Hawaiian volcanoes recycle rocks.

The slurpy orange lava erupting in Hawaii looks fresh, but it actually contains scraps of reused rock that covered the planet in its early years, according to French and U.S. scientists.

"We find in basalts erupting in Hawaii the traces of ancient sediments," says Francis Albarede of the Ecole Normale Superieure in Lyon. These sediments rained down on the ocean floor some 3 billion years ago and were eventually transported by plate tectonics into the planet's interior, propose Albarede and his colleagues in the Aug. 6 SCIENCE.

Ocean crust rides on giant plates that founder as they age, eventually sinking into Earth's mantle. For 3 decades, scientists have debated what happens to these former surface rocks. Clues collected in recent years have suggested that some volcanic rocks sink into the mantle and then return to the surface in the plume of hot rock feeding Hawaii's volcanoes.

The new study bolsters that idea, says Albarede, by showing that ocean sediments get dragged down and then rise back up along with the volcanic rocks. The evidence comes from isotopes of hafnium and neodymium. The isotopic ratios of lava collected in Hawaii more closely resemble values found in deep ocean sediments than in typical lavas erupting from seafloor fissures.

Many geoscientists suspect that the plume feeding Hawaiian volcanoes comes from the deepest part of the mantle. The new findings therefore suggest that surface material makes its way to the bottom of the mantle before getting recycled back to the surface, says Albarede. If so, the mantle must stir itself completely from top to bottom--a hotly debated issue among earth scientists (SN: 3/20/99, p. 180).

Others hesitate over the depth of recycling. "What the isotopes tell us fairly convincingly is that the Hawaiian plume contains recycled crustal material. I don't think they tell us where the recycled material comes from," says John Lassiter of the Max Planck Institute for Chemistry in Mainz, Germany.