Gneiss is a high-grade metamorphic rock defined by its characteristic banded structure. Light-colored minerals like quartz and feldspar alternate with darker, iron- and magnesium-rich minerals such as biotite or amphibole. Gneiss forms deep within the Earth’s crust under intense heat and pressure. The process by which solid gneiss turns into magma—molten rock found beneath the Earth’s surface—represents a reversal in the rock cycle, moving back toward the igneous stage. This transformation requires specific conditions found in the deep crust to overcome the strong mineral bonds.
The Necessary Conditions: Extreme Heat and Pressure
The transformation of gneiss into magma requires the rock to be subjected to extreme conditions typically found deep within the continental crust, often associated with mountain-building events. Crustal thickening, where tectonic forces pile rock masses on top of one another, effectively buries the gneiss to great depths where temperatures are significantly higher. The temperature increase with depth is described by the geothermal gradient, which dictates that rock deep in the crust is already very hot, sometimes reaching temperatures in excess of \(600^\circ\) Celsius.
The high temperature is the primary driver of melting, but the corresponding high pressure at these depths complicates the process. High pressure generally works to inhibit melting because it forces the mineral structures to remain compact and solid. Therefore, for melting to begin, the temperature must rise sufficiently to overcome the confining pressure, pushing the rock’s conditions past the solidus, which is the temperature at which melting first begins.
Partial Melting (Anatexis): How Gneiss Becomes Magma
The specific process by which gneiss melts is called anatexis, which refers to the partial melting of crustal rock. Unlike a rock mass that turns completely liquid at a single temperature, the multi-mineral composition of gneiss means different components melt selectively based on mineral stability.
The minerals with the lowest melting points, such as quartz and potassium feldspar, are the first to liquefy. These lighter, silica-rich (felsic) minerals begin to melt while the darker, iron- and magnesium-rich (mafic) minerals like biotite and amphibole remain solid. This selective melting explains why the resulting magma, derived from the liquid fraction, is generally more felsic and silica-rich than the original gneiss. The solid residue left behind after the melt is extracted is a refractory, mafic-rich rock known as the restite.
The partially melted gneiss becomes a migmatite, a hybrid rock containing both metamorphic and igneous components. The lighter, igneous-looking melt portions are called leucosomes, while the darker, solid residue portions are called melanosomes. The amount of melt produced is generally small. The liquid must exceed a certain fraction (estimated to be around 5 to 10 percent) to be able to separate from the remaining solid rock.
The Critical Role of Volatiles in Lowering Melting Points
A significant factor facilitating anatexis is the presence of volatile components, primarily water (\(\text{H}_2\text{O}\)). Water dramatically lowers the temperature required for the rock to melt, a phenomenon known as flux melting. The water acts like a chemical flux, weakening the bonds within the mineral structures. The addition of water can reduce the solidus temperature by \(100^\circ\) to \(200^\circ\) Celsius, allowing melting to occur at depths that would otherwise result only in high-grade metamorphism.
The water necessary for this process is not free liquid water but is chemically bound within the crystal structure of hydrous minerals found in the gneiss, such as micas and amphiboles. As the gneiss is heated, these hydrous minerals break down or dehydrate at temperatures lower than the dry melting point of the rock. The released water mixes with the surrounding rock, lowering its melting point and triggering the liquefaction of the nearby quartz and feldspar. This mechanism is particularly effective in high-pressure environments deep within the crust, where even a small amount of water can significantly promote the generation of magma.
The Journey of the Newly Formed Magma
Once the partial melt is generated, it must separate from the solid restite to become a mobile magma body. The liquid melt is significantly less dense than the surrounding solid rock. This density difference provides the buoyant force necessary for the magma to rise through the crust, collecting into larger bodies.
The buoyant magma moves upward through the crust, often exploiting existing fractures or forcing its way through the surrounding rock. If the magma cools and solidifies before reaching the surface, it forms a large, intrusive body of igneous rock called a pluton. This final step of cooling and crystallization completes the rock cycle, as the newly formed igneous rock, typically granite, is now available for subsequent weathering, erosion, and eventual metamorphism.