Mantle plumes—areas of unusually hot rock that can punch through the Earth’s crust to create volcanoes like the Hawaiian islands—have been thought to affect the motion of tectonic plates, but teasing out evidence for this has been difficult. A paper published in Nature may finally have provided a solid example of plumes in action.

The work relied on tracking plate motion using magnetic readings. The Earth’s magnetic poles commonly (in terms of geologic time) reverse themselves, with the magnetic north pole spending time at the geographic south pole. This orientation is captured by crystals of magnetite that are locked in place as magma solidifies into rock.

As a result of this process, stripes of normal and reversed polarity run parallel to the spreading centers on the ocean floor, where magma solidifies into new crust. Since the whole planet experienced the same magnetic events, variations in the width of each individual stripe tell you how much rock crystallized during that time period, which in turn tells you the velocity of plate motion at spreading centers.

The researchers used this information to build a more detailed timescale of the plate motions near Africa and India around 60 million years ago. At that time, Africa was rotating north toward central Europe, and India was an island continent on a collision course with Eurasia. When the significant events of this period were put on a single timeline, some remarkable coincidences emerged.

Around 67 million years ago, a mantle plume reached the surface, punching through southern India and causing an absolutely massive eruption that formed the Deccan Traps, a region of lava flows up to 2 kilometers thick that now cover an area of 500,000 square kilometers (stop for a moment and think about how much rock that is). At the same time, the northward motion of the Indian plate accelerated, more than doubling to a blistering (again, geologically speaking) 18 cm/yr. While this was going on, the velocity of the African plate slowed considerably.

Taken on their own, each of these observations could simply be coincidental with the time of the Deccan eruptions, but together they build an interesting case. Not only did these events all occur roughly 67 million years ago, they all ended around 15 million years after as the plates returned to their previous velocities and the Deccan eruptions ceased.

The researchers also found that changes in spreading rates are centered on the position of the mantle plume. It appears that the motion of the plume alters plate velocities by exerting force from below. The physics of this interaction have proven difficult to model, so there seems to be some uncertainty about whether the plume could have provided enough force to fully explain the observed changes in plate velocity, but the basic concept seems solid.

As an added teaser, the authors note that the kink in the Hawaiian island chain occurred around the same time. If you take a look at a map of seafloor topography, you’ll notice that the line of seamounts trailing behind the Hawaiian Islands takes a sharp bend to the north, which implies that the Pacific plate abruptly changed direction. The details of this are not entirely understood, but the authors write, “We speculate that the waning strength of the Deccan plume head influenced plate motions as far as the Pacific.”

That's a rather a big statement to drop in at the end of a paper. But the authors' approach should allow us to uncover similar interactions between plumes and plates in other locations, and that will undoubtedly impact our understanding of plate tectonics.

Nature, 2011. DOI: 10.1038/nature10174 (About DOIs).

Listing image by Image credit: NASA