Yellowstone, USA (a supervolcano case study)

Supervolcanoes are as their name suggests, super.  They are much larger than normal volcanoes and when they erupt, they can change the many characteristics of Earth.  They will eject at least 1,000km³ when they erupt compared to a normal volcano, which release around 1km³ of ash and rock.  Unlike regular volcanoes, they do not have the usual cone like structure, rather they occur in large depressions called calderas.  Calderas are formed when the magma chamber of a supervolcano collapses after an eruption leaving a large bowl shape in the ground.  The magma chamber under Yellowstone is currently 80km long, 40km wide and 8km deep so if and when an eruption occurs, the caldera that is left will cover roughly the same area as London.

10,000km³ of land erupted.  25 mile high ash cloud. Everything within a 100 mile radius of the eruption will be destroyed. The ash in the atmosphere could lower global temperatures by 20°C. 1,000 miles away, the layer of ash that will be deposited on the ground will be up to 10ft thick. Two thirds of the USA will be uninhabitable. 87,000 could be killed. 1 in 3 people affected would die. Huge impact on transport, energy, electricity, agriculture, and general production. The UK would receive the ash cloud 5 days after the eruption. The global economy will be placed under huge pressure and likely collapse due to the damage to the USA

This is a worst case scenario but if and when the volcano erupts, the effects will be considerable.