Why do Rocks Melt on Earth, Anyway?

Lava spattering from the Halema`uma`u lava lake at Kilauea on May 12, 2012. Image: HVO/USGS. I get a lot of questions here at Eruptions, but one of the more common themes is the properties of rocks - and specifically why they melt where they melt to produce magma? There are a lot of misconceptions out there about the interior of the Earth, namely that the tectonic plates that we make our home (both the continental and oceanic kinds) are sitting on a "sea of magma" that makes up the mantle. As I've said before, the mantle of the Earth, that layer of silicate rocks that starts at ~10-70 km depth and goes down to the outer core at ~2900 km depth that constitutes a large volume of the planet, is not molten, but rather a solid that can behave plastically. This means it can flow and convect, which is one ...