How can volcanic eruptions change the surface of the earth?
Volcanic eruptions result in the ejection of lava, which destroys plant-life and creates new rock on the surface when it cools.
A volcano is essentially a big hole (or vent) in the Earth's crust. There's a bunch of reasons for eruptions, but the main three are: newer, less dense magma floating up and out, pressure building and eventually releasing, and when a magma chamber is simply too full to hold more.
The resulting lava essentially obliterates any plant-life unfortunate enough to be in the volcanoes line-of-sight. However, volcanoes create more than they destroy; as the erupted magma cools, it produces rock. As the magma from an underwater volcano cools and erupts it forms islands, which is the origin of Hawaii!