Rock (geology) Study Guide

📖 Core Concepts Rock – any natural solid mass made of minerals or mineraloid material. Minerals vs Mineraloids – minerals have a crystalline lattice; mineraloids (e.g., volcanic glass) lack it. Silicate Dominance – silicate minerals (SiO₄ tetrahedra) form 95 % of the crust; silica amount dictates rock names. Rock Cycle – rocks continuously transform among igneous, sedimentary, and metamorphic through melting, erosion, burial, and recrystallization. Igneous Classification – based chiefly on silica content: felsic (high silica) vs mafic (low silica); alkali metal oxides are the next factor. Sedimentary Types – clastic (rock fragments), chemical (precipitated minerals), organic (biogenic material). Metamorphism – alteration of a pre‑existing rock (protolith) by heat + pressure without melting; produces foliated (layered) or non‑foliated textures. --- 📌 Must Remember Rock composition: grains of crystalline minerals ± mineraloids. Silica rule: ↑ silica → felsic (e.g., granite); ↓ silica → mafic (e.g., basalt). Igneous texture: slow cooling → coarse‑grained (intrusive); rapid cooling → fine‑grained (extrusive). Sedimentary grain size order: clay → silt → sand → gravel → cobble → boulder. Metamorphic conditions: T > 150–200 °C, P > 1500 bars. Metamorphic types: Contact – heat dominant (near magma). Burial (pressure) – deep burial, pressure dominant. Regional – high T + P, mountain‑building zones. Common non‑foliated rocks: marble ← limestone, quartzite ← sandstone. Radiometric dating: measures decay of unstable isotopes to assign absolute ages. --- 🔄 Key Processes Igneous Rock Formation Magma generation → temperature up / pressure down / composition change. Intrusive: magma cools slowly underground → large crystals. Extrusive: magma erupts as lava → cools quickly → fine crystals or glass. Sedimentary Rock Formation (Diagenesis) Weathering & erosion → transport (water, wind, ice, etc.) → deposition → compaction → cementation → solid rock. Metamorphic Transformation Protolith subjected to ↑T and/or ↑P → recrystallization (no melting) → new mineral assemblage & texture (foliated vs non‑foliated). --- 🔍 Key Comparisons Intrusive vs Extrusive Igneous Cooling rate: slow vs fast. Texture: coarse‑grained vs fine‑grained/glassy. Typical examples: granite vs basalt/pumice. Felsic vs Mafic Igneous Silica: high vs low. Color & density: light & less dense vs dark & denser. Clastic vs Chemical vs Organic Sedimentary Origin: broken rock fragments vs mineral precipitation vs organism remains. Common rocks: sandstone (clastic), evaporites (chemical), coal (organic). Foliated vs Non‑Foliated Metamorphic Texture: layered/banded vs massive. Typical formation: directed pressure (foliated) vs uniform pressure or recrystallization of pure minerals (non‑foliated). --- ⚠️ Common Misunderstandings “All rocks are solid minerals.” Mineraloids (e.g., volcanic glass) count as rock material despite lacking crystals. “Metamorphic rocks melt.” Metamorphism occurs without significant melting; melting produces igneous rock. “Silica content only matters for igneous rocks.” It also influences mineral stability in metamorphic rocks. “All sedimentary rocks are clastic.” Chemical and organic sediments are distinct categories. --- 🧠 Mental Models / Intuition “Silica = sweetness.” Think of felsic rocks as “sweet” (high silica) and mafic rocks as “bitter” (low silica). “Cooling speed = crystal size.” Slow‑cooling → big crystals (like a slow‑cooked stew); fast‑cooling → tiny crystals (flash‑freeze). “Pressure vs temperature dominance.” Contact → “heat lamp” (temperature dominates); burial → “pressing press” (pressure dominates); regional → “both oven and press.” --- 🚩 Exceptions & Edge Cases Volcanic glass (obsidian) – an extrusive igneous rock with a glassy, non‑crystalline texture despite rapid cooling. Metamorphic rocks formed at low‑grade conditions may retain remnants of original textures (e.g., slate from shale). Some rocks contain both felsic and mafic minerals (intermediate composition) – classification may require quantitative silica percentages not given in the outline. --- 📍 When to Use Which Identify rock type: Look at texture first (grain size, crystal size). Then check silica content (color, mineral assemblage). Choose classification method: For igneous – start with silica → felsic vs mafic → refine with alkali oxides. For sedimentary – assess origin (clastic vs chemical vs organic) → then grain‑size for clastics. For metamorphic – evaluate temperature vs pressure dominance → assign contact, burial, or regional. --- 👀 Patterns to Recognize Coarse‑grained + intrusive → deep, slow cooling → plutonic rock. Fine‑grained + extrusive → surface eruption → volcanic rock. Horizontal strata + fossils → sedimentary deposition environment. Banding + aligned minerals → foliated metamorphic (e.g., schist). Uniform mineral texture in metamorphic → non‑foliated (e.g., marble). --- 🗂️ Exam Traps “All igneous rocks are intrusive.” Extrusive rocks are equally igneous. Confusing “mafic” with “metamorphic.” Mafic describes silica‑low igneous composition, not metamorphic grade. Assuming any rock with fossils is sedimentary. While typical, some metamorphic rocks can preserve fossil outlines; the primary clue is the presence of layered strata. Choosing radiometric dating for any rock type. Only igneous (and some metamorphic) rocks contain suitable parent isotopes; sedimentary rocks are dated indirectly. ---