A volcano forms when molten rock (magma) rises up from below the Earth's crust and erupts onto the surface. The type of eruption (explosive or effusive) depends on factors like the viscosity and gas content of the magma. Explosive eruptions are more dangerous due to pyroclastic flows, ash falls, and their ability to significantly impact climate. Monitoring volcanoes through seismic activity, deformation measurements, and gas outputs can help scientists evaluate hazards and provide warning of impending eruptions to protect nearby populations.