The Earth’s crust is composed of three main rock types: igneous, metamorphic, and sedimentary. Igneous rocks form from the cooling of magma or lava, while metamorphic rocks are created when existing rocks are transformed by intense heat and pressure. Sedimentary rocks are not formed by significant heat and pressure; instead, this combination of forces generates metamorphic rock. Sedimentary rocks are the result of processes occurring near the Earth’s surface, relying on different mechanisms for their creation.
The Actual Formation of Sedimentary Rocks
Sedimentary rock formation begins with the breakdown of pre-existing rocks through a process called weathering, which can be physical or chemical. Physical weathering mechanically breaks rock into smaller fragments, while chemical weathering dissolves minerals or alters their composition. These fragments, known as sediment, are then transported by water, wind, or ice from their original location.
The moving agents eventually lose energy, causing the sediment to settle out and accumulate in layers, a process called deposition. This deposition frequently occurs in basins, like ocean floors, lake beds, or river deltas, where the layers stack up over geologic time. The process of turning these loose sediments into solid rock is called lithification, which encompasses both compaction and cementation.
Compaction occurs as the weight of the overlying sediment layers increases, squeezing the water out of the pore spaces and pressing the grains closer together. The pressure involved is solely from the overburden of accumulated layers, not the intense, directed pressure found deep within the crust. Following compaction, cementation takes place, where dissolved minerals carried by water precipitate in the remaining pore spaces.
Common cementing agents include quartz, calcite, and iron oxides, which act like a glue to bind the individual sediment grains into a cohesive, hard rock. The temperatures involved in lithification are generally low, corresponding to the geothermal gradient at shallow to moderate depths. This entire sequence of physical and chemical changes that turn sediment into sedimentary rock is collectively referred to as diagenesis.
Defining the Role of Heat and Pressure
Metamorphism, which means “to change form,” transforms existing igneous, sedimentary, or even other metamorphic rocks into a new type. This transformation occurs deep within the Earth’s crust under conditions far exceeding those found during the lithification of sediment. The intense heat and pressure required for metamorphism are what distinguish it from sedimentary processes.
The heat involved in metamorphism can range from around 300° to over 800° Celsius, often sourced from the geothermal gradient or contact with nearby magma bodies. This heat provides the energy needed to reorganize the rock’s internal mineral structure without melting it. The intense pressure, frequently caused by tectonic forces like plate collisions, is directed and non-uniform, unlike the simple vertical pressure of overburden.
This combination of intense heat and pressure causes the minerals within the parent rock to recrystallize, forming new, more stable minerals or causing existing minerals to realign. The realignment of platy or elongate minerals under directed pressure creates a layered texture called foliation, seen in rocks like gneiss and schist. This process fundamentally changes the rock’s texture and mineralogy, creating a denser, more compact material.
Completing the Picture The Rock Cycle
The rock cycle provides a framework for understanding how all three rock types are interconnected and can transition from one form to another over geologic time. This continuous process is driven by the Earth’s internal heat, which causes plate tectonics, and by solar energy, which powers surface processes like weathering and erosion. Rocks are constantly forced out of equilibrium, leading to transformation as they encounter new environments.
An igneous rock, formed from cooling magma, may be uplifted and exposed to the atmosphere, where it undergoes weathering to become sediment. That sediment can then be lithified into a sedimentary rock. Alternatively, the igneous rock could be buried deeply and subjected to high temperature and pressure, transforming it directly into a metamorphic rock.
A sedimentary rock, such as limestone, may be buried deeper still in a subduction zone or mountain-building event, leading to its transformation into a metamorphic rock like marble. If the conditions become extreme enough, the metamorphic rock could eventually melt to form magma, restarting the cycle to create a new igneous rock. Any rock type can be converted into any other type, depending on the geological forces acting upon it.