The deep Earth rock cycle involves the constant transformation of solid rock under extreme conditions, a process that can turn a metamorphic rock like gneiss into molten magma. Gneiss is a common rock deep within the continental crust. This article will explain the specific conditions of pressure and temperature, as well as the unique chemical mechanisms required, for gneiss to begin its journey into becoming liquid rock.
Gneiss: A Foundation of Metamorphism
Gneiss represents a high-grade metamorphic rock, meaning it has already endured intense heat and pressure deep within the Earth’s crust. Its defining feature is a characteristic banded or layered appearance, known as gneissic banding or foliation. These bands result from the segregation of minerals into alternating light and dark layers.
The light bands are typically rich in felsic minerals like quartz and feldspar, while the dark bands contain mafic minerals such as biotite mica and amphibole. This composition is broadly similar to granite, which is often the parent rock, or protolith, from which gneiss forms. Gneiss is already stable under conditions of high temperature and pressure, so transforming it into magma requires even more extreme energy input.
Gneiss does not melt uniformly at a single temperature. Instead, the rock must reach a point where its lowest-melting components begin to turn liquid, a process that is highly dependent on the environment. The stability of gneiss under deep crustal conditions sets a high bar for the temperatures needed to initiate melting.
The Geological Environments for Melting
The required conditions for melting gneiss—temperatures exceeding 650°C and very high pressures—are found primarily in two deep crustal settings. The most common setting is within continental collision zones, also known as orogenic belts. When continents collide, the crust thickens significantly, burying rock to depths where both pressure and temperature increase dramatically.
The immense weight of the overlying mountain chain traps heat, causing the deep crustal rocks to heat up over millions of years. This process brings the gneiss close to its melting point. The second setting involves the introduction of heat from below, where hot, mafic magma rises from the mantle.
When this mantle-derived magma intrudes into the cooler, overlying continental crust, it transfers heat through conduction. This thermal input can increase the temperature of the surrounding gneiss enough to trigger melting without any change in pressure.
Anatexis: The Process of Partial Melting
The transformation of solid gneiss into magma is specifically called anatexis, which is the partial melting of crustal rock. This process does not involve the entire rock turning liquid all at once. Instead, only a fraction of the rock melts, producing a magma that has a different chemical composition than the original gneiss.
This selective liquefaction occurs because the melting temperature of a rock is a range based on its different mineral components. The mixture of quartz and feldspar melts at a lower temperature than either mineral would alone. Since gneiss is rich in these felsic minerals, they are the first to melt, leaving the dark, mafic minerals behind as solid residue.
The presence of water or other volatile compounds, such as carbon dioxide, is often the deciding factor in triggering anatexis. This is known as wet melting or flux melting, and it acts like adding salt to ice. Water significantly lowers the solidus, or the temperature at which a rock begins to melt, by disrupting the crystal lattice structure.
If the gneiss is perfectly dry, it may require temperatures exceeding 900°C to melt, which are rarely achieved in the crust. However, if the rock contains even a small amount of water, the melting point can drop by hundreds of degrees, sometimes as low as 650°C. This small amount of water allows the silica-rich components to liquefy, forming a small percentage of melt that can then begin to migrate.
The Resulting Felsic Magma
Because the partial melting process preferentially selects the lower-melting, silica-rich components of the gneiss, the resulting liquid is a highly evolved, felsic magma. This melt is characterized by a high content of silica, typically over 65% by weight, and is rich in elements like potassium and sodium derived from the melted feldspars and quartz.
This high silica content makes the resulting magma highly viscous, which is a property that greatly influences its behavior. The remaining, unmelted portion of the gneiss, composed mostly of the higher-melting mafic minerals, is called the restite. This restite is left behind as a dense, dark residue, chemically distinct from the buoyant magma that separates and rises.
The ultimate fate of this newly formed felsic magma depends on its subsequent journey through the crust. If the viscous magma cools slowly while still deep underground, it will crystallize to form large bodies of intrusive igneous rock known as batholiths, with granite being the most common rock type. If the magma manages to rise quickly to the surface, its high viscosity and trapped volatile content often lead to explosive volcanic eruptions, which solidify into volcanic rocks like rhyolite.