Rock Cycle
Formation Processes · Rock Transformation · Continuous Recycling of Crust (28 Micro-Realms)
The Perpetual Lithospheric Recycling Machine
Rocks do not remain in their original form forever. Driven by internal mantle heat and external solar weathering, the Rock Cycle is a continuous, non-linear web through which old rocks are transformed into new ones—from primary Igneous solidification to Sedimentary lithification, Metamorphic recrystallization, and Subduction re-melting into magma.
I. Formation Processes: Magmatism, Denudation & Lithification
What is the Rock Cycle?
- Continuous process of change
- Old rocks become new
- James Hutton (1788) concept
Dual Energy Drivers
- Internal: Mantle radiogenic heat
- External: Solar & gravity
- Powers surface vs deep loops
Magma Generation
- Asthenosphere & lower crust
- Decompression & flux melting
- Starting reservoir of cycle
Solidification Process
- Cooling & crystallization
- Forms plutonic & volcanic
- First solid rock stage
Primary Parent Rocks
- Igneous = Primary rocks
- Other rocks form from them
- Ancestral crustal feedstock
Tectonic Uplift & Exposure
- Orogeny raises deep rocks
- Unroofing strips overburden
- Exposes rock to atmosphere
Weathering & Fragmentation
- Physical & chemical breakdown
- Turns rock into regolith
- Creates loose clastic detritus
Transport & Deposition
- Rivers, wind, ice, waves
- Carries fragments to basins
- Settles in horizontal layers
Burial Compaction
- Overlying sediment weight
- Expels trapped pore water
- Packs grains tightly together
Cementation & Lithification
- Silica/calcite mineral glue
- Binds loose sediment into rock
- Completes Lithification
II. Rock Transformation Pathways & Cross-Cutting Shortcuts
Igneous → Sedimentary
- Granite weathers on surface
- Quartz becomes Sandstone
- Feldspar becomes Clay/Shale
Igneous → Metamorphic
- Skips sedimentary stage
- Direct P–T recrystallization
- Granite $\to$ Gneiss · Basalt $\to$ Schist
Sedimentary → Metamorphic
- Deep burial & tectonic heat
- Limestone $\to$ Marble
- Sandstone $\to$ Quartzite
Sedimentary → Sedimentary
- Pre-existing sedimentary rock
- Eroded into new fragments
- Recycled into younger strata
Metamorphic → Sedimentary
- Exhumed metamorphic rock
- Broken down by denudation
- Source of new sedimentary rock
Metamorphic → Metamorphic
- Prograde P–T escalation
- Slate $\to$ Phyllite $\to$ Schist
- Re-metamorphism of crust
Anatexis (Complete Melting)
- Temperatures exceed solidus
- Metamorphic melts into magma
- Resets the entire Rock Cycle
Migmatite Boundary Rock
- Half-metamorphic, half-igneous
- Partial melting threshold
- Swirling gneiss + granite veins
Igneous → Magma Shortcut
- Igneous rock subducts directly
- Melts back into magma
- Bypasses surface weathering
Non-Linear Web Nature
- Not a one-way circle
- Any rock can become any other
- Multiple reversible shortcuts
III. Continuous Recycling of Crust (Plate Tectonic Conveyor)
Mid-Ocean Ridge Factory
- Divergent plate boundaries
- Upwelling mantle peridotite
- Birthplace of new basaltic crust
Subduction Conveyor (NCERT)
- Crustal rocks carried down
- Enters mantle via subduction
- Melts due to interior heat
Volcanic Arc Rebirth
- Subducted water lowers melting pt
- Andesitic magma rises upward
- Builds new continental crust
200-Myr Ocean Recycling
- Dense SIMA sinks completely
- Entire ocean floor recycled
- No ocean crust >200 Ma old
Continental Craton Survival
- Buoyant granitic SIAL ($2.7$)
- Resists subduction into mantle
- Preserves 4.0 Ga Archean rocks
Orogenic Root Recycling
- Continental collision zones
- Buries sediments 50–70 km
- Forges regional gneiss & granite
Carbon-Silicate Coupling
- Silicate weathering locks $\text{CO}_2$
- Subduction volcanism vents $\text{CO}_2$
- Stabilizes planetary climate
Conservation of Crustal Mass
- Crustal matter is conserved
- Creation equals destruction
- "No vestige of a beginning"
Core Distinctions
Solidification: Cooling and crystallization of liquid magma/lava into Igneous rock. Lithification: Low-temperature compaction and cementation of loose fragments into Sedimentary rock. Metamorphism: Solid-state recrystallization under high PVT without complete melting into Metamorphic rock.
2. Why the Rock Cycle is a Web, Not a Single CircleAn Igneous rock does not have to become Sedimentary first—it can be buried directly into a Metamorphic rock (Granite $\to$ Gneiss) or re-melted directly back into Magma. Similarly, Sedimentary and Metamorphic rocks can weather directly into new sediments.
3. Why Oceanic Crust is Young (<200 Ma) while Continental is Ancient (4.0 Ga)Dense basaltic Oceanic Crust ($3.0\text{ g/cm}^3$) is continuously subducted into the mantle and melted back into magma, whereas buoyant granitic Continental Crust ($2.7\text{ g/cm}^3$) floats atop the mantle and resists subduction.
NCERT Fig 5.5 Rock Cycle Matrix
Process → Product Conversion Key