The earth's layers.
Image courtesy of
Lawrence Livermore National Labs
In 1970 Russian geologists started drilling into the Kola Peninsula, near Finland, hoping to learn more about Earth’s enigmatic insides. After 22 years of digging, work had to stop when the crust turned gooey under the drill bit; at 356 degrees Fahrenheit, the underground rock was much hotter than expected at that depth. The result of the scientists’ grand effort: a tunnel as wide as a cantaloupe extending all of 7.6 miles down.
The Kola borehole is by far the deepest one ever dug, yet it reaches a mere 0.2 percent of the way to the core. The rest of Earth’s interior remains as frustratingly out of reach as it was three centuries ago, when astronomer Edmond Halley suggested that our planet was hollow and filled with life. His ideas seem laughable today, but the truth is, when it comes to the inner Earth, no one knows anything for sure. Might a massive crystal sit at the center? What about a natural nuclear reactor? Are we so sure that the textbook diagram of the Earth sliced open, with nested layers of yellow, orange, and red, reflects reality?
The questions are so compelling that they inspired one geophysicist to draw up blueprints for a journey to the center of Earth. Nobody is doing it just yet; it would require cracking open the ground and pouring in thousands of tons of liquid metal. But that and other far-fetched ideas may inspire the ambitious projects necessary to catch a glimpse of the core—a place just 3,950 miles below our feet and yet, in many ways, less accessible than the edge of the visible universe, 13.8 billion light-years away.
Geophysicists try to explore the architecture of Earth by studying seismic waves that shimmy through the planet. Every year more than a thousand earthquakes register at hundreds of seismic stations, sometimes making their way completely across the globe. The waves travel at differing speeds depending on the materials they flow through, which provides clues about the topography of the interior: Faster-moving waves, for instance, generally indicate denser rock. (It’s a bit like trying to identify a murder victim by examining the damage to the bullet.) The seismological data are combined with information about Earth’s internal density derived from the laws of gravity and with results from extreme-pressure experiments in which materials are squeezed between diamonds to pressures of millions of pounds per square inch.
From all of this indirect evidence, scientists have been able to conjure complex, if often still speculative, ideas about the world below. Here is the picture so far.
The Crust The thin skin of Earth ranges from three miles thick (under some parts of the ocean) to 40 miles thick (under the continents). The crust encompasses the brittle and shifting continental plates; it becomes scarred with mountains when the plates grind together or with deep ocean basins when the plates pull apart.
The Mantle This deep layer of warm rock accounts for two-thirds of the mass of our planet. The solid but pliable rock churns in slow motion, drawing heat from Earth’s center up toward the crust. There is enough turnover so that rock is constantly cycling through; pieces of crust have probably reached all the way down to the bottom of the mantle, about 1,700 miles below the surface. The underside of the mantle—the boundary between it and the liquid outer core—is probably rugged terrain. Think of it as the Earth’s surface turned upside down. The boundary’s mountains may approach the height of the Himalayas, and the constant activity may cause “avalanches,” flows of rock that cascade along the slopes of those inverted peaks.
The Outer Core Made of molten iron, nickel, and other ingredients yet to be determined, the churning liquid outer core may have the viscosity of water, streaming at possibly one to several miles per week with the turbulence of a gargantuan, slow-moving washing machine.
The Inner Core At the center of this spherical body of liquid is the inner core, a ball of iron alloy one-third the size of the moon. This metal ball is broiling hot at 11000 degrees Fahrenheit, comparable to the surface of the sun, but it remains solid because of the enormous weight of all the rest of Earth bearing down on it.
Life thrives on this planet partly because it is protected by the powerful magnetic field generated in the outer core. The swirling motions of the liquid metals there create the conditions for what is known as a geodynamo—a geologic electric generator. How this dynamo was initiated and how it works is mysterious, but it seems that the circulation of liquid metal through a magnetic field (which must have begun eons ago) causes a feedback loop of electricity and magnetism and unleashes a powerful electric current hundreds of miles wide. This current fills the core and is the source of tremendous magnetism; its poles, located roughly at the ends of Earth’s axis, mark magnetic north and south. Dangerous cosmic rays are deflected by the magnetic field back out into space, and our atmosphere remains robust because of this protection.
Although most geophysicists feel confident that they understand the topography of the inner Earth within a resolution of several hundred miles, it is possible that our assumptions from seismic data could be as inaccurate as were our assumptions about other planets when earthbound telescopes were our only source of observation. “If we have only remote information—the information we get from indirect methods—then we get only part of the picture,” says David Stevenson, a professor of planetary science at Caltech.
