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Earth's Layers

Earth is separated into compositionally different layers (cores, mantle, and crust) as shown on the previous page. A simple way to think of these different layers is to imagine a Tootsie Pop, which has a thin wrapper, a thick lollipop layer, and a soft tootsie roll center.
Well, Earth isn't as simple as a Tootsie pop. With Earth, the different layers are a result of:

• Differences in the abundance of chemical elements (and minerals, which are made of these elements) that have developed over 100s of millions of years.
• Differences in how these chemical elements respond to the intense heat and pressure within Earth.

The heat and pressure determines whether the material is solid, partially solid (mushy), or liquid -- and if it is solid -- what minerals will form.

Density and Convection

Two of the principles that drive plate tectonics are the principles of density and convection. Density is a measure of how much mass an object has referenced to a common size (i.e. a cubic inch of feathers weighs less than a cubic foot of sand). Density increases between the Earth's crust and core, where each layer has a higher density. As a result, the outer core floats on the inner core, the mantle floats on the core, and the crust floats on the mantle. (Need help understanding density? Click here for a short Quicktime video on density and how differences in density can cause materials to float over each other.)

Although the crust floats on the mantle, Earth's crust comes in two varieties: Continental and oceanic. Continental crust is the least dense bulk material within Earth, while oceanic crust is relatively denser. This is important because , when crust containing both types of crust collides, the more dense oceanic crust will likely be shoved or sink into the mantle.  Just to reiterate, the more dense crust will sink and the less dense crust will float over it.

Simply having crust broken up into plates doesn't explain plate tectonics. Energy is required to move these plates. That energy is obtained from mantle convection. Convection is a force created when significant temperature differences exist within a material. Basically, the hot material flows towards the cool material. The mantle is very hot, but some areas are relatively cooler and other areas relatively hotter. Any material, whether it's peanut butter or the Earth's mantle, will start to move under the force of convection. Since the plates float on the mantle, they are moved by the convective forces in the mantle, just as a floating canoe is carried by a river's current. (Need help understanding convection?  Click here for a short animation on heat transfer and convection.  Click on the “Convection” and play the animation.)

Lava lamps work under the force of convection and density, where a wax material moves up and down in an oil due to convection. The top of a lava lamp is relatively cool compared to the bottom of the lamp, which creates convection. The density of the wax changes with heat, being lighter than oil when it is hot and denser than oil when it is cool. (The oil's density doesn't change at the temperatures present). View the video below to observe how convection and density changes cause movement in a lava lamp (You can also view this Powerpoint slide from your text publisher to observe this phenomena).