A new study of the Earth's mantle beneath the ocean near Iceland provides the most convincing evidence yet that simple buoyancy of hot, partially molten rocks can play an important role in causing them to rise and erupt through the surface at mid-ocean ridges.
Published August 31 in the journal Science, the Georgia Institute of Technology study also shows that heat from a volcanic hotspot in Iceland can affect normal mantle convection activities at a nearby ridge.
The motion of the Earth's surface plates is driven by a convection cycle in which cold material sinks into the deep mantle and hot material rises toward the surface. At most mid-ocean ridges, scientists believe that hot rock rises passively to fill the gap created by the separation -- or spreading -- of the plates.
But a detailed analysis of seismic waves passing through regions of upwelling rock provides new evidence that another mechanism -- buoyancy much like that of a hot-air balloon -- helps drive partially melted rocks from the Earth's mantle up to the surface at these ridges.
The effect is especially pronounced at the Reykjanes Ridge, a portion of the mid-Atlantic ridge that gains significant heating from Iceland's volcanic hotspot. This additional heating adds 30-80 Kelvin to the mantle temperature there and may play an important role in powering the buoyancy at this location.
"These observations imply that the volcanic rocks erupting on the surface forced their way through the upper 60-100 kilometers of the Earth through the power of their own buoyancy," said James B. Gaherty, author of the paper and an assistant professor in Georgia Tech's School of Earth & Atmospheric Sciences. "You can envision this like a hot-air balloon that bursts through its hangar roof rather than waiting for the rooftop door to open. This contrasts with most spreading centers, in which the hot rocks reach the surface simply to fill the void left by the spreading plates."
Gaherty studied seismic waves from 17 earthquakes as they passed through the Reykjanes Ridge. Waves with vertical polarizations passed through the region at the speed expected. However, transversely polarized waves were delayed, providing Gaherty information about how the orientation of crystalline structures in the region may have been deformed by the mantle flow.
"The propagation speed of these waves provides information about two critical Earth parameters: the relative temperature of the rocks beneath the ridge and the crystalline structure or fabric embedded in the rocks as they have deformed during convection," he explained. "In this case the fabric is consistent with buoyancy-driven upwelling of the partially melted rock."
Because the ridge is adjacent to a volcanic hotspot on Iceland, the study also provides new information on how such heat sources affect ridges -- and may prompt reconsideration of existing models that explain such sea-floor spreading. For example, Gaherty found that heating from the Iceland hot spot extended to a depth of at least 100 kilometers.
Though Gaherty's study was confined to the Reykjanes Ridge, he believes buoyancy may also play a role in ridge dynamics for other areas.
"It provides unique observational evidence that buoyancy-driven upwelling is an important component of ridge dynamics, especially in environments where passive sea-floor spreading is too slow to accommodate melt production," he writes in the paper. "The presence of anomalous mantle fabric to about 100-kilometer depth implies that the hotspot modulates upper-mantle dynamics beneath the ridge to at least this depth."
Earth scientists have previously discussed buoyancy as a mechanism for powering volcanic upwelling, but direct evidence for it had been limited.
"This paper provides some of the most direct evidence to date," Gaherty said.
The study also provides additional information about the thermal anomaly associated with the Iceland mantle plume, which has been the topic of significant study. Further, it provides more evidence that slow-spreading ridge structures in the Atlantic differ in important ways from comparable but faster-spreading structures in the Pacific.
Gaherty's work focuses on understanding the connection between solid-state convection in the Earth's mantle -- a solid rock region extending from 30 to 3,000 kilometers in depth -- and surface deformation and plate tectonics.
Hot upwelling of volcanic rock from the mantle typically occurs in two environments: (1) mid-ocean ridges, which are linear chains of volcanic activity along the boundary where two plates move away from one another, and (2) "hotspots," which are point sources of high volcanic output associated with quasi-stationary and long-lived heat sources in the mantle.
The mid-Atlantic ridge is an example of the former; Hawaii, Iceland and Yellowstone National Park are examples of the latter.
The research was sponsored by the National Science Foundation. - By John Toon
[Contact: James Gaherty, John Toon]