created a model that revealed how fast seismic waves travel through different layers of the Earth.
They examined upper and lower regions of the lithosphere and found that waves traveled more slowly through the top layer than through the deeper layer, which sits around 140-200 kilometers below the surface.
According to Dalton, there are two possible explanations for the deeper layer’s higher velocity: Either the rocks contain diamond — a theory suggested by previous researchers — or the rocks in the area are cold. When rocks are more dense, they create a more stable layer, which allows the rocks to rotate faster.
Huang also repeated the experiment using only surface waves to determine whether these findings were a result of the two two-wave approach.
The results were surprising, and “more different than I expected,” Dalton said. Because surface waves cannot image abrupt changes in velocity, the surface-wave-only model placed the fastest rock closer to the surface, while the two-wave approach revealed two distinct layers, with the fastest rock sitting deeper.
Huang suggested future research could compare the Fennoscandian craton to others. “We actually don’t know whether the Fennoscandian craton is actually a pretty typical style for (a) craton, or just a certain example,” she said.
Future studies may explore if cratons are “all the same on different continents, or is there some diversity in cratons,” Dalton added. “If so, what’s responsible for that diversity, and how does it manifest in things that we can observe?”
Dalton also noted the possibility of applying the model developed from the two-wave approach to unreachable regions.
The ice in Antarctica, for example, prevents researchers from collecting the necessary seismic data, but the new data could provide information about the cratons. She added that the methods could also be used to examine the makeup of mineral deposits in cratons, which are necessary for industrial processes.
According to Christian Huber, professor of earth, environmental and planetary sciences, “our society is more and more depending on us finding rare earth elements or lithium for batteries.”
This type of research can have “direct society applications,” he added.
