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Images: Conduits Into Earth's Inaccessible Interior

A FOUNTAIN OF CLUES—Lava erupting on volcanic islands such as Hawaii is brought to the surface by narrow plumes of hot, buoyant rock originating in the mantle. The volcanic rocks contain chemical "fingerprints" that reveal their age, source, and formation—giving geochemists clues to the inner workings of the mantle.

The underlying "flow"of materials in the mantle drives geological phenomena at Earth's surface, ranging from earthquakes and volcanoes to the creation of mountains and oceans.

Shielding his face with an asbestos glove, WHOI geochemist Stan Hart uses a rock hammer to collect freshly erupted lava samples from Kilauea Volcano in Hawaii. The lava, orginating from Earth's mantle, provide chemical clues to the inner workings of the planet. (Albrect W. Hofmann.)

By analyzing radioactive isotopes in rocks from various volcanic islands, geochemists have determined that all islands are not alike. Most islands are made of mixtures of many rock types. But some islands are primarily composed of one of four chemically distinct rock types: EM1 (Enriched Mantle 1), EM2 (Enriched Mantle 2), HIMU (HIgh "MU," from the Greek symbol µ); and DMM (Depleted Mid-ocean ridge basalt Mantle). These chemical distinctions provide clues to understand the underlying mantle plumes that create the islands. (Bruce Heezen and Marie Tharp. ? 1977)