Metamorphism describes the transformation of an existing rock (igneous, sedimentary, or metamorphic) into a new type of rock. This process occurs deep within the Earth’s crust when the original rock, or protolith, is subjected to elevated temperature and pressure. The transformation involves physical and chemical changes, such as the growth of new minerals and the development of new textures, while the rock remains largely solid. Understanding the starting temperature is fundamental because heat drives the chemical reactions and mineralogical reorganization that define metamorphism.
Setting the Lower Temperature Boundary
The minimum temperature for the onset of true metamorphism is a transitional range, typically cited between \(150^{\circ}\text{C}\) and \(250^{\circ}\text{C}\). This range marks the shift from subtle, low-energy changes to the energetic, mineralogical reorganization of low-grade metamorphism. Geologists often arbitrarily set the boundary at \(200^{\circ}\text{C}\) for convenience, but the shift is gradual and depends on several factors.
The presence of water or other chemically active fluids is a major variable influencing this threshold. Fluids act as catalysts, allowing chemical reactions to occur more rapidly and at lower temperatures than in a dry system. This fluid-assisted transformation, known as hydrothermal metamorphism, can lower the effective temperature boundary closer to \(150^{\circ}\text{C}\).
The chemical composition of the original rock also determines where metamorphism begins. Rocks rich in hydrous minerals react and change at lower temperatures than stable, anhydrous rocks. Pressure is the third factor, as increased pressure from deep burial promotes the rearrangement of atoms into denser mineral forms. The interplay among temperature, pressure, and fluid content explains why the boundary remains a range.
The Process That Precedes Metamorphism
The geological process immediately preceding metamorphism is diagenesis. This term describes the physical, chemical, and biological changes that occur in sediment after deposition but before true metamorphic conditions. Diagenesis includes compaction (where overlying sediment squeezes out water) and cementation (where dissolved minerals bind sediment grains). These changes are mild and do not fundamentally alter the stability of the rock’s mineral components.
Diagenesis occurs at relatively shallow depths and low temperatures, generally below \(200^{\circ}\text{C}\) to \(250^{\circ}\text{C}\). The transition to very low-grade metamorphism is a continuum with no distinct break in the rock record. The upper temperature limit of diagenesis effectively defines the lower temperature limit of metamorphism.
While diagenetic changes involve the rearrangement of existing material, metamorphism signals the start of new mineral growth. Metamorphism involves the breakdown of minerals stable in the sedimentary environment and the growth of new mineral phases stable at higher temperatures and pressures. This chemical and structural reorganization distinguishes the two processes.
Mineralogical Markers of Initial Change
Geologists identify the start of metamorphism by the appearance of specific mineralogical markers, not just temperature readings. This is evident in fine-grained sedimentary rocks like shales, which are rich in clay minerals. Clay minerals, such as smectite and kaolinite, are stable during diagenesis but become unstable as temperatures rise.
As the rock crosses the minimum thermal threshold, clay minerals undergo progressive dehydration and structural ordering. For example, the disordered layers of smectite gradually convert into the more ordered structure of illite. This process is measurable using the Illite Kübler Index, which assesses illite crystallinity and provides an observable metric for the degree of very low-grade metamorphism.
The first appearance of new, non-clay hydrous minerals also marks the onset of metamorphism. Minerals like chlorite, prehnite, and pumpellyite begin to form, signifying that the rock has entered the very low-grade metamorphic zone (the anchizone). Chlorite, a green, sheet-silicate mineral, is a common product of this initial stage, especially in rocks rich in iron and magnesium. The growth of these specific minerals confirms the rock’s components have reacted chemically to the elevated temperature and pressure.