Partial melting is a fundamental geological process where a solid rock begins to liquefy, but only a fraction of its material turns into magma. This selective process generates almost all of the magma and volcanic rock found on Earth. It provides the mechanism for separating lighter, molten material from denser, solid rock within the mantle and crust. Understanding this process is key to explaining the diversity of igneous rock types and the chemical evolution of our planet.
The Mechanics of Partiality
A rock does not melt all at once, as a single component solid does, because it is an aggregate of many different minerals. Each mineral within the rock has its own unique melting temperature, governed by its chemical composition and the surrounding pressure. This difference in thermal stability means that when a rock is heated, melting occurs selectively, a process sometimes called fractional melting.
Minerals with lower melting points, typically those rich in silica and alkali elements such as quartz and some feldspars, are the first to liquefy. These components form the initial melt, which is always chemically distinct from the original solid rock. High-temperature minerals, like the iron- and magnesium-rich olivine and pyroxene, remain solid in the residue. The resulting magma is a mixture of the lowest-temperature components, explaining why the melt’s chemistry is more evolved than its source rock.
Triggering Partial Melting
Three primary physical conditions within the Earth’s interior can cause a rock to cross its melting threshold and begin to partially melt. These three mechanisms are responsible for nearly all magma generation globally. They involve changing the rock’s temperature, pressure, or composition to lower its melting point.
Decompression Melting
Decompression melting occurs where hot mantle rock rises toward the surface, such as beneath mid-ocean ridges. Although the temperature of the ascending rock remains nearly constant, the overlying pressure decreases significantly. Since the melting temperature of most rocks decreases as pressure drops, this reduction in confining pressure allows the rock to cross its melting point and liquefy.
Flux Melting
Flux melting is most common in subduction zones where one tectonic plate slides beneath another. The subducting oceanic plate carries water trapped within its minerals and sediments into the mantle. This water, along with other volatile compounds like carbon dioxide, acts as a flux, lowering the melting point of the surrounding mantle rock. The addition of volatiles effectively changes the chemical composition, causing the rock to melt at a lower temperature than it would if it were dry.
Heat Transfer Melting
Heat transfer melting happens when a body of hot magma rises and intrudes into cooler crustal rock. The heat from the intruding magma is transferred to the surrounding crust, raising its temperature until it melts. This process is commonly seen where basaltic magma, formed in the mantle, ponds beneath the continental crust, generating silica-rich magmas from the crustal rock above it.
The Chemical Outcome
The most significant result of partial melting is the chemical differentiation between the melt and the source rock residue. The magma produced is invariably more silica-rich than the parent material. For instance, partial melting of the ultramafic mantle rock, peridotite, yields a silica-poor, but relatively more silica-rich, mafic magma like basalt.
This difference in chemistry occurs because elements are partitioned based on their compatibility with the minerals remaining in the solid residue. Incompatible elements—those that do not easily fit into the crystal structure of the solid minerals—are preferentially swept into the liquid phase. Elements like potassium, uranium, and thorium become highly concentrated in the partial melt.
The separation of this silica-rich, incompatible element-rich liquid from the denser, refractory solid is known as magmatic differentiation. As the melt migrates upward, its chemical composition continues to evolve, creating the spectrum of igneous rock types—from basalt to andesite to granite—observed at the Earth’s surface. This process segregates and concentrates specific elements and compounds.
Geological Significance
Partial melting is the primary engine driving the chemical evolution of the Earth’s outer layers. It is the fundamental mechanism responsible for the creation of new material for the planet’s crust. Without this process, the Earth’s mantle would remain chemically undifferentiated, and the crust would not exist in its present form.
The repeated cycles of partial melting and subsequent magma ascent have led to the formation of the continental crust, which is chemically distinct from the oceanic crust. This process, occurring largely at subduction zones, generates the massive volumes of silica-rich magmas that solidify to form the granitic cores of continents. Partial melting is directly linked to plate tectonics and is the source for all global volcanism.