6.5.4: Rocks and the Rock Cycle
- Page ID
- 42668
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Transformation of Earth Materials
The rock cycle (shown in Figure \(\PageIndex{1}\)) illustrates the continuous transformation of rock-forming minerals both above and below the Earth's surface. The cycle begins with magma (molten rock), which cools and solidifies through the process of crystallization to form igneous rocks. Magma that reaches the surface is referred to as lava; its rapid cooling results in fine-grained textures, whereas magma that cools slowly deep underground typically develops large, visible crystals. Igneous rocks can eventually be forced back into the mantle to remelt into magma, be subjected to heat and pressure to become metamorphic rock, or be uplifted to the surface.
Figure \(\PageIndex{1}\) illustrates the rock cycle, showing how rocks and Earth materials are continuously transformed from one type to another through geologic processes. Five major components are shown as connected by arrows:
- Magma at the top
- Igneous Rock on the left
- Sediment at the lower left
- Sedimentary Rock at the lower right
- Metamorphic Rock on the right
Magma to Igneous Rock - An arrow labeled crystallization points from magma to igneous rock. As molten rock cools and solidifies, crystals form and create igneous rock.
Igneous Rock to Sediment- An arrow labeled weathering, erosion, and deposition points from igneous rock to sediment. Exposure at Earth's surface breaks rock into smaller particles that are transported and deposited as sediment.
Sediment to Sedimentary Rock - An arrow labeled compaction and cementation points from sediment to sedimentary rock. Layers of sediment are compressed and cemented together to form sedimentary rock.
Sedimentary Rock to Metamorphic Rock - An arrow labeled heat and pressure points from sedimentary rock to metamorphic rock. Burial and tectonic forces alter the rock without melting it, producing metamorphic rock.
Igneous Rock to Metamorphic Rock - Another arrow labeled heat and pressure points from igneous rock to metamorphic rock. Igneous rocks can also be changed by heat and pressure into metamorphic rocks.
Metamorphic Rock to Magma - An arrow labeled melting points from metamorphic rock to magma. If metamorphic rock is heated enough to melt, it becomes magma again.
The diagram also shows that:
- Metamorphic rock can be broken down by weathering, erosion, and deposition to form sediment.
- Igneous rock can melt directly into magma.
- Sedimentary rock can also be heated, pressured, or eventually melted as part of the cycle.
Once exposed at the surface, rocks undergo weathering and erosion , breaking down into sediment. As these layers of fragments accumulate, they undergo lithification —the combined process of compaction and cementation —to form sedimentary rock. These rocks are not permanent; they may be weathered back into sediment, buried and melted into magma, or transformed through metamorphism.
Metamorphic rocks are created when pre-existing rocks are altered by intense heat, pressure, or both. This pressure can result from the immense weight of overlying material (confining pressure) or from the powerful collision of lithospheric plates (directed pressure). If these rocks are pushed deep enough to melt entirely, they return to the cycle as magma. Alternatively, if they are uplifted to the surface, they are broken down by erosion to provide the raw material for new sedimentary rocks.