There are no impact craters on Io because it's so volcanically active that any impact crater is covered up by the flow of lava. Io has over 400 volcanoes, which makes it even more volcanically active than Earth. It is the most volcanically active body in the solar system.
Io's volcanism is the result of the stresses of being pulled between Jupiter and the other Galilean moons. The moon, which is unusual in that its crust is made of silicates instead of ice like the other gas giant moons, features a mixture of yellows, reds, blacks and other colors. This is because of the amount of sulfur and sulfur dioxide its volcanoes eject.