The Movie: The volcano Spock skydives into will destroy the entire planet of Nibiru if it erupts. Eep.
Real Life: Volcanoes are more than just zits on the faces of planets. They can wipe out entire species and drastically change their planet's structure and atmosphere. Even so, they cannot destroy a planet, and the one in the movie was too small to even do significant damage. If it had been a supervolcano, the stakes would be raised quite a bit, because those bad boys barf magma everywhere. They also cough a huge amount of gas into the atmosphere, which can blot out sunlight, rain down ash, and other apocalyptic stuff. We have many active supervolcanos right here on Earth, the most famous being Yellowstone National Park. Seriously, it's not in Yellowstone, it is Yellowstone. When it blows it could pose a huge threat to much of life on Earth, but for now, it's just a great hangout for bears, wolves, and the humans who love them. Sleep tight.