What defines a volcanic eruption?

A volcanic eruption is fundamentally defined by the discharge of magma, gases, and ash from a volcano. When pressure builds up within the Earth's crust due to accumulated magma, it eventually forces its way to the surface, resulting in an eruption. This process can eject not only molten rock (magma) but also volcanic gases and ash clouds that can spread over large areas, affecting climate and air quality.

The other options provide related geological concepts but do not encompass the essence of what an eruption actually entails. The movement of tectonic plates is significant in triggering volcanic activity but is not itself an eruption. Similarly, the formation of a volcano under the ocean describes the process of creating volcanic landforms rather than the act of erupting. The melting of rock underground can contribute to the formation of magma, leading eventually to an eruption, but it does not describe the eruption process itself. Thus, the correct answer highlights the key event that characterizes a volcanic eruption.