Earth's Mantle below the Oceans [Preview]

Samples collected from the ocean floor reveal how the mantle's convective forces shape Earth's surface, create its crust and perhaps even affect its rotation

Looking at a globe, one can easily imagine the continents and oceans as eternal, unchanging aspects of Earth's surface. Geophysicists now know that the appearance of permanence is an illusion caused by the brevity of the human life span. Over millions of years, blocks of Earth's rigid outer layer, the lithosphere, move about, diverging at mid-ocean ridges, sliding about along faults and colliding at the margins of some of the oceans. Those motions cause continental drift and determine the global distribution of earthquakes, volcanoes and mountain ranges.

Although the theory of plate tectonics is well established, the engine that drives the motion of the lithospheric plates continues to defy easy analysis because it is so utterly hidden from view. To confront that difficulty, I and other investigators have focused our research on the mid-ocean ridges. The ridges are major, striking locations where the ocean floor is ripping apart. Examination of the composition, topography and seismic structure of the regions along the mid-ocean ridges is yielding results that often run contrary to conventional expectations. More complicated and fascinating than anyone had anticipated, the chemical and thermal processes in the mantle below mid-ocean ridges dictate how new oceanic crust forms. Mantle activity may also cause islands to emerge in the middle of oceans and deep trenches to form at their edges. In fact, these processes may be so potent that they may even subtly affect the rotation of the planet.

The idea that Earth incorporates a dynamic interior may actually have its roots in the 17th century. In his 1644 treatise Principles of Philosophy, the great French philosopher Ren Descartes wrote that Earth had a central nucleus made of a hot primordial, sunlike fluid surrounded by a solid, opaque layer. Succeeding concentric layers of rock, metal, water and air made up the rest of the planet.

Geophysicists still subscribe to the notion of a layered Earth. In the current view, Earth possesses a solid inner core and a molten outer core. Both consist of iron-rich alloys and have temperatures reaching over 5,000 degrees Celsius and pressures well over a million times the pressure at the surface. Earths composition changes abruptly about 2,900 kilometers below the surface, where the core gives way to a mantle much less dense than the core and made of solid magnesium-iron silicate minerals. Another significant discontinuity, located 670 kilometers below the surface, marks the boundary between the upper and lower mantle (the lattice structure of the mantle minerals changes across that boundary because of the different pressure). An additional major transition known as the Mohorovicic discontinuity, or Moho, separates the dense mantle from the lighter crust above it. The Moho lies 30 to 50 kilometers below the surface of the continents and less than 10 kilometers below the seafloor in the ocean basins. The lithosphere, which includes the crust and the upper part of the mantle, behaves like a mosaic of rigid plates lying above a hotter, more pliable lower part of the mantle called the asthenosphere.

Making ridges from mantle
THIS ORDERED, layered structure might seem to imply that Earth's interior is static. On the contrary, the deep Earth is quite dynamic. Thermal energy left over from the time of Earth's formation, augmented by energy released through the radioactive decay of elements such as potassium 40, uranium and thorium, churns the material within Earth. The heat travels across Earth's inner boundaries and sets into motion huge convection currents that carry hot regions upward and cold ones downward. These processes ultimately cause many of the broad geologic phenomena on the surface, including mountain building, volcanism, earthquakes and the motions of continents.

Among the regions offering the best access to Earth's interior are mid-ocean ridges. These ridges dissect all the major oceans, winding around the globe like the seams of a tennis ball, for a total of more than 60,000 kilometers. The Mid-Atlantic Ridge is a part of that global ridge system. A huge north-south scar in the ocean floor, it forms as the eastern and western parts of the Atlantic move apart at a speed of one to two centimeters per year. In addition to the frequent earthquakes that take place there, the summit of the Mid-Atlantic Ridge spews out hot magma during frequent volcanic eruptions. The magma cools and solidifies, thus forming new oceanic crust. The ridge is higher than the rest of the Atlantic basin floor. At progressively farther distances from the ridge, the seafloor deepens with respect to sea level, presumably because the lithospheric plates that move away from the ridge contract as they gradually cool with age.

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