Rock cycle: igneous → weathered → sedimentary → metamorphosed → melted → igneous again. All paths possible.
The Rock Cycle
How all rock types are interconnected — Earth continuously recycles its crust
No starting point — all rock types interconvert. Igneous: magma cools → crystalline rock. Weathering + erosion: any rock → sediment → transport → deposition → lithification → sedimentary rock. Heat + pressure (no melting): any rock → metamorphic rock. Melting: any rock → magma → igneous. Shortcuts: igneous can directly metamorphose; sedimentary can melt. Timeframes: igneous crystallization (years to thousands of years); sedimentary lithification (thousands to millions of years); metamorphism and mountain building (millions of years). Plate tectonics drives the cycle: subduction, collision, rifting, volcanism.
Volcanic rocks made from fragmental material — the products of explosive eruptions
Pyroclastic: from explosive volcanic eruptions (high-silica, gas-rich magma). Ash: < 2 mm fragments. Lapilli: 2–64 mm. Blocks and bombs: > 64 mm (bombs = rounded in flight). Volcanic tuff: consolidated ash layers — can be very extensive (Yellowstone tuffs). Ignimbrite (welded tuff): hot ash flow (pyroclastic density current) deposits so hot it welds together. Pumice: frothy glass — so porous it floats. Volcanic breccia: angular fragments. Tephra: collective term for all airfall pyroclastic material. Fallout tephrochronology: widespread ash layers date events (Campanian Ignimbrite, Minoan eruption). Nuée ardente (pyroclastic surge): 700°C gas + rock flowing at 200+ km/h — most deadly volcanic hazard.
Weathering Types
Weathering: physical (breaks apart) vs chemical (changes composition). Climate controls rate — hot + wet = fastest.
Physical vs Chemical Weathering
How rocks break down at Earth's surface — the first step in the sedimentary cycle
Physical (mechanical) weathering: breaks rock into smaller pieces without changing chemistry. Frost wedging (freeze-thaw), thermal expansion/contraction, abrasion, exfoliation (pressure release — forming dome-shaped outcrops like Half Dome). Chemical weathering: alters minerals chemically. Hydrolysis (feldspar → clay + ions in solution — most important), oxidation (iron minerals → hematite, limonite → 'rust'), dissolution (calcite + CO₂ + H₂O → caves). Spheroidal weathering: corners weather fastest → rounded boulders. Climate: tropical wet = fast chemical weathering. Arctic = physical dominates. Differential weathering: less resistant rocks weather faster → creates topographic relief.
Limestone and Carbonates
Limestone: CaCO₃, formed from shells and coral. Effervesces with HCl. Karst topography from dissolution.
Limestone and Carbonate Rocks
The most important sedimentary rock group — biologically produced and tectonically recycled
Limestone: CaCO₃. Origins: bioclastic (shell fragments — coquina), reef (coral, algae), micrite (lime mud), chemical (travertine, tufa). Test: fizzes with dilute HCl (hydrochloric acid). Chalk: soft limestone from foraminifera (Cretaceous). Dolostone/dolomite: CaMg(CO₃)₂ — dolomitization by Mg-rich fluids. Karst: dissolution by slightly acidic rain → caves (Carlsbad Caverns), sinkholes, disappearing streams, springs. Stalactites/stalagmites: CaCO₃ precipitation from dripping water. Economic: building stone, cement (burned limestone → CaO → Portland cement), CO₂ sink, oil reservoir. Carbon cycle: limestone = largest geological carbon reservoir.
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🎓 Common Exam Questions
Q: Explain the rock cycle — how do the three rock types transform into each other?
A: Rock cycle: a continuous series of geologic processes that recycle Earth's materials. No rock type is permanent — each can transform into any other. Igneous formation: magma cools and crystallizes (intrusive) or lava solidifies at surface (extrusive). Weathering and erosion: igneous or metamorphic rock is broken down by physical and chemical weathering, transported, and deposited as sediment. Sedimentary formation: sediment is compacted and cemented (lithification) into sedimentary rock. Metamorphism: any rock subjected to heat and pressure (but not melting) transforms into metamorphic rock — minerals recrystallize into new stable phases. Melting: any rock taken deep enough melts into magma, restarting the cycle. Driving forces: plate tectonics (recycling crust through subduction), mantle convection (delivering heat), and solar energy (driving surface weathering and erosion). The cycle has no beginning or end — rocks are continuously recycled over millions to billions of years.
Q: Compare the classification of igneous rocks — what determines composition and texture?
