September 6, 2013
Newly Discovered Seismic Waves Reveal Earth’s Volcanic Hotspots
April Flowers for redOrbit.com - Your Universe Online
A previously unknown channel of slow-moving seismic waves in the Earth’s mantle has been discovered by a team of scientists from the University of California, Berkeley and the University of Maryland. These waves help to explain "hotspot volcanoes" that give birth to island chains such as Hawaii and Tahiti. The findings of this study have been published in Science Express.
Hotspot volcanoes form in the middle of tectonic plates, unlike volcanoes that emerge from collision zones. Current scientific theory for how mid-plate volcanoes form is that a single upwelling of hot, buoyant rock rises vertically as a plume from deep within Earth's mantle -- the layer between the planet's crust and core. This plume supplies the heat to feed volcanic eruptions.
This simple model does not easily explain every hotspot volcano chain, however. This suggests that a more complex interaction between plumes and the upper mantle is at play.
The research team says that these new-found channels of slow-moving seismic waves provide an important piece of the puzzle in the formation of these hotspot volcanoes and other observations of unusually high heat flow from the ocean floor.
Volcano formation along the edges of tectonic plates is closely tied to the movement of the plates themselves. The plates are created as hot magma pushes up through fissures in mid-ocean ridges and solidifies. The magma cools, hardens and gets heavier as the plates move away and eventually sink back down into the mantle at subduction zones.
Scientists have noticed large areas of the ocean floor that are warmer than the tectonic plate-cooling model suggests they should be. Some scientists have suggested that the plumes responsible for hotspot volcanism could also play a role in explaining these observations, but it was not entirely clear how.
"We needed a clearer picture of where the extra heat is coming from and how it behaves in the upper mantle," said Barbara Romanowicz, UC Berkeley professor of earth and planetary sciences and a researcher at the Berkeley Seismological Laboratory. "Our new finding helps bridge the gap between processes deep in the mantle and phenomenon observed on the earth's surface, such as hotspots."
The researchers created a computer model of the Earth’s interior by using a new technique – comparable to a CT scan - that takes waveform data from earthquakes around the world, and analyzing the individual "wiggles" in the seismograms.
Waves of energy produced by earthquakes, explosions and volcanic eruptions, can travel long distances beneath the planet’s surface. Their shape changes as they travel through layers of different density and elasticity. A global network of seismographs record these changing waveforms – allowing scientists to compare waveforms from hundreds of earthquakes recorded at locations around the world and make inferences about the structures through which the seismic waves have traveled.
Interpreting this information isn’t as easy as with a human CT scan, however, as we know much less about the structures beneath the surface of the Earth. "The Earth's crust varies a lot, and being able to represent that variation is difficult, much less the structure deeper below" said University of Maryland seismologist Vedran Lekic, an assistant professor of geology at the College Park campus.
Prior to this study, such analyses would have taken up to 19 years of computer time. Lekic developed a more accurate way to model waveform data while still keeping computer time manageable, resulting in higher-resolution images of the interaction between the layers of Earth's mantle.
The researchers found channels they called “low-velocity fingers” where seismic waves traveled unusually slowly. The fingers moved at depths of 120-220 miles below the seafloor in bands that stretched 600 miles wide and 1,200 miles apart from each other.
Typical seismic waves travel at speeds of 2.5 to 3 miles per second at these depths. The waves in the low-velocity fingers, on the other hand, move approximately 4 percent slower in average seismic velocity.
"We know that seismic velocity is influenced by temperature, and we estimate that the slowdown we're seeing could represent a temperature increase of up to [392 degrees Fahrenheit]," said Scott French, UC Berkeley graduate student in earth and planetary sciences. At such depths, absolute temperatures in the mantle are about 2,372F. French refined Lekic’s new methodology to identify the channels.
Previously, scientists have suggested theoretical channel formations similar to those revealed in the current study’s computer model that would affect plumes in the planet’s mantle, but it has never before been imaged on a global scale. The new findings reveal the size, depths and shape of these channels. The research team said that the fingers are also observed to align with the motion of the overlying tectonic plate, further evidence of "channeling" of plume material.
"This global pattern of finger-like structures that we're seeing, which has not been documented before, appears to reflect interactions between the upwelling plumes and the motion of the overlying plates," Lekic said. "The deflection of the plumes into these finger-like channels represents an intermediate scale of convection in the mantle, between the large-scale circulation that drives plate motions and the smaller scale plumes, which we are now starting to image."
"We believe that plumes contribute to the generation of hotspots and high heat flow, accompanied by complex interactions with the shallow upper mantle," said French. "The exact nature of those interactions will need further study, but we now have a clearer picture that can help us understand the 'plumbing' of Earth's mantle responsible for hotspot volcano islands like Tahiti, Reunion and Samoa."
Image Below: A map view of seismic shear-wave speed in Earth’s upper mantle. The warm colors highlight slow wave-speed channels. Where present, the channels align with the direction of tectonic-plate motion, shown as dashed lines. Credit: Berkeley Seismological Laboratory, UC Berkeley