Metamorphic rocks are transformed from a previous rock type (igneous, sedimentary, or older metamorphic) due to intense heat and pressure deep within the Earth’s crust. This process, called metamorphism, alters the rock’s mineral composition and texture without causing it to melt completely. If a rock melts, it ceases to be metamorphic and becomes igneous rock material. However, metamorphic rocks subjected to the most extreme conditions—at the boundary of the metamorphic and igneous realms—begin a process of partial liquefaction. Therefore, metamorphic rocks can melt, but only under intense conditions that push them beyond the highest grade of metamorphism.
The Role of Temperature, Pressure, and Fluids
The transition from solid metamorphic rock to a molten liquid is controlled by three primary factors: temperature, pressure, and the presence of volatile fluids. Required temperatures generally exceed 650°C to 750°C for common rock compositions. Rocks with a basaltic composition demand greater heat, often requiring 900°C to 1,200°C to initiate melting.
Pressure acts as a restraint on melting, as the weight of overlying rock layers compresses the material and inhibits liquefaction. However, high pressure is necessary to keep the rock buried deep enough to reach these extreme temperatures. The most significant factor influencing the onset of melting is the presence of volatile fluids, primarily water.
Water acts as a flux, dramatically lowering the rock’s solidus—the temperature at which the first melt appears. For example, a dry granitic rock might melt at 1,000°C, but with water present, it can begin to melt at approximately 650°C. This reduction occurs because water molecules weaken the mineral structure’s chemical bonds, facilitating the transition to a liquid state. Therefore, the melting of metamorphic rocks is most often facilitated by water-rich fluids, rather than by heat alone.
The Mechanism of Partial Melting (Anatexis)
When the necessary conditions are met—high temperature, high pressure, and the presence of water—the metamorphic rock undergoes anatexis, the partial melting of crustal rocks. Anatexis is not uniform because metamorphic rocks contain minerals, each with a different melting point. The rock melts selectively, starting with the minerals that have the lowest melting temperature.
This selective melting is governed by eutectic points, the lowest melting point combinations of minerals within the rock. Silica-rich minerals, such as quartz and potassium feldspar, typically melt first because they have lower melting temperatures than iron and magnesium-rich silicates. The resulting liquid is therefore highly felsic, meaning it is rich in silica and aluminum, often possessing a granitic composition.
As the low-melting point components liquefy, they leave behind a solid residue called restite, consisting of the higher-melting point minerals. This restite is enriched in minerals like biotite and garnet, which are rich in iron and magnesium. The resulting rock is called a migmatite, a hybrid showing a mixture of dark, unmelted metamorphic rock and lighter, newly formed igneous material. The amount of melt produced relates directly to the rock’s composition and the degree to which temperature exceeds the solidus.
Completion of the Rock Cycle: Magma Formation
The liquid material generated by anatexis separates from the solid restite and is classified as magma, completing the transition from the metamorphic to the igneous stage of the rock cycle. Since this magma is a product of partial melting, its chemical makeup differs significantly from the original metamorphic rock, being enriched in silica and other easily melted components. This molten material is highly buoyant due to its lower density compared to the surrounding solid rock.
The magma then begins to migrate upward through the crust. If this magma cools slowly deep underground, it crystallizes to form intrusive igneous rocks, such as granite. If the magma reaches the Earth’s surface through fractures or volcanic vents, it erupts as lava and solidifies quickly to form extrusive igneous rocks. The process of melting a metamorphic rock to create magma, which then cools to form an igneous rock, is the mechanism by which the rock cycle continually renews and differentiates the Earth’s crust.