National Geographic
National Geographic: Encyclopedia: Mantle
An in-depth look at the structure and composition of Earth's mantle, with lots of visuals. Covers lithosphere, Mohorovicic discontinuity, asthenosphere, transition zone, lower mantle, the d double-prime region, mantle convection, mantle...
Other
University of Texas Arlington: Isostasy, Gravity, Magnetism, and Internal Heat
An in-depth look at how isostasy works and the processes taking place during glaciation and the melting of glaciers. Looks at gravity and the effects of density differences within the crust and mantle, and at Earth's magnetic field and...
Other
Colorado Geological Survey: Gradient Map
This site provides geothermal maps of Colorado with the intent being to identify possible sources of geothermal energy. It explains how the temperature measurements were taken and the reasons the results must be considered to be...
University of Calgary
University of Calgary: Energy Education: Geothermal Gradient
A good, concise explanation of the geothermal gradient and how and why the Earth's temperature gets hotter and hotter with increasing depth. Includes two diagrams showing this and the heat transfer mechanisms that are taking place.