Igneous rock can definitively become sedimentary rock, a transformation that is a fundamental part of the planet’s continuous geological recycling system. Earth’s crust is not static; the material making up all rock types is constantly being broken down, moved, and reformed. This ongoing process of change and conversion between the three main rock classes—igneous, sedimentary, and metamorphic—is known as the Rock Cycle. The mechanism is driven by energy from the planet’s interior and external forces like the Sun, water, and atmosphere.
Defining Igneous Rock
Igneous rock forms directly from the cooling and solidification of molten material. This hot, liquid rock is called magma when it remains beneath the Earth’s surface and lava once it erupts onto the surface. The resulting igneous rock is classified based on where it crystallizes and its texture.
Intrusive rocks, like granite, form when magma cools slowly deep within the crust. This slow cooling allows mineral grains time to grow, resulting in a coarse-grained, crystalline texture. Conversely, extrusive rocks, like basalt, form when lava cools almost instantly upon exposure to the atmosphere. This rapid cooling prevents the formation of large crystals, yielding a fine-grained or glassy texture.
Breaking Down the Original Rock
The conversion of igneous rock into sedimentary rock begins with weathering. This process breaks the original rock into smaller fragments, or sediments, without moving the material away from its source. Weathering is categorized into two main types: mechanical and chemical.
Mechanical weathering physically breaks the rock into smaller pieces, increasing the surface area exposed to further breakdown. Frost wedging is a common example, where water seeps into cracks, freezes, expands, and forces the rock apart. Abrasion occurs when particles carried by wind, water, or ice grind against the rock surface, slowly wearing it down.
Chemical weathering alters the chemical composition of the rock, changing original minerals into new ones more stable at the Earth’s surface. Hydrolysis occurs when water reacts with minerals like feldspar, producing clay minerals that weaken the rock structure. Carbonation involves slightly acidic rainwater dissolving certain rock components, such as calcite, and carrying them away.
Once the rock is broken down by weathering, erosion transports the material. Wind, flowing water, glaciers, and gravity act as agents of erosion, moving these sediments away from the original rock formation. This movement is essential because forming a new sedimentary rock requires the accumulation of these transported fragments in a new location.
Forming the Sedimentary Rock
The transported fragments, now called sediment, eventually come to rest through deposition. Sediments are deposited in layers, often in low-lying areas like lake beds, floodplains, or ocean basins, where the transporting energy diminishes. As layers accumulate, the material at the bottom is subjected to increasing pressure from the overlying weight.
This pressure triggers lithification, the process that turns sediment into solid rock. The first step is compaction, where the immense weight squeezes the sediment grains closer together, forcing out trapped water. Compaction reduces the volume and brings the grains into contact, but the fragments are not yet permanently bound.
The final step is cementation, which binds the loose sediments together. Dissolved minerals, commonly silica or calcite, precipitate out of the water circulating through the pore spaces. These crystallized minerals grow between the sediment grains, cementing the fragments into a single, cohesive mass. The result is a clastic sedimentary rock, such as sandstone, shale, or conglomerate, completing the transformation.