The rock cycle connects Earth's surface and internal processes into one dynamic system.
Rocks are continuously broken down, transformed, and reformed, linking processes like volcanism, weathering, erosion, and mountain building.
The three major rock types are igneous, sedimentary, and metamorphic; they form under very different conditions.
The cycle is not a closed loop with a single starting point; it is better thought of as a network or web where any rock type can become another given the right conditions.
Earth has two main energy sources that drive the cycle:
Heat from interior: leftover heat from formation (approximately 4.5 × 10^9 years ago) and ongoing radioactive decay of unstable isotopes inside Earth. Heat drives mantle convection, which powers tectonics, volcanism, mountain building, and metamorphism.
Sun’s energy at the surface: solar radiation powers the hydrologic cycle (evaporation, precipitation, rivers) and fuels weathering and erosion, moving sediment across the landscape.
Compared to Mars, Earth has much more internal heat and an active hydrologic cycle, enabling a dynamic rock cycle; Mars has much less internal heat and no active hydrologic cycle, so its rock cycle is largely inactive.
In summary, Earth’s rock cycle is a dynamic interaction of surface and interior processes that can recycle any rock into another through time.
What is a Rock?
Rocks are naturally occurring aggregates of minerals, not pure substances.
A rock is made of many mineral grains stuck together.
Example: Granite is composed of quartz, feldspar, and mica.
This composition is a common exam point (granite example used repeatedly).
Igneous Rocks
Igneous rocks form when magma cools and crystallizes.
Bowen’s reaction series (conceptual framework linking temperature, mineral stability, and rock composition):
Discontinuous branch: olivine → pyroxene → amphibole → biotite, as temperature decreases, with each mineral crystallizing from the melt and then reacting to form the next.
Continuous branch: plagioclase feldspar changes from calcium-rich to sodium-rich variants as temperature drops.
Residual phase at low temperatures includes potassium feldspar, muscovite, and quartz.
Mineralogical implications:
Quartz and potassium feldspar often crystallize late and are commonly found together.
Olivine and pyroxene are common in basalts that solidify from hotter magma.
Silica content and rock classification:
Ultramafic: high Fe/Mg, low silica, e.g., peridotite (mantle-derived).
Mafic: ~45–50% silica; dark minerals predominate; e.g., basalt (extrusive) and gabbro (intrusive).
Intermediate: ~55–65% silica; e.g., andesite (extrusive) and diorite (intrusive).
Siltstone and shale: very fine grains; shale splits along bedding planes.
Chemical sedimentary rocks:
Halite (rock salt) from evaporated seawater.
Gypsum formed in evaporative environments.
Chert: microcrystalline quartz formed from silica-rich skeletal remains and detrital silica; valuable for recording chemistry and biology of ancient oceans and lakes.
Biochemical/organic sedimentary rocks:
Limestone: composed largely of shells and skeletal fragments cemented by calcite; chalk is a type of limestone from microscopic plankton remains.
Coal: formed from plant material, progressing from peat to lignite to coal to anthracite with burial, heat, and pressure.
Oil shales: organic-rich rocks that can generate oil and natural gas when buried and heated, stored as source rocks and can migrate into reservoirs.
Practical interpretation of sedimentary records:
Clastic rocks reveal transport and energy of environments (floods, rivers, deserts).
Chemical/biochemical rocks reveal water chemistry and biological activity.
Organic rocks reveal past life and ecosystem conditions.
Notable details:
Chalk and chert provide important paleooceanic records.
Sedimentary rocks form the surface record and preserve layered histories of environments.
Metamorphic Rocks
Metamorphism transforms rocks while remaining solid, driven by heat, pressure, and fluids.
Protolith concept:
The original rock from which a metamorphic rock formed; it can be igneous, sedimentary, or another metamorphic rock.
Agents of metamorphism:
Heat increases mineral stability and promotes recrystallization.
Pressure includes confining pressure (equal from all directions) and directed stress (unequal, common during collisions).
Fluids (water-rich) circulate through rocks, transporting ions and speeding reactions to form new minerals.
Time is important; long durations allow substantial mineral changes and growth.
Metamorphic textures:
Foliated: minerals align in parallel layers or bands due to directed pressure; shows directional stress.
Nonfoliated: lack of preferred orientation; typically dominated by a single mineral that recrystallizes into an interlocking mosaic.
Regional vs contact metamorphism:
Regional metamorphism occurs on large scales during tectonic collisions and mountain-building, producing foliation (eg, slate to phyllite to schist to gneiss).
Contact metamorphism is localized near heat sources such as intruding magma; results in nonfoliated rocks (eg, quartzite, marble).
Mineral stability and metamorphic grade:
Mineral assemblages reflect stability fields at higher temperatures and pressures; progression with increasing grade yields minerals such as garnet and kyanite in higher-grade rocks.
Shale can become slate (low-grade), then phyllite, schist, and finally gneiss at higher grades.
Limestone can become marble; sandstone can become quartzite.
Practical significance:
Metamorphic rocks record deep crustal history, including plate tectonics and mountain-building processes.
Textures and mineral assemblages provide clues to the pressure-temperature trajectory of rocks.
Quick field implications:
Slate exhibits slaty cleavage and breaks along plains; Schist shows shiny mica crystals; Gneiss displays distinct light-dark banding; Marble and Quartzite are resistant to weathering as building stones.
The Rock Cycle in Practice
The rock cycle is a dynamic, interconnected system rather than a simple loop.
Uplift exposes rocks to surface processes; subduction transports rocks downward, leading to metamorphism or melting.
Volcanic activity returns materials to the surface, closing the surface part of the cycle.
Plate tectonics is a key driver of the cycle: it creates mountain belts, volcanic arcs, ocean basins, and uplifted terrains.
The cycle links internal energy (Earth’s heat) with external energy (solar energy and hydrological cycle).
Practical implications and relevance:
Sedimentary rocks record surface environments and life; igneous rocks reveal magma evolution and crust formation; metamorphic rocks tell deep crustal histories and tectonic processes.
The rock cycle informs resource exploration, such as building stones (granite, marble, quartzite) and hydrocarbon resources (oil shales, coal-derived deposits).
The big takeaway is that the rock in your hand could have originated as lava, beach sand, or deep crustal rock, illustrating Earth as a system in constant change.
Quick Takeaways
The rock cycle demonstrates that Earth materials are constantly recycled through igneous, sedimentary, and metamorphic processes.
Each rock type forms in characteristic ways:
Igneous: crystallize from magma or lava.
Sedimentary: form from weathered material or precipitation; cementation and compaction lithify sediments.
Metamorphic: form from heat and pressure without melting; new minerals and textures arise.
Classification hinges on texture and mineral composition:
Texture refers to grain size and foliation.
Composition ranges from ultramafic to felsic (ultramafic, mafic, intermediate, felsic).
Rocks are records of Earth processes, plate tectonics, and past environments, including life.
Studying rocks is about reconstructing Earth’s history, often one rock at a time, by interpreting textures, mineralogy, and depositional or tectonic context.