The resulting unit, kg/ms2, is the same as N/m and is also known as the pascal (Pa), the standard unit of pressure (see appendix 1, "Units and Measurements," to understand more). Although here pressure is calculated in terms of pascals, many scientists refer to pressure in terms of a unit called the atmosphere. This is a sensible unit because one atmosphere is approximately the pressure felt from Earth's atmosphere at sea level, though of course weather patterns cause continuous fluctuation. (This fluctuation is why weather forecasters say "the barometer is falling" or "the barometer is rising": The measurement of air pressure in that particular place is changing in response to moving masses of air, and these changes help indicate the weather that is to come.) There are about 100,000 pascals in an atmosphere, so the pressure of the woman's high heel is about the same as 57.8 times atmospheric pressure.

What is atmospheric pressure, and what causes it? Atmospheric pressure is the force the atmosphere exerts by being pulled down toward the planet by the planet's gravity, per unit area. As creatures of the Earth's surface, human beings do not notice the pressure of the atmosphere until it changes; for example, when a person's ears pop during a plane ride because the atmospheric pressure lessens with altitude. The atmosphere is thickest (densest) at the planet's surface and gradually becomes thinner (less and less dense) with height above the planet's surface. There is no clear break between the atmosphere and space: the atmosphere just gets thinner and thinner and less and less detectable. Therefore, atmospheric pressure is greatest at the planet's surface and becomes less and less as the height above the planet increases. When the decreasing density of the atmosphere and gravity are taken into consideration, it turns out that atmospheric pressure decreases exponentially with altitude according to the following equation:

o where g is the gravitational acceleration of that planet, and p is the density of the atmosphere at the planet's surface.

Just as pressure diminishes in the atmosphere from the surface of a planet up into space, pressure inside the planet increases with depth. Pressure inside a planet can be approximated simply as the product of the weight of the column of solid material above the point in question and the gravitational acceleration of the planet. In other words, the pressure P an observer would feel if he or she were inside the planet is caused by the weight of the material over the observer's head (approximated as ph, with h the depth you are beneath the surface and p the density of the material between the observer and the surface) being pulled toward the center of the planet by its gravity g:

The deeper into the planet, the higher the pressure.

Density, pressure, and temperature all rise with depth in the Earth.

space. Heat flow through the crust can be measured, and it is higher where the lithosphere is thinnest and where there is volcanic activity. Fifty percent of heat flow through young continental crust is created by radioactive elements in the crust. Crustal materials are largely originally created from magmas, though they may have gone through enough further processing that they are now classified as metamorphic

Temperature and Pressure in the Earth with Depth

(1,000°C) (2,000°C) (3,000°C) (4,000°C) (5,000°C)

(1,000°C) (2,000°C) (3,000°C) (4,000°C) (5,000°C)

or even sedimentary. When a rock is partially melted, all the elements that do not fit easily into the rock's crystal structure rush into the melt. These are called incompatible elements. The major heat-producing radioactive elements are 238U, 235U (isotopes of uranium), 232Th (thorium), and 40K (potassium), and all of these are incompatible. Granite is an igneous rock that is thought to be produced by two or more sequential melting events (though no one knows definitely how granite is made, which makes it a significant mystery since there is so much granite on Earth). Granite contains especially concentrated incompatible elements, and is thus by far the most radioactive common rock. If you ever have access to a Geiger counter, try measuring the radioactivity of a building built of granite.

Heat is also produced by the growth of the solid inner core. Cooler material sinking to the inner core releases gravitational potential energy, and the solidification of that material also releases energy. This heat is called the latent heat of solidification and results from the fact that a solid is more ordered than a liquid: Converting a disordered system to an ordered system releases heat. Conversely, to melt a solid, this same amount of heat must be added, and it all goes into the physics of melting and not into raising the temperature of the solid.

Two Earth scientists, Adam Dziewonski of Harvard University and Don Anderson of the California Institute of Technology, decided about 20 years ago that there was enough information about the Earth's interior to make a model of temperature, density, pressure, and a number of other parameters for the interior of the Earth, from the surface to the center (see figure on page 54). This model is called the Preliminary Reference Earth Model, or PREM. There is now a movement in the Earth sciences to update this model with a new model called the Reference Earth Model, or REM, but for the moment, the data from PREM is a fine place to start.

Though very little material from deep inside the Earth is directly accessible to scientists, a number of techniques are used to learn more about the Earth's interior. Seismic waves take the equivalent of an X-ray of the planetary interior, slowing when passing through hot or wet material, and speeding when passing through cold, stiff material. Shear waves (S-waves) cannot pass through liquid, and so revealed the presence of the liquid outer core of the Earth. This same liquid outer core forms the basis of theories for the formation of the magnetic field.While the crust of the Earth is accessible and provides samples of melts of the planet's interior, the bulk, unmelted composition of the mantle has to be inferred from a few samples, from information about meteorite bulk compositions that are thought to mirror the starting materials for forming the Earth, and from the results of laboratory experiments at high pressures. Even for this best-known of planets there are many outstanding questions about its interior, and new hypotheses are regularly presented for the structure and composition of the deep Earth.

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