Heat Inside the Earth
We just discussed a lot of observations—a baseball-seam volcano through all the world’s oceans that is making seafloor and splitting and spreading continents and ocean basins. Explaining these observations will tell us much about the Earth and how it works.
The existence of volcanoes, bringing melted rock up from below, tells us that the Earth is hotter inside than at the surface. We learn the same in deep mines and drill holes—the Earth is warmer towards the center (once you get below the top thirty feet or so that are warmed a little by the summer and cooled by the winter). All rocks contain radioactive elements (mostly uranium, thorium, and radioactive potassium, but with some others). Radioactive decay of these makes heat in the Earth, much as radioactivity produces heat in nuclear power plants. Most of the Earth’s heat comes from this radioactive decay, although some heat is left over from when the Earth formed, or is being released as the core freezes, or as the core, mantle, and crust continue to separate and denser materials sink and give up heat from friction.
Heat is just the vibration of the atoms (or molecules, or ions; the little pieces) of which everything is made. More heat causes more vibration. Temperature is a measure of these vibrations; more vibration gives a higher temperature. Heat is moved around in three main ways: radiation, conduction, and convection.
Radiation
Vibrating atoms give off electromagnetic radiation. The electromagnetic radiation that comes from very hot atoms is visible to us and is called light; cooler atoms give off infrared or other electromagnetic radiation we cannot see. When an atom gives off electromagnetic radiation, the atom cools and slows down, unless it receives other energy to speed it up again. You soak up electromagnetic radiation if you lie out in the sun (and may get skin cancer as it damages the DNA in your skin, so doctors advise you to cover up with clothes or sunscreen), and you can feel electromagnetic radiation if you hold your hand off to one side of a hot stove burner. Electromagnetic radiation is a very efficient way to move heat from the sun to the Earth through space and our atmosphere, but radiation is a very inefficient way to move heat through rocks—the radiation doesn’t get very far before it is absorbed. (Thus, you cannot see far through most rocks.)
Conduction
If a rapidly vibrating atom (a hot one) sits next to a slowly vibrating atom (a cold one), collisions between the two will tend to slow down the fast one and speed up the slow one. This process is called conduction and moves heat energy from atom to atom. Conduction is a very rapid process over short distances. (If you foolishly touch a hot stove burner, you will almost instantly realize how quickly it makes the atoms in your skin vibrate rapidly, and how much damage can be done if they vibrate too rapidly and jump out of those places where they are supposed to be in your skin.) Conduction is a very slow process over long distances. Think of 1000 people standing in line. If one person accidentally bumps their neighbor, the response is almost immediate. But, it is rare indeed that the bumped neighbor will bump their neighbor who bumps their neighbor, rapidly moving the disturbance down the line. For the Earth, the distance from the center to the surface is roughly 10,000,000,000,000,000 atoms. The Earth is not old enough for a lot of the heat trapped at its center when it formed to have been conducted to the surface by this neighbor-bumps-neighbor-bumps-neighbor mechanism.
Convection

Convection is the third option for moving heat. Take something hot and move it from here to there. To get heat from the stove to your dinner table, you cook things on the stove and then carry them to the table, which is much more efficient than putting the food on the table and waiting for conduction or radiation to bring heat from the stove.
Nature has a special way of arranging this motion in many things. Heating causes almost all materials to expand because hotter molecules vibrate more rapidly and tend to bounce farther away from each other. This lowers the density of the material; its mass is the same, but it takes up more room. Low-density materials tend to rise, and high-density materials to sink. This leads to convection currents—a material is heated, rises, cools, and sinks. Typically, rising occurs in one place, then the material flows horizontally while it cools, and then it sinks and flows back to the rising point (see the convection figure above). (Technically, "convection" is used when the hot material moves because it was heated and expanded, and "advection" is used when the hot material moves for some other reason, such as you carrying the hot food from the stove to the table, but we follow most introductory texts in simplifying and letting you call it all "convection.") You may have seen the effects of convection currents in the air (we’ll talk about them later, but they produce wind, rain, etc.), and in the boiling water of a pot of spaghetti on the stove. Scientific evidence shows that convection currents occur in the Earth as well. This may seem odd at first because most of the Earth is solid rather than liquid. (Volcanoes come from a depth where there is a little bit of melted rock, but even there most of the rock is solid.) However, sufficiently hot rocks are soft enough to flow slowly. Again, we will discuss this later, but it is the same principle that allows a blacksmith to work hot iron into horseshoes or allows a chocolate bar to droop on a hot day.
Our modern picture of the Earth, then, is that it is heated inside, mostly by radioactivity. That heat cannot escape easily by radiation or conduction. When the Earth was young, the heat stayed in and warmed the planet. When the rocks became hot enough, they began to convect. The planet probably melted completely and convected vigorously, followed by solidification that slowed but did not stop the convection.
In convection, the hotter rocks rise and then spread. Rising occurs beneath the ridges in the oceans. There, the new seafloor is made and then rafted away on the spreading convection cells. Where a spreading zone passes under a continent, the continent is thinned and stretched and may be torn apart to make a new ocean. This is occurring under East Africa in the rift valleys, and in the Basin and Range of the western United States—including Death Valley—and occurred to open the Gulf of California, moving Baja away from the mainland. (There may be convection cells stacked on top of each other in the mantle, and other complexities, including much of the upward flow from deep in the Earth occurring as the hots spots we will meet soon—if we tried to cover all of the wonderful complexity at once in an introductory course, some of you would be overjoyed but many of you would be unhappy—but this is a good start.)

The Earth is layered chemically into a medium-to-high-silica crust, a low-silica mantle, and an iron core (well, there’s a good bit of nickel in the core, too). The Earth is also layered based on its ability to flow rather than break (see the Cross section of the Earth figure above). The lithosphere includes the crust and upper mantle. The lithosphere can flow a little in some places but usually breaks rather than flows if you hit it, squeeze it, or pull it with sufficient vigor. Below the lithosphere in the mantle is the asthenosphere, which generally flows rather than breaks, and from which many spreading-ridge volcanoes come. The mantle also has deeper flows-rather-than-breaks layers that we don't make you learn. And, the core has a liquid outer part and a solid inner part.
The lithosphere is broken into a few big pieces called plates. These float around on the convecting, soft asthenosphere. A plate may include just continental rocks, just the sea floor, or some of each. A plate map is given in chapter 3, in section 3.2 on Olympic National Park. The study of these plates and how they move and interact with each other is called “plate tectonics.”