A: Igneous rock classification uses two axes: Composition (felsic to mafic) and Texture (coarse to fine). Composition axis: Felsic (high Si, Al — quartz, K-feldspar, Na-plagioclase): granite (intrusive), rhyolite (extrusive). Light colored, low density. Intermediate: diorite (intrusive), andesite (extrusive) — hornblende, plagioclase. Mafic (high Mg, Fe, Ca — pyroxene, olivine, Ca-plagioclase): gabbro (intrusive), basalt (extrusive). Dark, dense. Ultramafic (almost no Si — olivine + pyroxene): peridotite (intrusive) — makes up the mantle. Texture axis: Coarse-grained (phaneritic): slow cooling, large crystals visible to eye — intrusive. Fine-grained (aphanitic): rapid cooling, crystals microscopic — extrusive. Glassy: quenched too fast for crystals — obsidian. Porphyritic: two crystal sizes (phenocrysts in groundmass) — two-stage cooling history. Vesicular: gas bubbles preserved — pumice (can float!), scoria.
Q: How are sedimentary rocks classified and what environments do they indicate?
A: Three main categories: (1) Clastic (detrital): classified by grain size. Conglomerate (pebbles, gravel greater than 2mm) — high energy rivers, beaches. Sandstone (sand, 0.0625-2mm) — rivers, beaches, deserts, shallow marine. Siltstone/shale (silt/clay less than 0.0625mm) — low energy: lake bottoms, deep ocean, floodplains. Shale is the most common sedimentary rock. (2) Chemical precipitates: rock salt (evaporite — restricted marine basin), gypsum (evaporite), chert (silica — deep ocean or hot springs), iron formations (BIFs — Precambrian oceans). (3) Biogenic/biochemical: limestone (shallow warm marine — reefs, carbonate platforms), chalk (pelagic foraminifera — open ocean), coal (swamp forests), phosphorite (upwelling zones). Sedimentary structures provide additional environmental clues: cross-bedding (currents), ripple marks (waves or currents), mudcracks (desiccation), graded bedding (turbidites). Reading ancient environments from rock sequences is called sedimentary facies analysis.
Q: Explain metamorphic grade and the Barrovian sequence of index minerals.
A: George Barrow (1893) mapped metamorphic zones in the Scottish Highlands and identified index minerals that define metamorphic grade — now called the Barrovian sequence: Chlorite zone (lowest grade, ~300-400 degrees C, low P): green color from chlorite. Fine-grained slate or phyllite. Biotite zone (~450 degrees C): biotite appears, rock becomes schist (visible foliation). Garnet zone (~500 degrees C): red garnet porphyroblasts appear in schist — very distinctive, used as abrasives. Staurolite zone (~550 degrees C): staurolite forms (cross-shaped twins — called fairy crosses in Appalachians). Kyanite zone (~600 degrees C): kyanite (blue bladed crystals) appears. Sillimanite zone (highest grade, ~650 degrees C+, high P): kyanite transforms to sillimanite (fibrous). At highest grades: migmatite (partial melting begins). Foliation increases through this sequence: slate (very fine) → phyllite (silky sheen) → schist (distinct mica foliation) → gneiss (banded, high-T). The specific index mineral at a given temperature depends also on pressure — three polymorphs of Al2SiO5 (kyanite, andalusite, sillimanite) occur at different P-T conditions.
Q: What is the difference between physical and chemical weathering and how do weathering products form soil?
A: Physical (mechanical) weathering breaks rock into smaller pieces without changing chemical composition: Freeze-thaw (frost wedging): water expands 9% when freezing, shattering rock in repeatedly frozen environments. Common in high mountains and arctic regions. Thermal expansion: daily heating/cooling cycles create stress fractures (especially dark rocks in deserts). Root wedging: plant roots grow into fractures and widen them. Abrasion: rocks collide during transport, rounding and reducing size. Salt crystallization: salt grows in pores, especially in arid coastal areas. Chemical weathering changes mineral composition: Hydrolysis: water and carbonic acid (H2CO3) react with silicate minerals — feldspar → clay minerals + dissolved ions (K+, Na+, Ca2+, Si). The most important weathering reaction. Oxidation: Fe2+ minerals (olivine, pyroxene) oxidize to Fe3+ (hematite, limonite) — produces red/orange/brown soil colors. Dissolution: CO2-rich water dissolves CaCO3 (limestone) — produces karst topography. Soil formation (pedogenesis): weathered mineral material + organic matter + time. Soil horizons develop (O, A, B, C, R). Warm, wet, old soils are deeply weathered (tropical laterites — iron-rich, nutrient-poor). Cold or dry soils are shallowly weathered.