Buoyancy causes volcanic rocks to rise.

• Position: Geology - Brief Article

A 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. It 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 the 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.

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[degrees] 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," notes James B. Gaherty, an assistant professor in the Georgia Institute of Technology's School of Earth & Atmospheric Sciences, Atlanta. "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...