How is granite formed from mantle rock?
Granite is an igneous rock, which means it forms when hot liquid rock cools. You are probably familiar with volcanic rocks (rocks that exit volcanoes in a liquid state and cool to solidity at the Earth’s surface). These rocks are extrusive; they come out of the earth, or are extruded. There is another kind of igneous rock called an intrusive rock (because it forms where liquid rock pushes its way in somewhere in the subsurface, or intrudes). Granite is an intrusive rock: it forms far underground. If you have ever looked at a lava rock, you probably did not see any crystals. That is because the crystals are tiny; the rock cooled too quickly for the crystals to grow large. But the rock is made of nothing but crystals (or glass, which forms when even the tiniest crystals do not have time to grow). Granite is made out of big crystals: you can easily see them with the naked eye. This is because the granite cooled slowly underground, shielded by the rock surrounding it so that its great heat
