Geology Study Guide

📖 Core Concepts Geology – scientific study of Earth (and other planetary bodies), its rocks, and the processes that change them over time. Mineral – naturally occurring chemical compound with a definite composition and ordered atomic arrangement. Rock – solid mass of one or more minerals/mineraloids; three major types: igneous, sedimentary, metamorphic. Plate Tectonics – lithosphere is broken into moving plates; motion driven by mantle convection. Relative Dating Principles – superposition, original horizontality, cross‑cutting relationships, inclusions, faunal succession, etc. Absolute (Radiometric) Dating – decay of radioactive isotopes (U‑Pb, Rb‑Sr, K‑Ar, C‑14, etc.) to determine rock ages. Rock Cycle – igneous → sedimentary → metamorphic → melt → new igneous (continuous). Seismic Imaging – travel‑time of P‑ and S‑waves reveals internal layers; S‑waves do not travel through liquid outer core. --- 📌 Must Remember Superposition: In an undisturbed sequence, older layers are below younger layers. Original Horizontality: Sediments are deposited horizontally. Cross‑cutting Relationships: A fault or intrusion is younger than the rocks it cuts. Inclusions: Clasts inside a sedimentary rock are older than the surrounding matrix. Faunal Succession: Fossil assemblages allow correlation of strata across distances. Plate Boundary Types: Divergent – creates new oceanic crust (mid‑ocean ridges). Convergent – subduction zones, volcanic arcs, mountain belts. Transform – horizontal slip (e.g., San Andreas Fault). Radiometric Half‑Lives (outline‑derived): U‑Pb (very long, > 10⁸ yr) → oldest rocks. K‑Ar (≈ 1.3 × 10⁹ yr) → volcanic rocks. C‑14 (≈ 5,730 yr) → organic material ≤ 50 ka. Specific Gravity: Ratio of mineral weight to weight of equal‑volume water. Steno’s Laws: Superposition, original horizontality, lateral continuity – foundation of stratigraphy. --- 🔄 Key Processes Rock Formation Igneous: Magma/lava cools → crystallization. Sedimentary: Weathering → erosion → transport → deposition → lithification. Metamorphic: Existing rock exposed to heat + pressure → new mineral assemblage, foliation. Plate Motion (Convection‑Driven) Heat from core creates mantle convection cells → drag lithospheric plates at surface. Relative Dating Workflow Observe contacts → apply superposition → note cross‑cutting features → identify fossils → assign relative ages. Radiometric Dating Steps Measure parent/daughter isotope ratios → apply decay equation \(t = \frac{1}{\lambda}\ln\left(1+\frac{D}{P}\right)\) → obtain absolute age. Deformation Mechanisms Compression → folding, thrust faulting, thickening. Extension → normal faulting, thinning, basin formation. Strike‑slip → lateral displacement along vertical faults. --- 🔍 Key Comparisons Igneous vs. Sedimentary vs. Metamorphic Origin: cooling magma vs. lithified sediments vs. recrystallization under heat/pressure. Texture: crystalline (often interlocking) vs. clastic/chemical vs. foliated/non‑foliated. Divergent vs. Convergent vs. Transform Boundaries Motion: plates pull apart vs. one plate subducts/overrides vs. slide past each other. Surface Features: mid‑ocean ridges vs. volcanic arcs & deep trenches vs. strike‑slip faults. U‑Pb vs. K‑Ar vs. C‑14 Dating Age Range: billions of years vs. millions of years vs. up to 50 ka. Typical Samples: zircon in igneous rocks vs. volcanic rocks vs. recent organic material. --- ⚠️ Common Misunderstandings “All minerals are hard.” Hardness varies; Mohs scale ranges from 1 (talc) to 10 (diamond). “All sedimentary rocks are layered horizontally.” Post‑depositional folding can tilt layers. “Radiometric dates are always exact.” They provide age ranges; assumptions about closed systems matter. “Transform faults create or destroy crust.” They mainly accommodate lateral motion; no net crustal gain/loss. “All volcanic rocks are young.” Some volcanic rocks can be ancient; age depends on radiometric results. --- 🧠 Mental Models / Intuition “Rock Cycle as a Circular Conveyor Belt”: Imagine rocks moving along a belt: melt → solidify (igneous) → break down → re‑deposit (sedimentary) → get buried & heated (metamorphic) → melt again. “Plate Boundaries as Joints in a Jigsaw Puzzle”: Divergent = pulling pieces apart, convergent = pushing pieces together (one slides under), transform = sliding pieces side‑by‑side. “Dating as a Stopwatch”: Relative dating tells you who started first; radiometric dating tells you exactly how long the timer has run. --- 🚩 Exceptions & Edge Cases Inclusions older than host rock – but an inclusion can be a younger mineral fragment (e.g., xenoliths) if captured during intrusion. Superposition fails in overturned folds – need structural analysis to restore original order. Carbon‑14 not usable for carbonate rocks older than 50 ka – alternative: uranium‑thorium dating for speleothems. Seismic wave speed anomalies can mislead about material state; rely on both P‑ and S‑wave behavior. --- 📍 When to Use Which Relative Dating – first step for any sedimentary sequence lacking datable minerals. U‑Pb – date zircon crystals in ancient igneous/metamorphic rocks (> 100 Ma). K‑Ar – date volcanic ash layers within sedimentary basins. C‑14 – date recent organic remains (archaeology, Holocene studies). Optically Stimulated Luminescence – determine last exposure of sand grains (e.g., dune or fluvial deposits). Ground‑Penetrating Radar – map shallow (< 30 m) subsurface features in unconsolidated sediments. Aeromagnetic Survey – locate magnetic minerals (e.g., iron‑rich igneous bodies) over large areas. --- 👀 Patterns to Recognize Triangular “V” of younger rocks over older – indicates a normal fault (extension). Inverted “U” of older rocks in the core of a fold – anticline. Linear magnetic anomalies alternating polarity – mid‑ocean ridge spreading history. Sharp change in seismic velocity at 410 km & 660 km – mantle phase transitions. Consistent fossil assemblages across regions – same stratigraphic interval (faunal succession). --- 🗂️ Exam Traps Choosing “C‑14” for a 200 Ma volcanic rock – tempting because it’s a dating method, but out of range. Assuming all igneous intrusions are younger than surrounding sedimentary rocks – intrusive relationships must be observed; some intrusions can be older than overlying sediments if later covered. Confusing “specific gravity” with “density.” Specific gravity is a ratio to water; density has units (kg m⁻³). Mistaking “transform” for “convergent” because both involve faulting – remember motion direction (horizontal vs. vertical). Believing “all minerals are identified by color.” Color can be misleading due to impurities; use streak, hardness, etc. ---