Why can volcanoes be both constructive and destructive?

Answer 1

The ash and lava that a volcano erupts can build up the land, which include the mountain of the volcano itself. In the case of underwater volcanoes, this process can created new islands. At the same time, the slopes of volcanoes are often unstable and prone to landslides, which are destructive. Very large eruptions can form large explosion craters and may even result in the volcano collapsing to forma depression called a caldera.