Final answer:
Volcanoes happen when plates separate due to decompression melting, where reduced pressure at diverging plate boundaries allows mantle material to melt and rise as magma. Convergent boundaries and mantle hotspots can also produce volcanoes through different processes.
Step-by-step explanation:
Volcanoes occur when tectonic plates separate because this movement creates conditions suitable for the formation of magma. As the plates diverge at places like the mid-ocean ridges, the mantle beneath experiences reduced pressure, leading to decompression melting. This process produces magma that rises to create new crust, resulting in volcanic activity. In contrast, volcanic activity at convergent plate boundaries, such as subduction zones, involves flux melting as water released from the subducting plate lowers the melting temperature of the overlying mantle, also causing magma to form.
Additionally, volcanoes can form away from plate boundaries due to mantle hotspots. These hotspots originate from deep within the Earth and create heat that can melt the mantle and crust, regardless of plate boundaries. An example is the Yellowstone hotspot, capable of producing significant eruptions as North America moves over it.
Understanding the different mechanisms of magma formation is essential to comprehending volcanic activity. Decompression melting occurs at divergent boundaries, flux melting at convergent boundaries, and mantle plumes can generate volcanoes at hotspots.