19.1 Solar-System Origin, Earth Interior & Atmospheric Layers

General geology begins at planetary scale because Earth's composition, heat, tectonics, atmosphere, water and mineral deposits are outcomes of its origin and evolution. The leading solar nebula model describes gravitational collapse of a cloud of gas and dust into a rotating disk about 4.6 billion years ago. Most mass formed the Sun; collisions and accretion built planetesimals and planets. Temperature in the disk helped separate rocky inner planets from volatile-rich outer bodies.

Accretion and Differentiation

Early Earth heated through impacts, compression, radioactive decay and segregation of dense material. Partial to extensive melting allowed differentiation:

• iron and nickel concentrated toward the core;
• magnesium- and iron-rich silicates dominated the mantle;
• lower-density silicate melts formed evolving crust; and
• volcanic degassing and later inputs contributed to atmosphere and hydrosphere.

Differentiation explains why crustal ore deposits require concentration processes. Most metals are scarce in average crust, but magmatic, hydrothermal, sedimentary and weathering systems locally concentrate them to potentially economic levels.

Compositional Layers

The Moho is the seismic boundary between crust and mantle. It is a compositional boundary, not simply the base of every rigid plate.

Mechanical Layers

• Lithosphere: rigid crust plus uppermost mantle, broken into tectonic plates.
• Asthenosphere: weaker, ductile upper mantle over which plates move; mostly solid, not a global liquid ocean.
• Mesosphere/lower mantle: stronger under pressure yet convects over geologic time.
• Outer core: liquid iron alloy.
• Inner core: solid iron alloy due to immense pressure.

Pressure, temperature and material behavior all change with depth. “Solid” mantle can flow slowly over millions of years while transmitting seismic shear waves.

Seismic Evidence

P waves are compressional and travel through solids and liquids. S waves are shear waves and do not propagate through liquids. Refraction, reflection, velocity changes and shadow zones reveal internal boundaries. The absence of direct S waves through the outer core supports its liquid state; P-wave behavior constrains the core boundaries. Surface waves travel along Earth and often cause severe earthquake shaking.

Seismic tomography uses travel-time variations to image faster and slower regions that may reflect temperature, composition or phase. It is an inference with resolution limits, not a photograph of the mantle.

Magnetic Field and Heat Engine

Motion of conductive liquid iron in the outer core sustains the geodynamo and Earth's magnetic field. Internal heat drives mantle convection, melting and plate tectonics, while gravity drives slab sinking. These processes create arcs, rifts, collision zones and hydrothermal circulation associated with mineralization.

Atmospheric Layers

Atmospheric layers are classified by temperature trend:

1. Troposphere: temperature generally decreases with height; most mass, water vapor and weather occur here.
2. Stratosphere: temperature rises upward because ozone absorbs ultraviolet radiation.
3. Mesosphere: temperature declines upward; meteors commonly ablate here.
4. Thermosphere: temperature rises as high-energy radiation is absorbed; includes much of the ionosphere.

The thin exosphere grades into space. Boundaries are tropopause, stratopause and mesopause.

Mining Connections

Tropospheric rainfall controls runoff, slope pore pressure, pit inflow, tailings water balance and erosion. Atmospheric stability affects dust dispersion. Regional tectonics drives earthquakes and magmatic arcs, while mantle-derived ultramafic rocks become hosts to chromite and lateritic nickel.

Concept Trap

Do not match compositional and mechanical terms as if identical. The lithosphere includes crust and upper mantle; the asthenosphere is mantle. The outer core is liquid because S waves do not cross it, but the mantle remains mainly solid despite its capacity for slow flow.

Continental and Oceanic Contrast

Continental crust is commonly thicker, more compositionally diverse and less dense than oceanic crust, so it is less readily subducted. Oceanic lithosphere cools, thickens and becomes denser with age before subduction. At convergent margins, sediment, altered basalt and water-bearing minerals enter the system; devolatilization, mantle melting and crustal interaction help produce arc magma. This is a process chain, not a rule that every volcano or ore deposit sits directly above one slab point.

Evidence, Not Direct Access

Earth's internal layers are inferred by combining seismic travel times and shadow zones with gravity, moment of inertia, magnetic behavior, high-pressure experiments and meteorite composition. Each observation constrains a model; no borehole reaches the mantle or core. On exam questions, distinguish a compositional boundary from a change in mechanical behavior and identify which wave or physical property supplies the evidence.