Metamorphic rock forms deep beneath the Earth’s surface under intense heat and pressure, representing a complete physical or chemical transformation of pre-existing rock material. These conditions create dense, durable rocks like gneiss, slate, and marble, which are stable underground but not at the surface. The journey from this hard, crystalline state back to loose fragments is a fundamental process within the Rock Cycle. This transformation requires the rock to be exposed to surface forces that break it down and carry the pieces away.
The Initial Breakdown: Weathering
Before a metamorphic rock can become sediment, tectonic forces must uplift and expose it to the atmosphere and hydrosphere. Once exposed, the rock is subjected to weathering, which is the in-place disintegration and decomposition of the solid mass. This breakdown begins when the pressure that confined the rock deep underground is released, causing the rock to expand and fracture in a process called mechanical exfoliation. These initial cracks provide pathways for water and air, accelerating decay.
Mechanical weathering continues to fracture the rock without changing its chemical composition. Frost wedging is a common process where water seeps into fractures, freezes, and expands by about nine percent, gradually prying the rock apart. Plant roots also contribute to this physical breakdown, growing into existing cracks and exerting pressure as they thicken. Abrasion, the physical scraping of rock by wind-blown particles or water-carried fragments, further reduces the rock into smaller fragments.
Chemical weathering simultaneously alters the rock’s mineral structure, especially as the surface area increases from mechanical fracturing. Hydrolysis, where minerals react with water, transforms minerals like feldspar (common in gneiss) into clay minerals. Oxidation, the reaction of rock minerals with oxygen, often results in the formation of iron oxides, which weaken the rock’s structure.
The dissolution of minerals, particularly in metamorphic rocks like marble, is highly effective. Carbonic acid, formed when atmospheric carbon dioxide dissolves in rainwater, reacts with the calcium carbonate in marble, dissolving it and carrying the material away in solution. This dual action of mechanical force and chemical alteration eventually reduces the solid metamorphic rock mass into individual fragments, ions, and clay particles.
Transporting the Fragments: Erosion and Movement
Once weathering has created fragments, erosion takes over to remove and transport this material from its original location. Erosion is distinct from weathering because it involves the movement of the material, not just its creation. The primary agents of erosion are gravity, running water, wind, and glacial ice, all of which move the weathered debris. Liquid water, in the form of streams and rivers, is a major force, carrying fragments both along the bottom as bed load and suspended within the flow.
The energy of the transporting agent dictates the size and distance the fragments can travel. Larger, heavier fragments, such as cobbles and pebbles, require high-energy environments like fast-moving mountain streams. As these fragments are moved, they continuously collide with one another and the streambed, causing their sharp, angular edges to be knocked off. Consequently, the further the material is transported, the smaller and more rounded the fragments become.
Glacial ice can carry an unsorted mix of fragments, from fine silt to large boulders, far from their source. Wind typically has only enough energy to transport fine-grained material like sand and silt over long distances. The movement of these fragments is a continuous process where both physical and chemical weathering still occur, gradually altering the shape and composition of the pieces as they move toward a final resting place.
Settling Down: Deposition and Sediment Formation
The final stage occurs when the transporting agent loses the energy needed to carry its load, causing the fragments to settle out of the flow. This process is called deposition, and the resulting accumulation of material is classified as sediment. Rivers, for example, deposit their sediment load when they slow down upon entering a lake or the ocean, or when they breach their banks.
The settling of fragments often results in a sorting effect, where the largest pieces are deposited first, closer to the source, while finer material like clay and silt is carried further out into calmer waters. Most material derived from the physical breakdown of metamorphic rock falls into the category of clastic sediment, which consists of physically broken fragments. The chemically dissolved material, such as ions from weathered marble, may also later precipitate out of the water to form chemical sediment. The accumulated layers of these loose fragments now represent the metamorphic rock’s new state as sediment.