July 27, 2006 — A passel of puny undersea volcanoes could be the undoing of a theory that's long held sway over gigantic volcanic hot spots like Hawaii and Yellowstone.
Hot spot volcanoes are far from the fiery edges of tectonic plates and are thought by many geologists to be created by mysterious plumes of hot rock welling up from the Earth’s mantle and melting the crust above it.
But the chemistry of the lava erupting from some teeny hot-spot volcanoes on the floor of the northwestern Pacific Ocean points to much shallower origins, say Japanese Earth scientists.
If they are right, cracking of the brittle 135-million-year-old Pacific Ocean crust as it flexes towards the Kuril and Japan Trenches could be enough to allow small amounts of molten rock to ooze up and generate the small volcanic features on the surface.
And if that is so, there could be a lot of undiscovered "petit spot" volcanoes, as they have been dubbed, on ocean floors around the world.
"A very important trigger of this study is that I had found 6-million-year-old lavas from the Japan Trench, where there is no hot spot and on the old and cold Pacific plate," wrote geologist Naoto Hirano of the Tokyo Institute of Technology and Scripps Institution of Oceanography at the University of California in San Diego, in an online interview with Discovery News.
That initial 1997 discovery of unusually young deep-sea lavas prompted Hirano and his colleagues to undertake more expeditions to map out the ocean floor in that region with side-scan sonar. They further explored any possible volcanoes with remotely operated submersible vehicles to find exactly what was expelling the molten rock.
They succeeded, finding several small volcanic cones ranging in size from one-thousandth of a cubic mile to a quarter of a cubic mile, all made of lavas no older than 8.5 million years old. Their results are featured in the July 27 online issue of Science Express.
What makes the discovery so potentially revolutionary is that that these small volcanoes appear to require no extraordinary heat from below to melt the rocks under the crust. Instead, the lava comes from areas between the crust and the underlying mantle – the malleable zone called the aesthenosphere – where partially melted rocks can exist without any help from "mantle plumes."
When the old, brittle oceanic crust above it is flexed, it cracks and creates conduits for the magma to move up and erupt.
"The volcanoes are so tiny, that the aesthenospheric melting already present is adequate," agreed geophysicist Marcia McNutt, who is president and CEO of the Monterey Bay Aquarium Research Institute. McNutt reviewed Hirano’s work for Science and wrote a commentary on the matter in the same issue.
"It’s not that I think this study proves there are no plumes at all," McNutt told Discovery News. It’s just that there are some volcanic places on Earth that are far from plate boundaries, which have never fit well into the mantle plume theory, she said.
"Now we start to look at all those chains of volcanoes that people forced into the plume model," McNutt said.
