Supervolcanoes

A supervolcano is a volcano capable of producing a volcanic eruption that could eject volcanic over a trillion tonnes of material over 1,000 cubic kilometers. This is thousands of times larger than most historic volcanic eruptions. Yellowstone erupts every 600 thousand years. Super volcanoes happen when magma in the Earth rises into the crust from a hotspot but is can’t break through the crust. Pressure builds in a large and growing magma pool until the crust is unable to contain the pressure. They can also form at convergent plate boundaries and continental hotspot locations (like Yellowstone).

There are 6 known volcanoes; Lake Toba in Indonesia, Long Valley in California, Lake Taupo in New Zealand, Valles Caldera in New Mexico, Aira in Japan, and Yellowstone in the USA.

Supervolcanic eruptions usually cover huge areas with volcanic ash and lava, and cause a long-lasting change to weather, like  triggering a small ice age sufficient to threaten the extinction of species. 73,000 years ago on the island of Sumatra there was a super-eruption of Toba, which caused the deforestation of much of Central India, 800 cubic kilometres of ash into the atmosphere (which has been found India, the Indian Ocean, the Bay of Bengal and the South China Sea), leaving a crater (now the world's largest volcanic lake) that is 100 kilometres long and 35 kilometres wide. It also caused an instant ice age, temperatures dropped by as much as 16 degrees centigrade and is thought to be one of the causes of the extinction of the dinosaurs. 

Primary Effects

•  Destruction of a 500km radius around the blast
• Ash would cover most of USA

Secondary Effects

• Ash would cause instant ice age
• Less sunlight means no food supply causing starvation on a massive scale
• Possible extinction of humans
• Release  of large amounts of gases that could alter the balance of the planet's carbon dioxide