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12/15/1996 08:00:00

Caltech Geophysicist Offers Evidence For New View of Earth's Inner Workings

SAN FRANCISCO—In two closely related presentations today at the annual American Geophysical Union conference, Caltech geophysicist Don Anderson will describe work suggesting a radical new interpretation of how Earth operates inside. The work is based on recently declassified satellite imagery as well as a revisiting of the issue of primordial helium (the 3He isotope) within Earth.

"It's becoming more and more clear that Earth is driven from above by motion of the lithosphere (the cold outer shell of Earth) and cold 'fingers' sticking down under continents into the mantle," Anderson said in an interview prior to the conference. "So rather than Earth being like a pot on a stove that gets heated from below and boils, it's more like a glass of iced tea where ice cubes cause cold downwellings in the liquid beneath it from thermal convection, and cause cracking in the 'lid' that permits volcanism."

Anderson's presentation of two lectures on the topic is appropriate because he has been working at the problem for the last eight months from two directions. The first, the satellite imagery evidence, is based on highly accurate global satellite gravity data compiled by David Sandwell of the Scripps Institution of Oceanography and Walter Smith of the National Oceanic and Atmospheric Administration.

These maps show that many hot spot tracks (chains of volcanoes) exist along preexisting cracks on the plate. Others exist where new cracks are forming as the oceanic plate bends to go under other plates—for example, at the Chile Trench and near Samoa.

According to Anderson's analysis of the maps, the evidence is compelling that there are five regions in the Pacific Ocean where hot spots of underwater volcanic activity can be associated with new fractures in Earth's crust. Two of the hot spots are located a few hundred miles to the west of Chile, the third is near Samoa, the fourth is on the Easter Microplate near Easter Island, and the fifth is near the Galapagos Islands. Many other "hot spot tracks" are along ancient fractures.

All of the hot spots were previously thought to be random outcroppings, Anderson says. In the old interpretation, these hot spots are caused by the molten mantle burning through the cooler mantle and the crust from a boundary near the core.

However, the nature of the data supplied by the satellite imagery allows the fabric of the crust at those points to be inferred. The structure of the seafloor suggests that these hot spots show fracturing of the crust. The conclusion, then, is that bending of the crust and locations of previous crustal boundaries (faults) are creating weak spots in the lithosphere, which in turn make the hot spots inevitable because hot mantle can penetrate and break through the plate. The fault lines are pressure-release valves.

Another helpful analogy Anderson offers is the polar oceans. In the Arctic and Antarctic, the oceans are not driven by heating from below, but by winds blowing across the surface and icebergs cooling parts of the surface. Earth's mantle, like the Arctic ocean, is driven from above, not below. Even though the ultimate source of energy is from primordial heat and radioactivity, the continents and tectonic plates form a surface template that controls the shape of convection and the locations of hot upwellings.

Anderson's other line of reasoning is based on a reinterpretation of the ratio of primordial helium, or 3He, in Earth's mantle to 4He, which is created by the decay of uranium and thorium. Primordial helium, having two protons and one neutron, was created in the early stages of the universe and is conventionally thought to have sat more or less unperturbed within Earth for billions of years. The 4He isotope, by contrast, has two protons and two neutrons, and has been created far more recently within Earth by the radioactive decay of uranium and thorium.

The previous geophysical interpretation was that a high ratio of primordial helium to 4He in basalt was evidence that the lava is an upgushing from the molten magma thousands of kilometers below the surface. Thus, the primordial helium ratio was thought to support the view that a volcanic eruption was solely the result of an upsurge of the lava from the primordial mantle, because the primordial store of helium could be envisioned as existing only at great depths.

It has always been a mystery, Anderson explains, how any part of Earth could have survived the violent impacts during accretion, or planet-building, without melting or vaporizing. That is, it's difficult to see how any part of Earth could have remained "primordial."

However, Anderson's new interpretation suggests that the ratio may not presuppose a relatively high amount of primordial helium, but rather a relatively low amount of 4He. This is in keeping with evidence that Earth's mantle just below the crust, the lithosphere, is low in uranium and thorium. Cracking of the lithospheric plate allows access to helium-rich "bubbles."

The new interpretation suggests that geological processes cause tectonic plates to stretch and break, and that volcanic lava from an eruption utilizes these weak zones rather than being the result of deep narrow hoselike upwellings of magma. The strange chemistry of hot-spot lavas, such as at Hawaii and Iceland, is the result of near-surface contamination (crustal, lithosphere, and ocean), rather than a property of the deepest mantle.

The heat of the magma is involved in the convection processes, Anderson notes, but the old idea of a chute of molten material blowing all the way from the deepest mantle to the upper surface can no longer be supported.

"Volcanoes with high ratios of helium isotopes can be interpreted as new cracks in the lithosphere," Anderson concludes, noting that the helium evidence further supports the "top-down" evidence from the new satellite imagery. "Volcanoes are more like grass growing through cracks in the sidewalk rather than tree roots which break the pavement.

"This turns everything upside-down."

Written by Robert Tindol