On average, between 50 and 60 volcanoes erupt each year around the world. This amount averages nearly one volcanic eruption per week.
Sometimes, one volcanic eruption can trigger other eruptions in volcanoes that are close by. Generally, the larger a volcano is, the more likely it is that its eruption will trigger other volcanoes into erupting at around the same time.
Most often, volcanic eruptions are brought on by earthquakes or tectonic movement. For example, in 1868, a large earthquake hit the island of Hawaii. The earthquake was almost immediately followed up by a small eruption from Kilauea's southwest rift zone. During that volcano, the Kilauea Iki crater filled with lava.