What if all the volcanoes in the world erupted at once? Well, the ash spewed into the atmosphere from the more massive volcanoes will block out most of the sunlight that reaches Earth, essentially throwing the planet into complete darkness. This event is known as a volcanic winter.
The lack of sunlight means there won’t be much in the way of photosynthesis. This means all plants and crops would die, leaving no food for us or the animals that thrive on plant life. Even if there are some crops that are able to survive the lack of sunlight, the acid rain that comes next will still wipe them out.