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Crustal Evolution and Geochemistry

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1,072prerequisites beneath it

Mantle Geochemistry Sm-Nd Isotope System +1 more→

Core Idea

Continental crust has formed and been recycled through Earth history, with geochemical evidence constraining when, how much, and by what mechanisms crust was generated. Nd model ages and detrital zircon U-Pb age distributions reveal episodic crustal growth, with peaks at ~2.7, 1.9, and 1.1 Ga associated with supercontinent assembly. The continental crust has a bulk composition approximating andesite (~60% SiO2), despite being generated primarily at subduction zones and hotspots. Intracrustal differentiation (partial melting, metamorphism, delamination of dense lower crust) has produced the stratified structure observed today: felsic upper crust, intermediate middle crust, and mafic lower crust. The secular evolution of crustal composition through time is recorded in shale composites, glacial tills, and river sediments that integrate large-scale crustal averages.

Explainer

The continental crust is unique to Earth among known planetary bodies and is the product of 4.5 billion years of geochemical differentiation. Understanding when it formed, how it evolved, and what controls its composition is one of the grand challenges of solid-Earth geochemistry.

The gross mechanism of crustal generation is well established: partial melting of the mantle produces basaltic magma, which is further differentiated through fractional crystallization, crustal assimilation, and re-melting to produce the range of igneous rocks that compose the crust. Subduction zones are the primary locus of crustal generation today, where hydrous melting of the mantle wedge above the subducting slab generates arc magmas. Over time, arc crust is thickened, metamorphosed, and differentiated, ultimately producing the vertically stratified continental crust with its andesitic bulk composition.

The growth curve of continental crust -- how much existed at each time in Earth history -- remains debated. End-member models range from early formation (most crust existed by 3 Ga, with subsequent recycling balancing new production) to continuous growth (crust accumulating steadily) to episodic growth (pulses associated with supercontinent cycles). Geochemical constraints come from multiple systems: Nd model ages constrain when mantle material was first extracted; U-Pb detrital zircon ages record the timing of crustal magmatism; oxygen isotopes in zircons distinguish juvenile versus reworked crust; hafnium isotopes in zircons provide another extraction-age proxy. The challenge is disentangling crustal production from crustal preservation -- much crust that formed may have been recycled back into the mantle by subduction.

Secular changes in crustal composition through time are subtle but real. The Archean crust had a higher proportion of mafic-ultramafic rocks (greenstone belts) and a distinctive granitic association (TTG -- tonalite-trondhjemite-granodiorite) formed by partial melting of hydrated basalt. Post-Archean crust is dominated by calc-alkaline granodiorite-granite generated in subduction settings. These compositional shifts reflect changing tectonic regimes, mantle temperature, and the style of crustal differentiation through Earth history.

Practice Questions 3 questions

Prerequisite Chain

Understanding Zero → The Number Zero → Counting to Five → Counting to 10 → Counting to 20 → Counting a Set of Objects Up to 20 → Cardinality: The Last Number Counted → Matching Numerals to Quantities → Subitizing Small Quantities → Addition Within 10 → Number Bonds to 10 → Addition Within 20 → Doubles and Near Doubles → Doubles Facts Within 10 → Near Doubles Facts Within 20 → Mental Math Strategies for Addition → Mental Math: Adding and Subtracting Tens → Addition Within 100 → Repeated Addition as Multiplication → Multiplication as Equal Groups → Multiplication: Arrays → Basic Multiplication Facts (0s, 1s, 2s, 5s, 10s) → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Multiplication and Division Fact Families → Relationship Between Multiplication and Division → Division Facts as Inverse of Multiplication → Remainders and Quotients in Division → Division Word Problems → Multi-Step Word Problems → Solving Multi-Step Word Problems → Multiplication Word Problems → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Integers and the Number Line → Comparing and Ordering Integers → Absolute Value → Adding Integers → Subtracting Integers → Multiplying Integers → Dividing Integers → Unit Rates → Proportions → Percent Concept → Converting Between Fractions, Decimals, and Percents → Operations with Rational Numbers → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Sine, Cosine, and Tangent Ratios → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Indefinite Integrals → Basic Integration Rules → Riemann Sums → Definite Integral Definition → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Work and Circulation → Line Integrals of Scalar and Vector Functions → Fundamental Theorem for Line Integrals → Conservative Vector Fields → Conservative Vector Fields and Potential Functions → Curl and Divergence of Vector Fields → Curl and Divergence → Divergence Theorem → Electric Flux and Divergence Theorem → Gauss's Law: Integral Form and Meaning → Solving Problems with Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → Postulates of Special Relativity → Time Dilation → Length Contraction → Lorentz Transformation → Relativistic Velocity Addition → Relativistic Momentum and Energy → Mass-Energy Equivalence and E=mc² → Photons as Particles with Energy and Momentum → Planck-Einstein Relation: Energy and Frequency → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Observables and Quantum Operators → Commutators and Commutation Relations → Quantum Angular Momentum → Quantum Mechanical Treatment of Hydrogen → Solving the Schrödinger Equation for Hydrogen Atom → Quantum Numbers → Electron Configuration → Periodic Trends → Covalent Bonding → Electronegativity and Bond Polarity → Ionic Bonding → Lewis Structures → VSEPR Theory and Molecular Geometry → Molecular Geometry and Electron Pair Geometry → Molecular Polarity and Dipole Moments → Intermolecular Forces → States of Matter and Phase Changes: Melting, Boiling, and Sublimation → Gas Laws and the Ideal Gas Equation → Gas Stoichiometry and Volume-Volume Calculations → Thermochemistry and Enthalpy → Heat Capacity and Calorimetry → Entropy and Molecular Disorder → Spontaneity and ΔG → Entropy and Gibbs Free Energy → Chemical Equilibrium → Statistical Mechanics: Ensembles and the Boltzmann Distribution → Partition Function: Definition and Properties → The Canonical Partition Function and Thermodynamic Derivation → Free Energy and Thermodynamic Relations from Partition Functions → Legendre Transformations and Thermodynamic Potentials → Chemical Potential and Partial Molar Properties → Phase Equilibrium and Coexistence Conditions → Clausius-Clapeyron Equation → Phase Diagrams and Clausius-Clapeyron Equation → Chemical Potential and Thermodynamic Equilibrium → Geochemical Thermodynamics → Trace Element Geochemistry → Partition Coefficients → REE Patterns in Geochemistry → Mantle Geochemistry → Crustal Evolution and Geochemistry

Longest path: 189 steps · 1072 total prerequisite topics

Prerequisites (3)

Mantle Geochemistryhard Sm-Nd Isotope Systemhard U-Pb Geochronologysoft

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