UC Berkeley researchers discovered previously unknown zones of slow seismic waves, which may help determine the process of the formation of hotspot volcanoes.
Taking seismic wave data from multiple earthquakes from around the world, the researchers developed a 3-D map of the interior structure of Earth through analyzing the speed of seismic waves in the mantle rock.
While most volcanoes form at the edges of tectonic plates when they collide, hotspot volcanoes form in the middle of the plates. These can give rise to island chains, such as Hawaii.
The model maps changes in speed of the seismic waves in terms of certain factors like temperature, according to Scott French, a UC Berkeley graduate student who is a lead author in this project.
“Mantle rocks that have anomalously low seismic velocities can be interpreted to have anomalously high temperatures,” French said. “This is how we interpret the seismic velocities that we see in our model.”
By measuring the velocity of seismic waves, the researchers were able to locate hot upwelling from the deep mantle and its behavior when it moves to the shallow mantle. They discovered that the waves traveled abnormally slowly in certain channels in the upper mantle of the Earth, known as “fingers”, into which mantle plumes radiate.
“This study sheds new light on what happens to hot plumes rising from the deep mantle as they get closer to the surface,” French said. “Understanding this ‘plumbing’ of the upper mantle has important implications for understanding the formation of hotspot volcanoes like Hawaii.”
The pattern of low seismic velocity fingers in the model, which has never been seen before, suggests that these hot upwellings are deflected laterally and travel beneath the tectonic plate for thousands of kilometers, French said.
The average seismic velocity is typically about 2.5 to three miles per second at 120 to 220 miles below the seafloor, but these channels were 4 percent slower, according to a UC Berkeley press release.
French said there are multiple theories about this issue, but as of now, there is no agreed general explanation for the phenomenon. Although this research showed the contribution of plumes to hotspot formation, the other aspects of this complex interaction are still unclear, he noted.