Plate tectonics Study Guide

📖 Core Concepts Plate Tectonics – The lithosphere is broken into a limited number of rigid plates that move ≈ 0–10 cm yr⁻¹ over the ductile asthenosphere. Lithosphere vs. Asthenosphere – Lithosphere = cool, rigid, conducts heat; Asthenosphere = hotter, partially molten, transfers heat by convection + conduction. Plate Boundaries – Three types: Divergent (plates separate, new crust forms), Convergent (plates converge → subduction or collision), Transform (plates slide past one another). Driving Forces – Slab pull (weight of a sinking slab), Ridge push / gravitational sliding (elevated ridge topography), Mantle convection, Slab suction and viscous mantle traction. Evidence – Magnetic striping symmetric about ridges, age‑progressive oceanic crust, Wadati–Benioff seismic zones, fit of continental margins, ophiolites. --- 📌 Must Remember Plate motion rates: 0–10 cm yr⁻¹ (typical). Oceanic lithosphere thickness: 6 km at ridges → >100 km at old/subducting sections. Continental lithosphere thickness: 200 km (highly variable). Density contrast: Oceanic crust ≈ 3.0 g cm⁻³; Continental crust ≈ 2.7 g cm⁻³ (Si‑Al richer). Slab pull = strongest individual driver for plates attached to a subducting slab (e.g., Pacific Plate). Magnetic stripes are mirror‑symmetric about ridge crests and record geomagnetic reversals. Transform motion is right‑lateral (dextral) in most global examples (e.g., San Andreas). Most active volcanoes lie along convergent margins; the “Ring of Fire” circles the Pacific. --- 🔄 Key Processes Seafloor Spreading Upwelling mantle creates ridge crest → basaltic magma solidifies → new oceanic crust. New crust moves outward, cools conductively, thickens, and becomes denser. Subduction Dense oceanic plate bends, descends into mantle at trench. Generates slab pull, slab suction, and a Wadati–Benioff seismic zone (40°–60° dip). Transform Fault Motion Horizontal shear; no crust creation or destruction. Motion direction recorded by offset markers (e.g., stream channels). Plate Reconstruction Magnetic stripes → relative motion since Jurassic. Hotspot tracks → absolute motion back to Cretaceous. Paleomagnetic poles → latitude & rotation (no longitude). --- 🔍 Key Comparisons Divergent vs. Convergent vs. Transform Divergent: crust creation, shallow quakes, volcanic ridges. Convergent: crust destruction (subduction) or thickening (collision), deep quakes, volcanic arcs. Transform: crust neither created nor destroyed; strike‑slip earthquakes. Slab Pull vs. Ridge Push Slab Pull: force from sinking slab weight; strongest for plates with active subduction. Ridge Push: gravitational sliding of elevated ridge lithosphere; generally weaker, acts on all plates. Oceanic vs. Continental Crust Thickness: 6–100 km vs. 200 km. Density: higher vs. lower. Age: oceanic crust ≤ 200 Ma, continental crust can be > 3 Ga. Heat Transfer Lithosphere: pure conduction. Asthenosphere: convection + conduction (nearly adiabatic). --- ⚠️ Common Misunderstandings “Continental drift = plate tectonics.” Drift is the observation; plate tectonics explains how and why plates move. Ridge push alone drives plates. It contributes but slab pull (and mantle flow) dominates for most plates. All plates move at the same speed. Plates attached to a subducting slab (e.g., Pacific) move faster than those lacking slab pull. Transform faults create new crust. They only accommodate lateral motion; crust is neither created nor destroyed. Oceanic crust is thicker than continental crust. It is actually much thinner. --- 🧠 Mental Models / Intuition Conveyor‑Belt Model: Mid‑ocean ridges are the “factory”; oceanic plates are the belt that cools, thickens, and eventually “recycles” at trenches. Iceberg Analogy (Wegener): Continents are the visible tip; the underlying plate (lithosphere) moves like an iceberg drifting on the ocean (asthenosphere). Tape Recorder: Magnetic stripes are a record of Earth’s magnetic polarity reversals, just as a tape records sound. --- 🚩 Exceptions & Edge Cases Plates without subduction (e.g., African Plate) move slower; ridge push and mantle drag dominate. Intraplate volcanism (e.g., Hawaiian hotspot) occurs away from boundaries due to mantle plumes, not plate interactions. Early Earth (Archean) stagnant‑lid – higher mantle temperatures may have prevented modern plate behavior. Older oceanic lithosphere can exceed 100 km thickness, contradicting the “thin oceanic crust” simplification. --- 📍 When to Use Which | Situation | Preferred Explanation / Tool | |-----------|------------------------------| | Estimating relative plate motion (≥ Jurassic) | Magnetic stripe symmetry & spreading rates | | Determining absolute plate path (≤ Cretaceous) | Hotspot track (assume fixed mantle plume) | | Assessing plate velocity | GPS/ satellite data; consider slab‑pull contribution if subduction present | | Identifying past boundary locations | Ophiolite occurrence + magnetic anomalies | | Explaining intraplate volcanism | Mantle plume model (water‑weakening, plume head) | | Choosing dominant driving force | Presence of a subducting slab → slab pull; otherwise ridge push + mantle drag | --- 👀 Patterns to Recognize Symmetric magnetic stripes → active seafloor spreading center. Age gradient of oceanic crust (youngest at ridge, oldest toward trench). Wadati–Benioff zones: deep, planar seismicity dipping 40°–60° beneath trenches. Clusters of shallow earthquakes & volcanoes along divergent ridges; deep earthquakes & volcanic arcs along convergent margins. Linear offset of geomorphic features (streams, ridges) → transform fault. --- 🗂️ Exam Traps “All transform faults are right‑lateral.” Some are left‑lateral (sinistral); always check motion sense. Choosing slab pull for a plate without subduction. That plate’s motion is dominated by ridge push or mantle flow. Assuming every volcanic arc marks a subduction zone. Some intraplate volcanoes (e.g., Hawaii) are plume‑related. Interpreting a thick oceanic lithosphere as continental crust. Thickness alone does not define crust type; density and composition matter. Treating ridge push as a “pushing” force from the ridge crest. It is actually a gravitational sliding component of the topographic gradient. ---