What is the name of the process where a substance melts under pressure?

The process where a substance melts under pressure is known as pressure melting or pressure-induced melting. In scientific terms, this phenomenon is better described by the term "melting point depression." When pressure is applied to a solid, its melting point can decrease, causing it to melt at a lower temperature than it would under normal atmospheric conditions. This occurs because the applied pressure can disrupt the crystal lattice structure of the solid, making it easier for the solid to transition into a liquid state.

The concept of melting under pressure is dependent on the particular properties of the substance in question. Water is a well-known example that behaves somewhat unusually; it has a property whereby increasing pressure can actually raise its melting point, which is not the case for most materials. This is due to the unique nature of water's solid form, ice, being less dense than its liquid form.

To understand this further, we can look at a principle called Le Chatelier's principle, which suggests that a system will adjust to counteract the change applied to it. In the context of melting under pressure, when pressure is applied to a solid, the system will try to minimize this pressure by changing to a state that occupies less volume (typically, solids have less volume than liquids). Therefore, if increasing pressure causes a substance to have a lower volume in liquid form (which isn't often the case as most substances expand when melting), the melting point is reduced, and the solid will melt at a lower temperature.

Phase diagrams can also provide insight into how pressure affects the state of a substance. A phase diagram shows the preferred physical states of matter at different temperatures and pressures. The line in the diagram that separates solid and liquid phases is known as the melting curve or liquidus line, which can slope negatively or positively depending on whether pressure increases or decreases the melting point.

In geological processes, pressure melting is an important concept, as it describes the behavior of ice in glaciers and the mantle in Earth's interior. The melting of ice under the weight of a glacier can lead to the formation of subglacial lakes, and pressure-induced changes in the Earth's mantle can affect volcanic activity and plate tectonics.