Sedimentary rocks are a major category of rock formed by the accumulation and cementation of fragments of older rocks, minerals, and organic matter. This process is fundamentally different from the formation of igneous rocks, which crystallize from molten material, or metamorphic rocks, which change due to heat and pressure. The creation of layered sedimentary rock involves a sequential process that begins with the breakdown of existing landmasses. The final rock layers embed a detailed history of the Earth’s surface conditions at the time of their formation.
Creating the Raw Material
The journey to forming a sedimentary layer begins with the deterioration of pre-existing igneous, metamorphic, or older sedimentary rocks exposed at the Earth’s surface. This initial stage is known as weathering, involving two distinct types of breakdown. Mechanical weathering physically fractures the parent rock into smaller pieces, such as when water freezes and expands within rock cracks (frost wedging), which increases the exposed surface area of the rock. Chemical weathering alters the mineral composition through reactions with water, oxygen, or acids, often dissolving minerals into water-soluble ions. Once the rock material is broken down, erosion takes over, involving the removal and transportation of the resulting fragments and dissolved ions away from the source area.
The Mechanics of Deposition
The transportation of weathered material, now called sediment, is carried out primarily by moving fluids like water, wind, or ice. Water is the most widespread agent, carrying sediment in suspension or rolling it along the bottom of a river or ocean floor. Wind transports finer particles, such as sand and dust, while glaciers carry the largest and most varied fragments, from fine rock flour to massive boulders.
Deposition occurs when the transporting medium loses the energy required to hold the sediment. For example, a decrease in flow velocity causes material to settle out of the water column due to gravity. Larger and heavier particles, like pebbles and coarse sand, require more energy to keep moving, so they are the first to drop out of suspension.
As the fluid continues to slow, progressively finer particles, like silt and clay, settle in more tranquil environments, such as lake beds or deep ocean basins. This differential settling based on particle size is a form of sorting that results in distinct layers of material. The accumulation of these settled materials, one layer on top of another, forms the foundation of the future sedimentary rock.
From Sediment to Solid Rock
The transformation of loose, unconsolidated sediment into solid rock is a process known as lithification. This process begins as new layers of sediment pile up, increasing the pressure on the material below. The weight of the overlying layers compresses the sediment grains together, dramatically reducing the empty space, or porosity, between them.
This phase is called compaction, and it also squeezes out much of the water trapped in the pore spaces. Following compaction, the second major step, cementation, takes place. Water carrying dissolved minerals, such as silica or calcium carbonate, flows through the remaining pore spaces. As this mineral-rich water precipitates, or crystallizes, in the gaps between the sediment grains, it acts like a natural glue, binding the fragments into a coherent, solid rock mass.
Understanding Stratification
The most recognizable feature of sedimentary rock is its layered appearance, or stratification, with each layer being called a stratum or bed. The flat surfaces separating these distinct layers are known as bedding planes. These layers form because the conditions of deposition, such as the type of sediment or the energy of the transporting fluid, change over time, creating abrupt boundaries between beds.
Geologists use two fundamental principles to interpret the history preserved in these layers. The Principle of Original Horizontality suggests that layers are initially deposited in nearly horizontal beds because sediment settles under gravity. If a layer is found tilted or folded, it indicates powerful Earth forces disturbed it after lithification. The Principle of Superposition states that in an undisturbed sequence of strata, the oldest layer is found at the bottom, and the youngest layer is at the top.