Metamorphism is the process where a rock’s mineral composition or texture changes due to physical and chemical conditions different from those under which it originally formed. This transformation occurs while the rock remains in a mostly solid state, distinguishing it from melting to form igneous rock. Burial metamorphism is a specific type of transformation driven primarily by the immense weight of overlying material. It relies on the straightforward increase of heat and pressure that comes with deep interment, occurring without the strong directional forces associated with major mountain-building events. It acts as a gradual bridge between the low-temperature changes of diagenesis and the high-grade transformations of regional metamorphism.
Geological Prerequisites: Deep Sedimentary Basins
The setting for burial metamorphism is a deep sedimentary basin, a geological structure where sediments accumulate continuously and rapidly over millions of years. These basins, often found along passive continental margins or within continental interiors, require a long-term, stable environment for massive layers of rock to stack up. The sheer volume of deposited material sets the stage for the metamorphic process.
As new layers of sediment pile onto older ones, the Earth’s crust gradually sinks, a process known as subsidence. This constant sinking accommodates the accumulating weight, pushing the deepest rocks farther into the crust. The base of these sediment stacks can reach depths exceeding 8 to 15 kilometers, far beyond the shallow depths associated with simple compaction and lithification.
The rocks that undergo this process are typically sedimentary protoliths, such as shales, mudstones, and sandstones. For burial metamorphism to begin, the rocks must be pushed to depths generally greater than 2,000 meters, where pressure and temperature effects become significant enough to trigger solid-state mineral reactions. The intensity of the transformation increases steadily with depth within this thick column of rock.
The continuous, uninterrupted nature of deposition and subsidence is necessary for this type of metamorphism. If sedimentation were to stop or if tectonic forces caused uplift and erosion, the rocks would not reach the required pressure and temperature conditions.
The Mechanism: Pressure and Temperature Gradients
The two primary forces that drive burial metamorphism are pressure and temperature, both of which increase systematically with depth. The pressure component results from the weight of the overlying rock column, known as lithostatic or confining pressure. This pressure is uniform, meaning it is applied equally in all directions, which is a defining characteristic of burial metamorphism.
This confining pressure causes the rock volume to decrease, leading to compaction and a significant reduction in porosity. The weight of the overlying material is measured in gigapascals (GPa) or kilobars and increases linearly with the depth of burial. Importantly, the uniform nature of this pressure means the rock does not experience the strong, one-sided squeezing force known as differential stress, which is common in mountain-building settings.
The thermal component is governed by the geothermal gradient, the rate at which temperature increases within the Earth’s crust. While the average gradient is approximately \(30^{\circ}\text{C}\) per kilometer, burial metamorphic settings often exhibit a lower gradient, typically \(15^{\circ}\text{C}\) to \(20^{\circ}\text{C}\) per kilometer. This lower gradient reflects a relatively stable, non-tectonic environment without nearby magma intrusions.
The temperatures reached are low to moderate, generally peaking below \(300^{\circ}\text{C}\). This range is high enough to destabilize certain minerals and trigger chemical reactions but is significantly cooler than temperatures found in regional or contact metamorphism. The combination of moderate heat and uniform pressure defines the unique pressure-temperature path of this metamorphic process, which relies on the gradual transfer of heat through conduction.
Resulting Mineralogical and Textural Transformations
The onset of burial metamorphism marks the point where the low-temperature processes of diagenesis begin to transition into true metamorphic reactions. This transition typically occurs when the rock reaches temperatures between \(150^{\circ}\text{C}\) and \(200^{\circ}\text{C}\). At these conditions, the parent rock, or protolith, begins to undergo transformative changes to its internal structure.
One of the most significant textural transformations is the loss of water and porosity, a process called dewatering. The increasing lithostatic pressure physically squeezes out fluids, and the rising temperature chemically drives off water molecules bound within the crystal structure of clay minerals. This expulsion of water leads to the formation of a much denser, less porous rock.
A defining textural outcome is the general absence of a strong foliation, or layering, in the resulting metamorphic rock. Because the pressure is applied uniformly in all directions, there is no strong differential stress to flatten and align mineral grains into the planar fabric characteristic of slate or schist. The rock remains largely non-foliated, retaining some of the original sedimentary structures.
Mineralogically, the transformation involves the conversion of unstable, water-rich minerals into new, more stable phases that can exist at the elevated pressure and temperature. For example, common clay minerals like smectite change into new clay minerals such as illite and chlorite. In many lower-grade burial metamorphic rocks, especially those formed from volcanic sediments, a suite of low-temperature minerals known as zeolites will begin to crystallize. The formation of these stable mineral assemblages, such as the zeolite facies, is the concrete evidence that the rock has crossed the threshold from a diagenetic to a metamorphic state.