Why are people often surprised when a volcano erupts?

Because there isn't a long warning, they sometimes happen while people are sleeping, and the ground starts shaking and you see lava flowing out of a volcano which is pretty rare

It is really difficult to provide reliable predictions of when it's going to erupt, is the right answer.

Explanation:

A volcano is a break in the outside of a planetary-mass thing, for instance, Earth, that concedes boiling lava, volcanic ash, and fumes to flee from a magma body underneath the exterior.

The volcanoes of the Earth occur because the crust of the earth is split into 17 major, rigorous tectonic bases that glide on a boiling, fainter sheet in its mantle. The most important reason why people often get surprised when a volcano erupts is that it is really difficult to provide reliable predictions of when it's going to erupt.

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