The burial of rock fragments, or sediment, marks the beginning of a profound geological transformation. Loose materials like sand, mud, and gravel begin a long journey from surface deposits into the solid structure of rock. This process is driven by the increasing weight and heat from overlying material, fundamentally altering the physical and chemical nature of the fragments. Understanding this transformation reveals how the Earth’s crust is continually recycled, changing soft, unconsolidated sediment into the hard rock layers that make up the planet’s surface.
The Immediate Effects of Burial
As layers of sediment accumulate, the weight of the material above presses down on the fragments below, a process known as compaction. This physical stress is the first effect of burial, reducing the overall volume of the deposit. The overburden pressure forces the individual sediment grains closer together, leading to a significant reduction in the open spaces, or porosity, between them.
This squeezing action is particularly effective in fine-grained sediments like mud and clay, which often have high initial porosity. The pressure physically expels the air and water (pore water) that filled the voids between the fragments. In some sandstones, mechanical compaction can be so intense that softer rock fragments are deformed and mashed against each other. This physical rearrangement accounts for a substantial loss of the original pore space, creating a denser, more tightly packed material.
The Chemical Transformation: Cementation
Following compaction, the chemical process of cementation begins, acting as the primary agent of binding. This involves the precipitation of dissolved minerals from groundwater circulating through the remaining pore spaces. These mineral solutions crystallize in the gaps between the sediment grains, effectively gluing the fragments together.
The composition of this natural “glue” varies depending on the chemical environment. The most common cements are silica (quartz) and calcite (calcium carbonate). Iron oxides and clay minerals can also act as cementing agents, giving the resulting rock distinctive colors and properties. This precipitation process fills the remaining pore space, permanently bonding the formerly loose fragments into a rigid structure.
Diagenesis and the Completion of Lithification
The combined physical and chemical changes (compaction and cementation) are the primary mechanisms of lithification, the process that transforms loose sediment into solid rock. This entire spectrum of alteration that occurs after deposition, but before the onset of metamorphism, is grouped under the term diagenesis. Diagenesis includes all physical, chemical, and biological changes, such as the dissolution and formation of new minerals, that alter the rock’s mineralogy and texture.
The final product of this diagenetic process is a sedimentary rock, such as sandstone or shale. Diagenesis operates over millions of years at relatively low temperatures, typically extending to depths where temperatures are less than 100°C to 200°C. Although lithification marks the point where the sediment becomes a cohesive rock, diagenesis continues to subtly modify the rock’s composition and texture even after solidification.
Deep Burial and Metamorphic Change
For rock fragments buried at the deepest levels, conditions pass the threshold of diagenesis and enter the realm of metamorphism. This transformation occurs as temperatures and pressures become significantly higher than those found in sedimentary environments. Metamorphism involves changes to the rock’s mineral composition and texture without completely melting the material.
Burial metamorphism, a lower-grade form, commonly takes place in sedimentary basins where rocks are buried beneath more than 2,000 meters of overlying sediment. Increased heat at these depths can cause clay minerals in shales to recrystallize or transform quartz-rich sandstone into quartzite. The intensity increases with depth, leading to a progression of rock types; for example, shale might sequentially transform into slate, phyllite, schist, and finally gneiss as the grade of metamorphism rises.