Hills are also formed by deep erosion of areas that were raised by disturbances in the earth's crust. Erosion forms hills by carrying away all of the soil on a mountain, causing a hill to be left behind. Humans also make hills by digging soil up and dumping it in a giant pile. Volcanoes are also another way that hills are formed. Volcanoes form hills when they erupt. During the eruption, volcanic ash is spewed through the air; after the eruption, the lava or molten rock hardens and builds up a thick layer of lava rock. The ash falls on the hardened lava causing a layer of ash to form on the hill. When rain falls, this layer of ash mixes with the rainwater to form black colored water. This black water will freeze causing the lava rock to crack and crumble and eventually erode to form a hill.