Geology tricks that make rock types stick

Texture reveals cooling rate: coarse (plutonic) = slow cooling, large crystals (granite, gabbro). Fine-grained (volcanic) = fast cooling, small crystals (rhyolite, basalt). Glassy (obsidian) = too fast to crystallize. Porphyritic: two crystal sizes = two cooling stages. Vesicular (scoria, pumice): gas bubbles. Composition (felsic→mafic→ultramafic): Felsic — high Si, K, Na, Al — granite (intrusive), rhyolite (extrusive). Intermediate — andesite/diorite. Mafic — low Si, high Fe, Mg — basalt (extrusive), gabbro (intrusive). Ultramafic — peridotite (mantle). Felsic = light-colored; mafic = dark.

Clastic (detrital): gravel → conglomerate/breccia; sand → sandstone; silt → siltstone; clay → shale (most common sedimentary rock). Sorting and rounding reflect transport distance. Chemical: mineral precipitation from water — halite (evaporite), chert (silica), travertine (CaCO₃). Biochemical/organic: shells + organisms → limestone (most common chemical), chalk (foraminifera), coal (plant matter). Sedimentary structures: cross-bedding (current direction), graded bedding (density current), ripple marks, mud cracks, fossils. 75% of rocks at Earth's surface are sedimentary (but only 8% of volume). Sequence stratigraphy: sea level changes create characteristic stacking patterns.

Foliated: minerals aligned under directed pressure → layering. Slate (low grade) → phyllite → schist (medium, visible micas) → gneiss (high grade, banding). Index minerals indicate metamorphic grade: chlorite (low), biotite, garnet, staurolite, kyanite, sillimanite (high). Non-foliated: no directed pressure, or no platy minerals. Marble (metamorphosed limestone — calcite recrystallizes). Quartzite (metamorphosed quartz sandstone — very hard). Hornfels (contact metamorphism — heat only, no pressure). Contact metamorphism: small scale, around igneous intrusions. Regional metamorphism: large scale, in mountain belts. Blueschist: high pressure, low temperature — subduction zones.

N.L. Bowen (1922): systematic experiments on basaltic melt cooling. Discontinuous branch: olivine (highest T) → Ca-pyroxene → Ca-Na-pyroxene → amphibole → biotite. Continuous branch: Ca-rich plagioclase → progressively Na-rich plagioclase. Converge: K-feldspar → muscovite → quartz (last, lowest T). If crystals removed as they form (fractional crystallization): mafic basalt magma → andesite → rhyolite. Explains: granite (felsic) from basaltic source by fractional crystallization. Goldich Dissolution Series (weathering stability): reversed from Bowen — olivine weathers fastest, quartz slowest.

Granite: quartz (>20%) + K-feldspar + plagioclase + biotite ± hornblende. Coarse-grained (slow cooling in batholiths). Color: pink/red (K-feldspar) or gray. Granodiorite: more plagioclase than K-feldspar (most common 'granitic' rock). Diorite: no quartz, plagioclase + hornblende. Syenite: K-feldspar, little quartz. Batholiths: enormous intrusive bodies (Sierra Nevada, Coast Ranges) — form by partial melting of continental crust or fractional crystallization. Granite is unique to Earth: Moon, Mars = basaltic. Granite requires plate tectonics to form (continental collision, subduction, crustal thickening → melting).

Carboniferous period (359–299 Ma): vast tropical swamp forests (Lepidodendron, Sigillaria). Plant debris accumulated in swamps (anoxic → no decay). Burial → pressure → temperature → grade increases. Peat: 50–60% carbon, partially decomposed — still forming today (bogs). Lignite (brown coal): ~70% carbon, low energy. Bituminous: ~80% carbon, most common, coking coal. Anthracite: ~90–95% carbon, highest grade, hardest, cleanest burning. Most coal = Carboniferous or Permian age (Gondwana coalfields). Burning coal releases carbon fixed 300+ Ma ago → atmospheric CO₂ increase (anthropogenic climate change).

Cross-bedding: inclined layers within horizontal strata — current direction (dunes, rivers, beaches). Can reconstruct paleocurrent direction. Graded bedding: coarse at base → fine at top — turbidite (submarine avalanche). Bouma sequence: characteristic graded turbidite package. Ripple marks: symmetric (waves, bidirectional) vs asymmetric (current, unidirectional). Mud cracks (desiccation): polygonal cracks = periodic wetting and drying → tidal flat or ephemeral lake. Bioturbation: animal burrowing disturbs lamination → trace fossils. Flame structures: soft sediment deformation. Stromatolites: layered microbial mats (Precambrian common, rare today). Each structure tells you about ancient environment.

George Barrow (1893, Scottish Highlands): mapped zones of increasing metamorphic grade using index minerals. Chlorite zone: low grade (~200–300°C). Biotite zone: moderate (~350°C). Garnet zone: ~450°C — first garnet crystals. Staurolite zone: ~550°C — cross-shaped crystals (fairy cross stones). Kyanite zone: high pressure, moderate T. Sillimanite zone: ~650°C+ — highest grade. Al₂SiO₅ polymorphs: kyanite (high P) vs andalusite (low P, contact) vs sillimanite (high T) — pressure-temperature indicator. Pressure facies: zeolite, prehnite-pumpellyite, blueschist (subduction), eclogite (deep subduction, very high P).

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.

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: 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.