2. Volcanoes

3. Magma Reaches the Earth’s Surface KEY CONCEPT:When a Volcano erupts, the force of the expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the vent. The volcano has a pocket of magma below the surface called a magma chamber. A long tube, called a pipe, connects the magma chamber to the surface. The opening at the top of the pipe is called the vent. When it leaves the volcano magma becomes lava.

4. Kinds of Eruptions KEY CONCEPT:Geologists classify volcanic eruptions as quiet or explosive. How a volcano erupts depends on the magma. A quiet eruption moves slowly because it is low in silica and flows very easily. An explosive eruption has magma that is high in silica and does not flow easily. Magma builds up until it explodes out of the vent.

5. Stages of Volcanic Activity KEY CONCEPT: Geologists often use the term active, dormant, or, extinct to describe a volcano’s stage of activity. Active volcanoes are erupting or showing signs that it will erupt soon. Dormant volcanoes are no longer erupting, but can again in the future. Extinct volcanoes are not likely to erupt any longer.

6. Where Volcanoes Form KEY CONCEPT: Volcanoes form along the boundaries of the Earth’s plates. A Volcano is a weak spot in the crust where melted material comes to the surface. Most Volcanoes happen at plate boundaries. Lava can build up to form islands and mountains. Some Volcanoes form above a hot spot when magma erupts through the crust and reaches the surface.