What is the primary cause of volcanic eruptions?

The primary cause of volcanic eruptions is the movement of molten rock due to tectonic activity. When tectonic plates give way or shift, they can create pathways for magma, formed from partially melted rock within the Earth’s mantle, to rise towards the surface. As this molten rock, or magma, ascends, it can accumulate within a magma chamber beneath a volcano. The process is influenced by the tectonic setting, such as subduction zones, rift zones, or hotspots, where significant geological activity occurs.

When the pressure of the magma becomes too great, or when gases dissolved in the magma form bubbles, it can lead to an explosive eruption or a more effusive release of lava.

In contrast to the other options, while the pressure from gases trapped in rocks contributes to the dynamics of an eruption, it is fundamentally the movement of molten rock due to tectonic activities that drives the process itself. The movement of water beneath the surface does not directly lead to volcanic eruptions, and the cooling of the Earth’s mantle is a result rather than a cause.