Free The Planets by Dava Sobel Page B

wood to rest myself. It came on suddenly, and lasted two minutes; but the time appeared much longer. The rocking of the ground was most sensible. The undulations appeared to my companionand myself to come from due east; whilst others thought they proceeded from south-west; which shows how difficult it is in all cases to perceive the direction of these vibrations. There was no difficulty in standing upright, but the motion made me almost giddy. It was something like the movement of a vessel in a little cross ripple….”
    Indeed, the continents themselves are voyaging. They ride as passengers aboard great slabs of the earth’s crust in constant motion. In 1912, German geologist Alfred Wegener explains that the east coast of South America complements the western edge of Africa because the two continents are pieces of the same jigsaw puzzle. Once in a prehistoric era they lay cheek by jowl, part of a single land mass Wegener calls “Pangaea” (“All-earth”), surrounded by the waters of “Panthalassa” (“All-sea”), before geological forces pulled them apart.
    Today the Old World and the New continue to recede from each other along a still-widening rift in the mid-Atlantic, where molten material wells up from inside the Earth and lays down new ocean floor. As the Atlantic spreads, the Pacific shrinks. Under the restless coasts of Peru, Chile, Japan, and the Philippines, old, cold ocean floor is plunging back into Earth’s infernal interior, to theaccompaniment of earthquakes and volcanoes, and sometimes catastrophic tsunamis.
    The ocean bottom undergoes constant recycling, and no part of it is older than two hundred million years. The continents, in contrast, stay topside through the ages, eroded but still intact after four billion years. Instead of sinking under each other, the continents wrinkle when the stress of contact deforms their crust: The Appalachian Mountains testify to an ancient collision between Africa and North America, while the ongoing pressure on the Himalayas continues, even now, to increase their altitude.
    Modern explorations conducted by submarine and spacecraft reveal the true, apolitical network of Earth’s borderlines, hidden underwater. Mid-ocean ridges and complementary coastal trenches divide the surface of the globe into a mosaic of some thirty plates, each one carrying a piece of a continent, a part of a sea floor. The mosaic pattern changes as plates separate, collide, or grind sidewise past one another, impelled by the pent-up residual heat of the Earth’s violent birth and ongoing radioactive decay.
    The seismic shocks that pierce the Earth during earthquakes permit the deepest possibleintrospection. They suggest the continents and ocean floors cast only a thin skin, or crust, around the planet. This crust slims to a slender mile under some ocean areas, while the continental crust averages a thickness of twenty miles plus, yet the crust in its entirety accounts for only one-half of one percent of Earth’s mass. The great bulk of the planet (about two-thirds of its mass) consists of the rocky yet fluid mantle roiling between the crust and the core. At the center of the Earth, part of the iron-nickel core has already cooled to a solid ball. Seismologists can hear it rotating independently inside the still-molten outer core, turning almost one second a day faster than the rest of the world.
    Like the hidden levels of the inner Earth, the invisible layers of Earth’s atmosphere have also been charted, from low in the troposphere, up through the stratosphere and mesosphere to the top of the thermosphere. The magnetic field and radiation belts surrounding Earth can be mapped from space. Also from space, a network of global positioning satellites can pinpoint locations—even individuals—on the planet with centimeter accuracy, while laser beam reflectors planted on the Moon by Apollo astronauts gauge the exact Earth-Moon distance.
    Earth’s place in space is now known to such confident