A Hotspot Alternative

A Hotspot Alternative

The rise of a surface-related explanation is turning things upside down

The best science comes from the competition of ideas. That happened in the middle of the last century when geologists were won over to plate tectonics, and it's happening today as competition is heating up between two theories about hotspots.

The idea that the crust moves through a cycle of formation and destruction梬hat we know today as plate tectonics梒ame to be accepted in the mid-1960s. But at that time there was also an older, competing idea that instead the whole Earth is expanding. The predictions that each theory made were put to the test, and expansion theory withered away.

Then the hotspot problem arose. One thing that plate tectonics explains is why chains of volcanoes occur where crustal plates are being subducted down into the mantle. But there's a scattered set of leftover volcanoes that occur elsewhere. The places they occur are called hotspots.

The widely accepted theory about hotspots is that they arise from long-lived plumes of hot material that rise from deep in the mantle, perhaps from its very base. A new plume builds up a large body of magma on the underside of a crustal plate, then bursts through. The result is a huge series of eruptions of basalt rock, creating what geologists know as Large Igneous Provinces or LIPs. After that, the plume settles down to a steady output of lava. As the crustal plate moves across the plume top, it pushes up strings of volcanoes the way a candle flame burns holes in a sheet of paper passed over it.

This is a fine theory.

It's easy to explain to people and it accounts for a wide set of observations.

Hotspots seem to have a source at a fixed location, beneath the wayward motions of the surface plates.

Some of their chemistry suggests an origin isolated from the rocks of the upper mantle and crust.

Plumes seem like an upward-moving counterpart of the downward rain of relatively cold, subducted crustal plates into the deepest mantle.

Lab experiments using suitable fluids, like glycerine and corn syrup, produce similar-looking plumes.

And plumes suggest ideas for further research, always something that scientists like.

Seismic tomography (the technique that yields "ultrasound baby pictures" of the deep Earth) shows the kind of magma conduits predicted by plume theory, but only in the shallow mantle. The deep mantle is harder to image, so the relative lack of direct support for deep plumes doesn't trouble plume theorists yet.

However, there is a body of theory that explains hotspots not as plumes from the deep, but mostly upwellings from the upper mantle梚ts top 400 kilometers梒aused from above by the stirring of the crustal plates and surface-based cooling. A recent article in Geotimes summarized this school of thought to a wide audience, citing fresh evidence. Clearly the debate is gaining visibility.

Earth scientist extraordinaire Don L. Anderson (of Caltech) has been a key player in this alternative hotspot hypothesis. He argues that the original hotspot theorists made assumptions about the upper mantle that new facts have overturned梩he whole upper mantle is hotter and more fluid than once thought.

Making magma happens easily in the upper mantle, Anderson says, simply by releasing pressure, for example during continental breakups. He traces many hotspots backward in time to starting points at the edges of continents which have since moved away. And he calculates that the insulating effect of thick continental plates builds up heat beneath them, which eventually comes out in hotspots. There is no reason to call for hot magmas from any deeper in the Earth, he believes.

He's not alone. Marine geophysicist Marcia McNutt (MBARI) argued in a recent study that features of the Marquesas hotspot usually explained by hot plumes are really of shallow origin, due to relatively buoyant rocks just beneath the crust. A team led by seismologist Eugene Humphreys (Univ. Oregon) has looked in detail at the Yellowstone hotspot and reported, "Where once a plume origin seemed natural, we now consider a nonplume explanation to be at least as attractive."

Most damningly for plume theory, though, seismic imaging around three major, classic hotspots has not found clear evidence of magma conduits below the transition zone (400 to 660 km deep). The Hawaii hotspot will soon be studied with a large seafloor seismic array to try to focus on that region. At the Iceland hotspot, a team led by Gillian Foulger (Univ. Durham, UK) and Bruce Julian (USGS) reported that the magma conduit appears to bottom out around 400 km. And at the Yellowstone hotspot, Humphreys' team found that the transition zone is cool, not hot.

You can read more about all of this yourself on the Web. See the links below. While some alternative theories, like the expanding Earth, have vanished like an overmatched boxer in a lopsided prize fight, this scientific match looks like a good one.

小邪泄褌褍锌褉邪胁谢褟械褌褋褟褋懈褋褌械屑芯泄 uCoz