Download Earths Interior Article Bryson

Excerpt from A Short History of nearly Everything by Bill Bryson
Exploring Earth’s Interior
We know amazingly little about what happens beneath our feet. It is fairly remarkable to think
that Ford has been building cars and baseball has been playing World Series for longer than we have
known that the Earth has a core. We understand the distribution of matter in the interior of our Sun far
better than we understand the interior of the Earth.
The distance from the surface of Earth to the center is 3,959 miles, which isn’t so very far. It has
been calculated that if you sunk a well to the center and dropped a brick into it, it would take only 45
minutes for it to hit the bottom. Our own attempts to penetrate toward the middle have been modest
indeed. One or two South African gold mines reach to a depth of two miles, but most mines on Earth go
no more than about a quarter of a mile beneath the surface. If the planet were an apple, we wouldn’t yet
have broken through the skin.
Until slightly under a century ago, what the best-informed scientific minds knew about Earth’s
interior was not much more than what a coal miner knew-namely, that you could dig down through soil
for a distance and then you’d hit rock and that was about it. Then in 1906, an Irish geologist, while
examining some seismograph readings from an earthquake in Central America, noticed that certain
earthquake waves had penetrated to a point deep within the Earth and then bounced off at an angle, as if
they encountered some kind of barrier. From this he concluded that the Earth has a core. Three years
later a Croatian seismologist studying seismic wave data from an earthquake noticed a similar odd
deflection, but at a shallower level. He had discovered the boundary between the crust and the mantle
(the layer just below the crust). In 1936 a Danish scientist, studying seismographs of earthquakes in New
Zealand, discovered that there were two cores. An inner core we now believe to be solid and an outer
core that is thought to be liquid.
By the 1960s U.S. scientists had grown sufficiently frustrated by how little they understood of the
Earth’s interior that they decided to try to do something about it. They got the idea to drill through the
ocean floor (the continental crust was too thick) to the boundary between the crust and mantle to extract a
piece of the Earth’s mantle. The project became a disaster. The hope was to lower a drill through 14,000
feet of Pacific Ocean water off the coast of Mexico and drill some 17,000 feet through relatively thin
crustal rock. According to one oceanographer, drilling from a ship in open waters is like trying to drill a
hole in the sidewalk of New York City from the top of the Empire State Building using a strand of
spaghetti. Every attempt ended in failure. The deepest they penetrated was only about 600 feet. In 1966
with rising costs and no results, Congress ended the project.
Four years later, Soviet scientists decided to try their luck on dry land. They chose a spot on
Russia’s Kola Peninsula, and set to work with the hope of drilling to a depth of 15 kilometers. The work
proved harder than expected. When they finally gave up, 19 years later, they had drilled to a depth of
12.2 kilometers (7.6 miles). Bearing in mind that the crust of the Earth represents only about 0.3 % of the
planet’s volume and that the Kola hole had not cut even 1/3 of the way through the crust, we can hardly
claim to have conquered the interior.
Earth’s Layers
So how much do we know about what’s inside the Earth? Very little. Scientists agree that the
world beneath us is composed of four layers: a rocky outer crust; a mantle of hot, viscous rock; a liquid
outer core; and a solid inner core. We know that the surface is dominated by silicates, which are
relatively light and not heavy enough to account for the planet’s overall density. Therefore there must be
heavier stuff inside. We know that to generate our magnetic field somewhere in the interior there must be
a concentrated belt of metallic elements in a liquid state. That much is universally agreed upon. Almost
everything beyond that – how the layers interact, what causes them to behave in the way they do, what
they will do at any time in the future – is a matter of at least some uncertainty.
The crust and part of the outer mantle together are called the lithosphere, which in turn floats on
top of a layer of softer rock called the asthenosphere, but using a term like “floats” is not entirely
accurate. To say that the lithosphere floats on top of the asthenosphere suggests a degree of easy
buoyancy that isn’t quite right. Similarly it is misleading to think of the rocks as flowing in anything like
the way we think of material flowing on the surface as lava. The rocks are viscous, but only in the same
way that glass is. It may not look it, but all the glass on Earth is flowing downward under the relentless
drag of gravity. Remove a pane of really old glass from the window of a European cathedral and it will
be noticeably thicker at the bottom than at the top. That is the sort of “flow” we are talking about. The
hour hand on a clock moves about ten thousand times faster than the “flowing” rocks of the mantle.
So all that can be said is that at some slightly indeterminate point as we head toward the center of
Earth we leave the asthenosphere and plunge into pure mantle. Considering that it accounts for 82% of
the Earth’s volume and 65% of its mass, the mantle doesn’t attract a great deal of attention, largely
because the things that interest Earth scientists and general readers alike happen either deeper down (as
with magnetism) or nearer the surface (as with earthquakes). We know that to a depth of about 100 miles
the mantle consists predominantly of a type of rock known as peridotite, but what fills the space below is
uncertain.
Beneath the mantle are the two cores, a solid inner core and a liquid outer core. Needless to say,
our understanding of the nature of these cores is indirect, but scientists can make some reasonable
assumptions. They know that the pressures at the center of the Earth are sufficiently high, something over
three million times those found at the surface, to turn any rock there solid. They also know from Earth’s
history that the inner core is very good at retaining heat. Although it is little more than a guess, it is
thought that in over four billion years the temperature at the core has fallen by no more than 200 degrees
F. No one knows exactly how hot the Earth’s core is, but estimates range from something over 7,000
degrees F to 13,000 degrees F, about as hot as the surface of the sun.
The outer core is in many ways even less well understood, though everyone is in agreement that it
is fluid and that it is the origin of magnetism on our planet. The theory originated in 1949 that this fluid
part of the Earth’s core revolves in a way that makes it, in effect, an electrical motor, creating the Earth’s
magnetic field. The assumption is that the convecting fluids in the Earth act somehow like the currents in
wires. Exactly what happens isn’t known, but it is felt pretty certain that it is connected with the outer
core spinning and with it being liquid. Bodies that don’t have a liquid core, like the Moon and Mars, do
not have magnetism.