When magma exits the earth's crust onto the earth's surface, a volcano is formed. Volcanos are most usually located in areas where tectonic plates meet. Tectonic plates are sections of the earth's crust that move against each other.
When lava is released onto the surface of the earth it hardens and can build land over time. Islands like Iceland and Hawaii were produced by volcanic land building.
When volcanos erupt they can impact living things by destroying habitat. Near active volcanos, habitats can change and impact the types of species that are able to live in the area. Certain types of bacteria live near volcanic vents in the ocean floor.