why do volcanoes erupt? and why do people not have a warning for volcanic eruptions?

1.) Pressure is built up over a large amount of time and so that's why when theres a volcano it's able to explode. 2.) You never know when it might happen. (how much pressure there is)

Volcanoes erupt due to the pressure they exert on the upper layers. The pressure is due to the large amount of heat they produce. People do not have a warning for volcanic eruptions because we cannot predict actually when a volcano is going to erupt.