The hidden mantle...
Earth's middle region, the mantle, is where the crust that repaves the ocean's floor is born. The mantle is therefore a focus of our expedition to the unusually tall underwater mountain, where ocean crust may be evolving in a special way.

A hot, high-pressure underworld that scientists can study only remotely using such tools as earthquake wave detectors (seismographs), the mantle is thought to begin 6-8 kilometers below the floors of oceans. It starts about 35 kilometers underneath continents, which have much deeper roots.

Its total thickness is believed to be about 3,000 kilometers (1,900 miles), spanning the distance from our planet's outer crust to its inner core. That's "almost half way to the center of the Earth," said Cann. The temperature may reach as high as 3,000 degrees centigrade (more than 5,000 degrees F).

Animation: Layers of the Earth
Get to know the compositional and physical layers of the earth.
http://earthguide.ucsd.edu/mar/dec5/earth.html

Scientists recognize that the mantle has different layers, based on how those different regions conduct seismic waves. The uppermost section, which also includes the crust, is called lithosphere. Underneath that is the asthenosphere, which extends about 700 kilometers (435 miles) deep.

Because of higher temperatures, materials in the asthenosphere start to soften and flow despite the high pressures there, though much more slowly than molasses. According to Cann, their viscosity, or stiffness, is similar to that of window glass, which, despite appearances, actually flows at an infinitesimally slow pace.

Mantle material from the asthenosphere migrates upward to ocean floor spreading centers, where plates are pulling apart along zones like the Mid-Atlantic Ridge. As the mantle rises there, where the temperature is about 1,300 degree centigrade, it decompresses and part of it begins melting.

Melt derived there is less dense than materials around it. So its chemistry begins to change, with different fractions separating off at different temperatures and densities, and less dense materials, in general, rising to the top.

Seawater that percolates down through opening cracks or faults can further alter the evolving minerals in important ways after they crystallize.

What emerges are chemically distinct rocks that - if the scientists are lucky - can reveal much about their mineralogical histories. It's as close as stones get to speaking. That's why investigators on this expedition are carefully collecting rock samples from the slopes of the underwater mountain during Alvin dives and dredging runs.

Today's pages: The Hidden Mantle | Rocky clues | Testing hypotheses, & Imagery | Surprises!