There are three main types of lava: a'a, pahoehoe and pillow lava. Each one flows at a different rate and has a different mineral composition.
A'a is the slowest flowing type of lava. Its flow is characterized by its rough outer surface that, once hardened, turns into a sharp, spiny surface that makes travel difficult. The second kind of lava is pahoehoe; it is the fastest flowing lava. These thin lava flows can stretch for miles and form a crust that is very smooth. The last kind of lava is pillow lava, which is normally found underwater. As it erupts and cools due to the water, cracks form and lava oozes out, producing a crust that resembles a pillow.