EARTH'S INTERIOR
Key Concepts
- The Earth has four layers: a crust, mantle, outer core,
and inner core.
- The Earth is layered because it underwent differentiation
when it was young and molten.
- The Earth's lithosphere is fractured into plates
which are moving relative to each other.
(1) The Earth has four layers: a crust, mantle, outer
core, and inner core.
Starting from the outside and moving inward.
- (i) Crust: Solid rock (mostly
low-density silicon-rich rock). Under the ocean, the crust is only 5
kilometers thick. Under the continents, the crust is up to 35 kilometers
thick; the rock of which continents are made is slightly lower in density
than the rock of the ocean floor; thus, continents stick up above the ocean
floor, like marshmallows floating in a mug of cocoa.
- (ii) Mantle: Partly made of solid
rock, partly made of semisolid (or plastic) rock. The rock of the mantle is
higher in density and richer in iron than the rock of the crust. The
behavior of the plastic mantle is much like that of Silly Putty. Subject it
to gentle forces, and it flows like a liquid; subject it to a sharp blow,
and it fractures like a solid. The mantle is about 2900 kilometers thick.
- (iii) Outer core: Liquid metal (mainly
iron and nickel). The outer core is about 2200 kilometers thick.
- (iv) Inner core: Solid metal. The
inner core is a sphere about 1300 kilometers in radius.
How do we know that the Earth is layered in
this manner? After all, the deepest wells that have ever been drilled are only
15 kilometers deep or so; a mere pinprick compared to the Earth's radius of 6400
kilometers.
Most of what we know about the internal layering of the
Earth comes from the study of seismic waves, the waves that
travel through the Earth whenever the crust is disturbed by an earthquake.
Earthquakes create two types of seismic waves:
- P waves (Primary, or Pressure
waves): The P waves are nothing more than sound waves which travel through
the Earth. Sound waves, remember, consist of alternate high-pressure and
low-pressure regions, and can travel through both liquids and solids.
- S waves (Secondary,
or Shear waves): The S waves are transverse waves. That is,
in an S wave, individual atoms move from side to side, transverse to the
direction in which the wave travels. Within an S wave, there are no pressure
differences. An S wave, unlike a P wave, can only travel
through a solid; it can't travel through liquids.
Consider an earthquake in Chile. A geologist in Mongolia
(directly opposite Chile) will detect P waves with his seismograph, but will not
detect S waves. ``Aha!'' he says (or the Mongolian equivalent). ``There must be
a layer of liquid between me and Chile, through which the S
waves cannot travel.'' A more detailed analysis of the arrival times and
amplitudes of seismic waves reveals the details of the internal structure of the
Earth.
The temperature, density, and pressure of the Earth all
increase as you approach the center. It is a bit surprising that the outer core
of the Earth consists of liquid metal, but the hotter inner
core consists of solid metal. After all, we usually expect a
material to melt as we raise its temperature, but in the
Earth's core, it's the hot stuff that's solid and the cooler stuff that's
liquid. In fact, it is the very high pressure in the inner core
that keeps it solid. One way to convert a liquid to a solid is to cool it down;
another way is to compress it until the atoms are squeezed together. The
pressure at the center of the Earth is very high; about 3 million times
atmospheric pressure at sea level. This pressure is high enough to keep the
inner core solid, despite the fact that its temperature is roughly 5200 Kelvin
(nearly the same temperature as the Sun's surface).
(2) The Earth is layered because it underwent
differentiation when it was young and molten.
Why is the central 3500 kilometers of the Earth (the inner
and outer core) made of metal, while the outer 2400 kilometers (the mantle and
crust) is made of rock? Why isn't it a jumble of rock and metal mixed together,
reflecting the jumble of rocky and metallic planetesimals that collided to form
the Earth?
When the Earth was very young, still in the process of
being formed, it was heated up by having planetesimals constantly slamming into
it. Consequently, the Earth was hot enough to be molten all the way through,
from surface to center. In a liquid body, dense stuff sinks and less dense stuff
floats. Thus, when the Earth was liquid, the dense stuff (iron and nickel, at
7000 kg/m3) sank to the middle, while the less dense stuff (rock, at
3000 kg/m3) rose to the top. Thus, the Earth is differentiated
(that is, divided into layers, with denser layers on the bottoms) because it was
entirely liquid at one time in its history.
(3) The Earth's crust is fractured into plates which are
moving relative to each other.
The crust and the upper, solid layer of the mantle together
make up the lithosphere. The lithosphere is solid, but it is
also brittle and thin. It rests on top of the semisolid mantle like a thin skin
on a bowl of pudding. The semisolid mantle is in constant motion. Because it is
heated from below by the hot metal of the outer core, the semisolid rock of the
mantle is constantly moving in convection currents. (You can
see convection as you heat a pot of soup on a stove. Hot soup in the middle of
the pot wells upward, moves sideways to the wall of the pot, then sinks again as
it cools down.) The slowly moving material of the mantle drags the lithosphere
along with it. At times, the brittle lithosphere can't stand the stresses
involved, and it cracks. Currently, the Earth's lithosphere is broken up into 16
large sections, or plates. Here in Ohio, we are in the
middle of the North American Plate.
The plates are moving relative to each other, as they are
dragged along by the motions in the mantle. The study of these moving plates is
referred to as plate tectonics. As an example, North America
and Europe, which are located on different plates, are moving apart from each
other at a speed of about 3 centimeters/year. This is not breathtakingly rapid,
to be sure (it's less than the rate at which your fingernails grow), but a speed
of 3 cm/year will carry you completely around the Earth in a billion years.
Since the Earth is over 4 billion years old, there has been enough time for the
continents to have been completely shuffled around. 200 million years ago, all
of the continents were joined together in a single super-continent, which
geologists have named Pangaea. For the past 200 million years or so, ever since
the Americas split away from Europe and Africa, the Atlantic Ocean has been
steadily widening.
The familiar pattern of the continents was very different
200 million years ago, and will be very different 200 million years from now,
thanks to the effects of plate tectonics.
The boundaries between plates are geologically active.
Most volcanoes are located close to plate boundaries, as are the epicenters of
most earthquakes. There are different types of plate boundaries, depending on
whether the plates are being pulled apart or shoved together.
- Mid-ocean rift: Plates are moving
apart, and lava is welling upward from the mantle to create new crust
between them. The Mid-Atlantic Ridge, which runs down the center of the
Atlantic, is a mid-ocean rift.
- Subduction trench: A thin ocean plate
and a thick continental plate are being shoved together. The denser ocean
plate is shoved underneath the continental plate, where it melts and is
recycled back into the mantle. The deep trench just off the coast of Chile
is a subduction trench.
- Mountain-building zone: Two
continental plates are shoved together in a head-on collision. The
continental crust buckles upward to form a very high mountain range. The
Himalayas, located where the Indian subcontinent is colliding with Asia, are
a mountain-building zone.
- Transverse fault: Two plates are
slipping past each other. Since there's a lot of friction between plates,
they don't slide smoothly; instead, they stick together until the pent-up
forces finally break the jam, releasing the stored energy in the form of an
earthquake. The San Andreas Fault in California is a notorious example of a
transverse fault. Los Angeles is being carried northward at a rate of 1
kilometer every 12,000 years.
An excellent illustrated overview of plate tectonics is
available from the United
States Geological Survey.
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