2.1 Earth's structure and composition

Earth's structure is like a giant onion, with layers that get denser as you go deeper. The crust, mantle, outer core, and inner core each play a unique role in shaping our planet's behavior and appearance.

Understanding Earth's composition is key to grasping how our planet works. From the rocky crust we live on to the molten core generating our magnetic field, each layer's makeup influences everything from plate tectonics to climate.

Earth's Internal Structure

Layered Structure of Earth

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• Earth comprises four main layers each with distinct properties and compositions
  • Crust (outermost layer)
  • Mantle (largest layer)
  • Outer core (liquid layer)
  • Inner core (solid sphere)
• Boundaries between layers marked by significant changes in seismic wave velocities (discontinuities)
• Lithosphere consists of crust and uppermost mantle behaves as a rigid layer divided into tectonic plates

Characteristics of Earth's Layers

• Crust varies in thickness from 5-70 km composed primarily of silicate rocks
• Mantle extends from base of crust to depth of about 2900 km made up of dense, iron and magnesium-rich silicate rocks
• Outer core extends from about 2900 km to 5150 km depth composed mostly of liquid iron and nickel
• Inner core has radius of about 1220 km composed primarily of solid iron and nickel

Composition of Earth's Layers

Crustal Composition and Structure

• Crust composed mainly of light elements (silicon, aluminum, oxygen) forming minerals like quartz and feldspar
• Oceanic crust
  • Thinner (5-10 km) and denser
  • Composed mainly of basaltic rocks (dark, fine-grained igneous rocks)
• Continental crust
  • Thicker (30-70 km) and less dense
  • Composed primarily of granitic rocks (light-colored, coarse-grained igneous rocks)

Mantle and Core Composition

• Mantle predominantly composed of silicate minerals rich in iron and magnesium (olivine, pyroxene)
• Upper mantle contains partially molten layer called asthenosphere allowing for plate tectonic movements
• Lower mantle solid due to immense pressure despite high temperatures
• Outer core liquid composed mainly of iron and nickel with some lighter elements (sulfur, oxygen)
• Inner core solid due to extreme pressure despite temperatures comparable to surface of Sun (~5400°C)

Density and Temperature in Earth's Interior

Density Distribution and Its Effects

• Earth's layers arranged according to density densest materials at center least dense at surface
• Density increases with depth due to gravitational compression and changes in composition
• Density contrasts between layers create buoyancy forces influencing mantle convection and plate tectonic processes
• Average densities of Earth's layers
  • Crust: 2.7-3.0 g/cm³
  • Mantle: 3.3-5.7 g/cm³
  • Outer core: 9.9-12.2 g/cm³
  • Inner core: 12.6-13.0 g/cm³

Temperature Gradients and Their Implications

• Temperature generally increases with depth in Earth phenomenon known as geothermal gradient
• Adiabatic temperature gradient in mantle responsible for mantle convection driving plate tectonics
• Balance between increasing pressure and temperature with depth determines physical state (solid or liquid) of Earth's layers
• Liquid outer core's convection driven by temperature differences generates Earth's magnetic field through geodynamo process
• Estimated temperatures within Earth's interior
  • Base of crust: ~200-400°C
  • Base of mantle: ~2,900°C
  • Outer core: ~4,400-5,700°C
  • Inner core: ~5,400-6,000°C

Rocks and Minerals in Earth's Crust

Rock Types and Formation Processes

• Rocks in Earth's crust classified into three main types each formed through distinct geological processes
  • Igneous rocks form from cooling and solidification of magma or lava
    • Extrusive (at surface) (basalt, pumice)
    • Intrusive (below ground) (granite, gabbro)
  • Sedimentary rocks form from deposition and lithification of sediments often in layers may contain fossils (limestone, sandstone)
  • Metamorphic rocks result from transformation of existing rocks due to changes in temperature, pressure, or chemical environment (marble, schist)
• Distribution of rock types varies between oceanic and continental crust reflecting different formation processes and tectonic histories

Mineral Composition and Distribution

• Minerals naturally occurring, inorganic solids with definite chemical composition and crystal structure
• Distribution of minerals in crust influenced by factors
  • Original composition of parent rock
  • Temperature and pressure conditions during formation
  • Presence of fluids during formation
• Common crustal minerals and their compositions
  • Quartz (SiO₂)
  • Feldspar (KAlSi₃O₈, NaAlSi₃O₈, CaAl₂Si₂O₈)
  • Mica (complex silicates with potassium, aluminum, and other elements)
  • Olivine ((Mg,Fe)₂SiO₄)
  • Pyroxene ((Ca,Na)(Mg,Fe,Al)(Si,Al)₂O₆)