Volcanoes are the result of pressure forcing molten material from the earth's core through a weakness in the earth's crust.

Magma / Lava is deposited on the outside of the earth's crust, volcanic ash and gases are spewed into the atmosphere.

Occasionally a volcano "explodes" with catastrophic results. The mass is flung into the atmosphere leaving a crater. If in the sea, a resulting tsunami can cause widespread damage. As with Krakatoa, the ash and gases can affect the whole world for many years. Sunshine in areas struggles to penetrate the clouds, and surface temperatures fall.

Ponder This ...

Although we have not recently had a "Krakatoa" what  impact does volcanic activity have in local regions, and the world as a whole ?