Composition of the Earth: Comprehensive Notes

Mechanical vs. Chemical Definitions of Earth’s Interior

  • Rheology & Mechanical Layering

    • Rheology = study of how matter flows when it behaves plastically rather than elastically.

    • Earth split mechanically into 5 layers:

    • Lithosphere

    • Asthenosphere

    • Mesospheric mantle (lower part of upper mantle in some texts)

    • Outer core

    • Inner core

  • Chemical Layering

    • Earth split chemically into 5 layers:

    • Crust

    • Upper mantle

    • Lower mantle

    • Outer core

    • Inner core

Lithosphere & Asthenosphere

  • Lithosphere

    • Rigid, brittle outer shell (crust + upper-most, coolest mantle).

    • Thickness: 70-100\,\text{km}.

  • Asthenosphere

    • Hotter, highly viscous, ductile upper-mantle zone beneath lithosphere.

    • Critical for plate tectonics & isostatic adjustments.

The Crust

  • General Characteristics

    • Outermost, thinnest layer; only about 1\% of Earth’s total mass.

    • Thickness is age-dependent: older crust = thicker.

  • Thickness by Setting

    • Oceanic crust: 5-10\,\text{km}.

    • Continental crust (average): 35\,\text{km}.

    • Beneath major mountains: up to 60\,\text{km}.

  • Composition by Weight

    • Oxygen: 46.6\%

    • Silicon: 27.7\%

    • Aluminum: 8.13\%

    • Iron: 5\%

    • Calcium: 3.6\%

    • Sodium: 2.8\%

    • Potassium: 2.6\%

    • Magnesium: 2.1\%

    • Combined: 98.5\% of crustal mass.

  • Temperature & Activity

    • Average crustal temperature: 870^{\circ}\text{C}.

    • Almost all earthquakes originate here.

Mohorovičić Discontinuity (Moho)

  • Boundary separating crust from mantle.

  • Identified by Andrija Mohorovičić in 1999* (in transcript; historically 1909).

  • Seismic velocity jump reflects increased density.

  • Average depth: 8\,\text{km} beneath oceanic crust; 32\,\text{km} beneath continental crust.

The Mantle

  • Mass & Composition

    • Contains about 80\% of Earth’s mass.

    • Elements: iron, aluminum, calcium, magnesium, silicon, oxygen.

  • Temperature

    • Average: 3\,700^{\circ}\text{C}.

    • Temperature increases with depth; the gradient is called the geothermal gradient.

  • Geothermal Gradient

    • Defined as temperature increase per unit depth.

    • Primary driver of differing rock behaviors with depth.

Upper Mantle

  • Depth: surface-Moho down to 660\,\text{km}.

  • Rocks: peridotite & dunite (ultramafic, rich in olivine & pyroxene).

  • Visco-elastic; can flow slowly but still capable of brittle failure where coolest.

Lower Mantle

  • Depth: 660\,\text{km} to about 2\,200\,\text{km}.

  • Hotter, softer; deforms plastically, does not fracture.

  • Convection currents here transfer heat upward, feeding upper-mantle dynamics.

The Core

  • Overall

    • Primary internal heat source via radioactive decay.

    • Composition: predominantly iron–nickel alloy.

Outer Core

  • Physical State: liquid.

  • Mass: ~4\% of combined mantle-crust mass.

  • Composition: iron, some nickel, \approx 10\% sulfur & oxygen.

  • Temperature: 5\,000^{\circ}\text{C} → remains molten.

Inner Core

  • Physical State: solid due to immense pressure crystallizing super-hot iron.

  • Thickness: 1\,250\,\text{km} (≈ 70\% the Moon’s diameter).

  • Differential Rotation

    • Spins up to 0.20^{\prime\prime}\,\text{longitude} / \text{yr} faster than overlying Earth.

  • Role in Magnetism

    • Heat from inner core drives convection in liquid outer core.

    • Rotational convection of molten iron generates geomagnetic field via the dynamo effect.

Earth’s Magnetic Field (Conceptual Summary)

  • Produced by moving liquid iron in outer core.

  • Field lines emerge near geographic poles but magnetic poles wander.

  • Compass detection relies on alignment of needle with magnetic field.

Key Terms & Concepts for Exam Review

  • Rheology: flow behavior under stress.

  • Lithosphere vs. Asthenosphere: rigid shell vs. weak, ductile layer enabling plate motion.

  • Geothermal Gradient: temperature increase rate with depth.

  • Peridotite / Dunite: ultramafic upper-mantle rocks rich in olivine & pyroxene.

  • Moho: seismic boundary marking crust–mantle transition.

  • Convection Currents: mantle & outer-core heat transport loops.

  • Dynamo Effect: mechanism transforming kinetic energy of fluid iron into planetary magnetism.

*Always verify dates; transcript cites 1999 but accepted discovery year is 1909.