Earth Interior Structure and Seismic Wave Dynamics Study Guide

Gravitational Density Increase: Rock density increases systematically toward the center of the planet as a result of gravity. Deeper Earth materials inherently possess higher density.
Upper Mantle Density: Upper mantle rocks have an average density of $3.3\,\text{g/cm}^3$ .
Lower Mantle Density: Lower mantle rocks have an average density of $5.6\,\text{g/cm}^3$
Mineral Phase Changes: Rocks within the lower mantle undergo mineral phase changes as their constituent minerals are compressed under extreme hydrostatic pressures at depth.

Diagnostic Utility: Seismic waves provide a primary technique to investigate and "see into" the planet's interior.
Factors Affecting Travel Times and Velocities:
- Travel times of seismic waves vary according to the physical and mineralogical properties of the materials through which they propagate.
- Seismic waves travel at their highest velocities through stiff, rigid rocks.
- Wave velocities vary systematically based on rock composition.
- Overall velocity of seismic waves increases with increasing depth inside Earth.
Types of Body Waves:
- Primary (P) Waves: Compressional waves that propagate through both solid and liquid mediums. P waves always travel faster than S waves.
- Secondary (S) Waves: Shear waves that propagate exclusively through solids. S waves cannot travel through liquid mediums.

Reflected and Refracted Energy: When a seismic ray encounters a boundary between Earth materials with differing physical properties (such as rock and water), the seismic energy splits into reflected waves and refracted (bent) waves.
Refraction Rules:
- Velocity Increase: When the velocity of a seismic wave increases upon crossing from one layer into another, the wave refracts (bends) toward the boundary separating the layers.
- Velocity Decrease: When the velocity of a seismic wave decreases upon crossing into another layer, the wave refracts (bends) away from the boundary separating the layers.

Concentric Layering: Abrupt changes in seismic wave velocities occur at specific depths, leading seismologists to determine that Earth is structured into distinct concentric shells defined by chemical composition or physical properties.
Crust:
- The comparatively thin outer skin of Earth, ranging from $3\,\text{km}$ ( $2\,\text{miles}$ ) to $70\,\text{km}$ ( $40\,\text{miles}$ ) in thickness.
- Composition is dominated by rocks ranging from mafic to felsic in nature.
- Continental Crust:
- Thickness ranges between $30\,\text{km}$ and $70\,\text{km}$ .
- Dominated by low-density felsic rock types.
- Oceanic Crust:
- Continually formed at mid-ocean ridges.
- Averages $7\,\text{km}$ in thickness.
- Composed predominantly of basalt and gabbro (dominated by mafic rock).
- Possesses an average density of $3.0\,\text{g/cm}^3$ , making it distinctly denser than continental crust.
Mantle:
- A solid rocky shell extending from the base of the crust down to a depth of approximately $2900\,\text{km}$ ( $1800\,\text{miles}$ ).
- Composed predominantly of ultramafic rocks.
Core:
- An iron-rich sphere located at Earth's center with a total radius of approximately $3500\,\text{km}$ ( $2200\,\text{miles}$ ).
- Represents approximately $\frac{1}{6}$ of Earth's total volume, but accounts for $\frac{1}{3}$ of Earth's total mass due to its high density.

Lithosphere:
- The outermost, rigid mechanical shell consisting of the crust and uppermost mantle.
- Overlies the weaker asthenosphere below.
- Broken into discrete, dynamic slabs known as tectonic plates that move relative to one another.
Asthenosphere:
- Weak layer located directly beneath the lithosphere.
- Mechanically detached from the lithosphere, allowing the overlying tectonic plates to move across its surface.
Mesosphere (Lower Mantle):
- A rigid layer extending from the base of the asthenosphere to the outer core.
- Despite its rigidity under short-term stress, rocks within this layer are capable of very gradual plastic flow over geological timescales.
Outer Core:
- A liquid layer composed primarily of an iron-nickel alloy.
- Inferred to be liquid based on the complete absence of S waves passing through this layer.
Inner Core:
- A solid metallic sphere with a radius of approximately $1200\,\text{km}$ .
- Composed of iron-nickel and behaves as a rigid solid.
- Continually growing in size as Earth cools, solidifying liquid material from the outer core at its boundary.
- Rotates faster than and moves independently of the overlying mantle and crust.

Mohorovičić Discontinuity (Moho):
- The boundary separating the crust from the underlying mantle.
- Discovered in 1909 following the observation of a sudden jump in P-wave velocities beneath the base of continents.
- P-wave velocities increase abruptly at the Moho, causing seismic waves to refract as they cross the boundary.
Core-Mantle Boundary and Seismic Shadow Zones:
- At distances beyond $100^\circ$ from an earthquake epicenter, P and S waves become absent or extremely weak, creating a seismic shadow zone.
- The complete blockage of S waves beyond $100^\circ$ demonstrates that the outer core is liquid.