Magma Origins
Three main layers compose the Earth: the crust, the mantle and the core. The crust is the surface and the underground material near the surface. The mantle is a liquid layer, filled with fiery molten rock. The core is a scorching, solid center that is the center gravity for the planet. In the rock cycle, rocks originate as magma. When magma breaks through the crust it becomes lava. It begins to cool immediately, the speed of which causes the minerals in the magma to crystallize as they solidify.
Slowing Particles
Heat is essentially just the movement of molecules. The faster the molecules of an entity move, the hotter the entity becomes. The molecules in magma are moving exceptionally fast to exist at such high temperatures. However, bodies of magma do not have a standard temperature. Some are hotter than others. The magma that cools the fastest, is the magma with the fastest moving molecules -- the hottest magma. When magma does cool, the hottest magma cools the most quickly. Crystallization is, first, the sudden declination of fast-moving molecules.
Crystal Size
The speed at which magma cools determines the size of crystals formed during crystallization. Magma that cools quickly forms very fine crystals that are not visible to the naked eye. If the magma was relatively cool in its liquid state, it cools more slowly. As a result, it forms larger crystals than hot magma forms when it cools. The longer it takes molten rock to solidify back into solid rock, the larger its crystals are when it is finally solid again.
Solid Crystals
As solid rock masses crystallize from cooled magma, the force of the Earth pushes the rocks above the surface or below. Though magma is beneath the surface, it is exceptionally far down into the planet. An extensive part of the crust exists beneath the topmost portion of the layer on which humans live. Massive, solid rocks can sit beneath the surface, still far above the hot mantle. Rocks beneath the surface are intrusive rocks. Those above the surface are extrusive rocks. Crystallization ends with rocks becoming one of these two rock bodies.