Rock cycle Study Guide

📖 Core Concepts Rock Cycle – Continuous transformation among sedimentary, metamorphic, and igneous rocks; driven mainly by plate tectonics and the water cycle. Igneous Formation – Melted rock = magma (below surface) → lava (above surface). Cooling rate determines texture: slow = coarse‑grained (intrusive), fast = fine‑grained or glass (extrusive). Metamorphism – Re‑crystallization under high T & P (regional) or high T, low P near an intrusion (contact). Produces foliation in regional settings. Sedimentation – Weathering → transport → burial → lithification. Produces clastic, biogenic, and precipitate sedimentary rocks. Plate‑Tectonic Drivers – Spreading ridges (new basaltic magma), subduction zones (fluid‑induced melting, arc volcanism), continental collision (mountain‑building → erosion). Water’s Role – Enhances weathering, transports ions, lowers mantle melting point in subduction zones, and links the rock cycle to the carbon cycle. --- 📌 Must Remember Intrusive vs. Extrusive: Intrusive = slow cooling, coarse grains (e.g., granite). Extrusive = rapid cooling, fine grains or glass (e.g., basalt, obsidian). Regional Metamorphism → foliation; Contact Metamorphism → recrystallization near heat source, may cause metasomatism. Three sedimentary types: Clastic – from broken rock fragments. Biogenic – from organisms (fossils, shells). Precipitate – from mineral precipitation (evaporation). Subduction‑zone melt: Fluids from the slab lower mantle wedge melting point → silica‑rich magmas farther from the arc. Wilson Cycle – Cycle of ocean basin opening → spreading → subduction → closure, repeatedly recycling crust. Water‑mediated weathering is strongest on igneous & metamorphic rocks near the surface. --- 🔄 Key Processes Magma Generation Subduction: slab releases H₂O → mantle wedge melts → buoyant magma rises. Spreading ridge: mantle upwelling produces basaltic magma. Igneous Rock Formation Intrusive: magma cools > 10 °C/Myr → large crystals. Extrusive: lava cools < 10 °C/min → fine crystals; instantaneous → glass. Metamorphism Regional: tectonic compression → temperature ↑ + pressure ↑ → foliation. Contact: intrusion heat → temperature ↑, pressure ≈ constant → recrystallization, metasomatism. Sedimentary Cycle Weathering (chemical + mechanical) → erosion → transport → deposition → burial → compaction + cementation (lithification). Plate‑Tectonic Cycle (Wilson) Ridge → new ocean crust → slab moves → subduction → melt → volcanism → sediment → uplift → erosion → new sediments. --- 🔍 Key Comparisons Intrusive vs. Extrusive – Slow cooling → coarse grains vs rapid cooling → fine grains/glass. Regional vs. Contact Metamorphism – Large‑scale, high P & T, foliation vs localized, high T low P, no foliation. Clastic vs. Biogenic vs. Precipitate Sedimentary Rocks – Derived from rock fragments vs organismal remains vs mineral precipitation from solution. Spreading Ridge vs. Subduction Zone Magma – Primarily mafic basaltic melt vs more silica‑rich arc magmas due to fluid‑induced melting. --- ⚠️ Common Misunderstandings All igneous rocks are volcanic. Wrong: intrusive igneous rocks never reach the surface. Metamorphism only occurs deep in the mantle. Wrong: contact metamorphism can happen near the crust. Sedimentary rocks are always “soft.” Wrong: lithification can produce hard, quartz‑rich sandstones. Water only acts in the surface weathering stage. Wrong: subduction‑zone fluids are crucial for mantle melting. --- 🧠 Mental Models / Intuition “Heat‑Pressure Slider” – Visualize a slider moving from low (weathering) to high (metamorphism). The farther right, the coarser the grain size (intrusive) or the stronger the foliation. “Rock Life Cycle” – Think of rocks as a “career”: start as magma, become igneous, get weathered, become sediment, lithify, maybe metamorphose, and possibly melt again. “Water as a Catalyst” – Just as a catalyst speeds a chemical reaction, water lowers melting temperatures and speeds mineral dissolution. --- 🚩 Exceptions & Edge Cases Metasomatism – Even in contact metamorphism, fluids can introduce new elements, altering rock chemistry beyond simple recrystallization. Eclogite Formation – Not all subducted basalt becomes eclogite; only under very high pressure (> 1.5 GPa) and moderate temperature. Obsidian – Glassy extrusive rock forms only when cooling is essentially instantaneous (e.g., lava contacting water/air). Continental Crust Subduction – Rare but can occur in “under‑thrust” scenarios; generally, continental crust is buoyant and stays at the surface. --- 📍 When to Use Which Identify rock texture → decide intrusive vs. extrusive (coarse = intrusive, fine/glass = extrusive). Presence of foliation → choose regional metamorphism; absence of foliation but proximity to an intrusion → choose contact metamorphism. Rock fragments with rounded grains & matrix → clastic sedimentary; fossils or shells → biogenic; evaporite crystals (e.g., halite) → precipitate. Arc volcanoes → expect silica‑rich lavas; mid‑ocean ridges → basaltic lavas. --- 👀 Patterns to Recognize Coarse‑grained + deep‐plutonic setting ⇒ intrusive. Fine‑grained + surface flow ⇒ extrusive. Parallel mineral bands ⇒ regional metamorphism. Contact aureole around intrusion ⇒ contact metamorphism. Rounded, sorted grains + cement ⇒ clastic sedimentary. Abundant fossils or organic layers ⇒ biogenic sedimentary. Mineral crystals filling voids after evaporation ⇒ precipitate sedimentary. --- 🗂️ Exam Traps “All volcanic rocks are basalt.” – Ignoring silica‑rich (andesite, rhyolite) extrusives formed in subduction zones. “Metamorphism always produces foliation.” – Contact metamorphism often lacks foliation. “Only subduction zones generate magma.” – Spreading ridges also produce basaltic magma. “Sedimentary rocks cannot become igneous directly.” – Through melting, sedimentary rocks can generate magma (e.g., crustal anatexis). “Water only affects surface processes.” – Fluids in subduction zones are a primary melt driver. ---