Volcanoes form where weak spots or breaks in the earth's crust allow magma to push toward the surface. When the pressure of gas and magma becomes too great, the volcano erupts. Magma may pour through the vent opening in lava flows or shoot into the air as dense clouds of gas and dust (ash) fall.

Volcanic eruptions can generate mild to moderate earthquakes, mudflows, flash floods, tsunamis, and huge ash clouds that can create intense lightning storms.

In the United States, the chance of eruptions that could damage populated areas is greatest in the active volcano range of the Pacific Rim. The danger area around a volcano can extend hundreds of miles.