A volcano is an opening in the Earth's surface through which hot melted rock, called magma, can escape. When magma reaches the surface, it is called lava. Some volcanoes erupt gently, and the lava simply flows down the sides of the mountain. Others explode violently, throwing ash and rocks high into the air. Most volcanoes are found near the edges of the large plates that make up the Earth's crust, because this is where the plates move and push against each other. The 'Ring of Fire' around the Pacific Ocean is a famous example. Eruptions can be dangerous and can destroy towns, yet volcanic soil is rich in minerals, so farmers often grow crops on the slopes. Scientists watch many volcanoes carefully with special instruments, which helps them warn people before an eruption begins.

Question: In the last sentence, what does 'which' refer to?