The behavior of rocks under varying pressure and temperature conditions is a fundamental aspect of geology. Understanding how rocks respond to changes in these factors is crucial for comprehending geological processes, including metamorphism and magma generation. One common question in geology is whether a decrease in pressure can cause rocks to melt.
Pressure and Melting Points of Rocks
To comprehend the interplay between pressure and rock melting, we must first grasp the basics of rock properties. The melting point of a rock is influenced by various factors, including composition, pressure, and temperature. As pressure increases, a rock’s melting point also rises due to the increase in the cohesive forces holding its constituent minerals together. Conversely, when pressure decreases, the melting point decreases, potentially leading to partial melting.
Partial Melting: A Fundamental Process
Partial melting is a process that occurs when a heterogeneous rock undergoes melting, but only a fraction of its minerals actually transform into a melt phase. This phenomenon takes place in rocks with varying mineral compositions and melting points. When a rock is subjected to reducing pressure, certain minerals may reach their respective melting points while others remain solid. This selective melting leads to the formation of a melt phase within the original rock matrix.
Decompression Melting at Mid-Ocean Ridges
One of the prime examples of rock melting due to pressure decrease can be observed at mid-ocean ridges. These are underwater mountain ranges where tectonic plates are diverging, causing a decrease in pressure on the underlying mantle rocks. The mantle is composed of peridotite, which has a high melting point due to the high pressure at depth. However, as the mantle material rises towards the ocean floor along the mid-ocean ridge, the pressure decreases significantly. Consequently, the peridotite begins to experience decompression melting, resulting in the generation of basaltic magma.
The Formation of Magma Chambers in Subduction Zones
Another case where pressure decrease leads to rock melting is at subduction zones. Here, one tectonic plate is forced beneath another, leading to an increase in pressure on the descending plate. However, as the plate descends into the hotter mantle, it gradually heats up, causing rocks to undergo partial melting. The newly formed magma, being less dense than the surrounding solid rock, rises and accumulates in magma chambers above the subducting plate. This process can eventually lead to volcanic activity, forming arcs of volcanoes on the Earth’s surface.
The following table Melting Points of Common Rocks under Varying Pressure
| Rock Type | Melting Point at 1 atm (°C) | Melting Point at High Pressure (°C) |
| Granite | ~ 1215 | ~ 800-950 |
| Basalt | ~ 1200 | ~ 1000 |
| Peridotite | ~ 1400 | ~ 1200-1300 |
| Gabbro | ~ 1250 | ~ 950-1000 |
| Shale | ~ 800 | ~ 550-700 |
The Role of Water in Rock Melting
Water plays a crucial role in rock melting, especially in subduction zones and continental rifts. When water is present in a rock, it significantly reduces its melting point, allowing the rock to melt at lower temperatures and pressures. This is why subduction zones, where hydrated oceanic crust descends into the mantle, are often associated with intense volcanic activity due to the presence of water facilitating partial melting.
The relationship between pressure and rock melting is a complex and essential aspect of geology. While an increase in pressure generally raises a rock’s melting point, a decrease in pressure can induce partial melting under specific circumstances. This phenomenon is observed in diverse geological settings, such as mid-ocean ridges and subduction zones.
