June 27, 2013
Upwellings In Earth’s Mantle Have Remained Stable Over Geologic Time
April Flowers for redOrbit.com - Your Universe Online
A new study from the University of Hawaii - Manoa's School of Ocean and Earth Science and Technology (SOEST) reveals that the large-scale upwelling within Earth's mantle mostly occur in only two locations: beneath Africa and the Central Pacific.
Clinton Conrad, associate professor of geology at SOEST, led the team of researchers who found, despite dramatic reconfigurations of tectonic plate motions and continental locations on the Earth's surface, the upwelling locations have remained remarkably stable over geologic time.
"For example," said Conrad, "the Pangaea supercontinent formed and broke apart at the surface, but we think that the upwelling locations in the mantle have remained relatively constant despite this activity."
Throughout his career, Conrad has studied patterns of tectonic plates. He noticed, on average, that the plates were moving northward. "Knowing this," explained Conrad, "I was curious if I could determine a single location in the Northern Hemisphere toward which all plates are converging, on average." Conrad found this convergence point in eastern Asia. Then he wondered if other special points on Earth could characterize plate tectonics as well. "With some mathematical work, I described the plate tectonic 'quadrupole', which defines two points of 'net convergence' and two points of 'net divergence' of tectonic plate motions."
The research team computed the plate tectonic quadrupole locations for present-day plate motions, finding that the locations of net divergence were consistent with the African and central Pacific locations where scientists believe the mantle upwellings occur today. The findings of this study were published in the journal Nature.
"This observation was interesting and important, and it made sense," said Conrad. "Next, we applied this formula to the time history of plate motions and plotted the points - I was astonished to see that the points have not moved over geologic time!"
The team was able to infer upwelling flow in the mantle must also remain stable over geologic time because plate motions are merely the surface expression of the underlying dynamics of the Earth's mantle. "It was as if I was seeing the 'ghosts' of ancient mantle flow patterns, recorded in the geologic record of plate motions!" said Conrad.
Many aspects of geologic change on Earth's surface are governed by the mantle dynamics. The knowledge that mantle upwelling centers on two locations and remains stable will provide a framework for understanding how mantle dynamics can be linked to surface geology over geologic time. Scientists are now able to estimate how individual continents have moved relative to the two upwelling locations, for example, allowing them to tie specific events seen in the geologic record to the mantle forces that caused the events.
For solid earth scientists, the study findings open up a big question: What processes cause these two mantle upwelling locations to remain stable within a complex and dynamically evolving system such as the mantle? The team observed that the lowermost mantle beneath Africa and the Central Pacific seems to be composed of rock assemblages that are different than the rest of the mantle. They question if it is possible that these two anomalous regions at the bottom of the mantle are somehow organizing the flow patterns for the entire mantle, and if so, how?
"Answering such questions is important because geologic features such as ocean basins, mountains belts, earthquakes and volcanoes ultimately result from Earth's interior dynamics," Conrad described. "Thus, it is important to understand the time-dependent nature of our planet's interior dynamics in order to better understand the geological forces that affect the planetary surface that is our home."
Geophysicists can predict surface uplift and subsidence patterns as a function of time using the mantle flow framework defined by this new research. Both local and global changes in sea level are caused by such vertical motions of continents and the seafloor. The team would like to use this new understanding of mantle flow patterns to predict sea level changes over geologic time. Conrad hopes that by comparing these predictions to observations of sea-level change, they are able to develop new constraints on the influence of mantle dynamics on sea level.