2. The difference between the density of the entire planet and the density of the crust indicates the core must be as dense as iron.

3. Half of the meteorites that fall to Earth are composed of iron and nickel, and if meteorites are a sample of what the inner planets are made of, then the Earth must contain a lot more iron than is found at the surface.

3. The Earth has a strong magnetic field that could only be generated by a large moving mass of a conducting metal like iron.

4. Radioactive elements are those that give off energy when they spontaneously lose particles from their nuclei and change into a different element.

5.1 A lot of heat is required to keep the outer core liquid under the enormous pressures that must be present at that depth.

5.2 The core heat probably comes from heavy radioactive elements that settled to the cores when nearly the entire planet was molten rock early in its formation.

6. A convection current is a movement of material due to differences in temperature.

7. The movement seen in an unstirred cup of coffee is due to convection currents caused by the cooler coffee along the sides descending, and the warmer coffee in the middle rising.

8. The heat of the cores moves upward through the planet, and in some places, this flow of heat produces plumes of slowing moving convection currents of solid rock in the asthenosphere.

2. The upward moving asthenosphere convection currents push up the lithosphere above them, creating the midocean ridges.

3. The convection currents flow under the rigid lithosphere plates away from the midocean ridges, causing the plates to pull apart at the midocean ridges.

4. The movement of the plates on the currents of rock in the asthenosphere causes the continents on the plates to move.