Geology: Deep Structure, Composition, and Magnetic Fields of Earth

Studying the interior of the Earth presents a significant challenge because humans lack direct access to it.
Despite this lack of access, understanding the interior is essential because processes observed at the Earth’s surface are driven by heat generated within the Earth.
Results of internal processes include:
- Volcanism.
- Earthquakes.
- Many of the Earth’s surface features.
Knowledge of the interior is primarily gathered through indirect means, primarily by using seismic data to determine internal structure.
Early 1900s Discovery: Scientists discovered that seismic waves generated by earthquakes help distinguish the properties of the internal layers.
Seismic Wave Properties:
- The velocity of these waves (Primary and Secondary waves, or P and S waves) changes based on the density of the materials they travel through.
- Seismic waves do not travel in straight lines; they are reflected and refracted, indicating the Earth is not homogeneous throughout.

Seismic waves are energy waves generated during earthquakes that propagate through the Earth as wave fronts from their point of origin.
Primary Waves (P-waves): These are compressional waves that move back and forth like an accordion.
Secondary Waves (S-waves): These are shear waves that move material in a direction perpendicular to the direction of travel, similar to snapping a rope.
Wave Velocity and Density: The velocity of both P and S waves increases as the density of the material they travel through increases ( $v \propto \rho$ ).
Behavior in Liquids:
- Most liquids are less dense than their solid counterparts.
- Seismic velocity is dependent on density; therefore, the presence of liquid phases affects wave behavior.
- S-waves cannot travel through liquids because the side-to-side (shear) motion cannot be maintained in fluids. This absence of S-waves in certain areas indicates that the outer core is liquid.
Refraction and Shadow Zones:
- If the Earth were completely homogeneous, waves would travel in straight lines. Instead, they are refracted (bent) as their direction and velocity alter when entering materials of different densities.
- Shadow Zones: Areas along the Earth where no seismic waves are detected due to refraction.
- P-wave Shadow Zone: Exists from $103^o$ to $143^o$ from the earthquake focus (origination point).
- S-wave Shadow Zone: Exists in areas greater than $103^o$ from the earthquake focus.

Bulk Earth Density: Based on the Earth's travel through space, the average density is determined to be $5.52\,\text{g/cm}^3$ .
Crustal Density: Rocks analyzed at the surface typically have densities between $2.5\,\text{g/cm}^3$ and $3\,\text{g/cm}^3$ .
Core Density: Because the surface density is lower than the average, the interior must be denser. The core region is estimated to have a density of $9\,\text{g/cm}^3$ to $13\,\text{g/cm}^3$ .
Chemical Makeup of the Bulk Earth:
- Iron ( $Fe$ ): $\sim 32\%$
- Oxygen ( $O$ ): $\sim 30\%$
- Silicon ( $Si$ ): $\sim 16\%$
- Magnesium ( $Mg$ ): $15\%$
Chemical Makeup of the Surface (Crust):
- Oxygen ( $O$ ): $\sim 47\%$
- Silicon ( $Si$ ): $\sim 28\%$
- Aluminum ( $Al$ ): $\sim 8\%$
- Lesser amounts of Iron ( $Fe$ ), Calcium ( $Ca$ ), Sodium ( $Na$ ), Potassium ( $K$ ), and Magnesium ( $Mg$ ).
Silicates: Minerals composed of silicon and oxygen; these are the most important minerals in the crust.
Iron Distribution: Much of the Earth's iron has migrated to the core, which explains the major increase in density in that region compared to the crust.

Continental Crust:
- Density: $\sim 2.7 - 2.9\,\text{g/cm}^3$
- Thickness: $\sim 20 - 70\,\text{km}$
- Composition: Felsic rocks.
Oceanic Crust:
- Density: $\sim 3.0\,\text{g/cm}^3$
- Thickness: $\sim 8 - 10\,\text{km}$
- Composition: Mafic rocks.
Mantle:
- Density: $\sim 3.4 - 5.6\,\text{g/cm}^3$
- Thickness: $\sim 2,885\,\text{km}$
- Composition: Ultramafic rocks.
Outer Core:
- Density: $\sim 9.9 - 12.2\,\text{g/cm}^3$
- Thickness: $\sim 2,200\,\text{km}$
- Composition: Iron, some sulfur, nickel, oxygen, silicon.
Inner Core:
- Density: $\sim 12.8 - 13.1\,\text{g/cm}^3$
- Thickness: $\sim 1,220\,\text{km}$
- Composition: Iron, some sulfur, and nickel.
Lithosphere: The outer, rigid part of the Earth, consisting of the upper mantle, oceanic crust, and continental crust.
Asthenosphere: Located just beneath the lithosphere; it behaves plastically and flows rather than remaining rigid.

Definition: A relatively recent imaging technology used to generate detailed 3D models of the Earth’s interior.
CAT Scan Metaphor: Similar to medical CAT scans where X-rays rotate to create cross-sectional images, seismic tomography stacks repeated scans of seismic waves to produce a three-dimensional image.
Applications: Used for petroleum exploration near the surface and for imaging the planet as a whole.
Thermal Convection Models: Tomography models depict mantle temperature variations:
- Blue areas: Represent cool mantle material.
- Red areas: Represent warm mantle material.
- Thin red areas: Represent rising plumes.

Generation: Produced by electrical currents created by thermal and compositional currents moving within the liquid outer core, coupled with the Earth’s rotation.
Structure: Similar in shape to a large bar magnet.
Magnetic vs. Geographic Poles: The ends of the magnetic field are close to, but not exactly at, the geographic poles. Consequently, a compass points to magnetic north, not geographic north.
Protective Role: The magnetic field deflects most solar wind (hot gases called plasma). Without this protection, these damaging rays would harm life on Earth.
Magnetic Field Distortion: As solar wind approaches, the side of the field closest to the Sun is pushed in, while the opposite side stretches out.
Aurora Borealis (Northern Lights): Magnificent light displays created by solar storms causing disturbances within the magnetic field.
Magnetic Reversals: The field changes constantly and has undergone numerous polarity reversals in the past; the exact reasons for these are not well understood.

Paleomagnetism: The study of the record of remnant magnetism preserved within certain rock types.
Iron-bearing Minerals: Minerals forming from lava align with Earth's magnetic field, acting as a record of the field at the time of formation.
Curie Point: The specific temperature above which minerals lose their magnetism. If minerals remain below this point, iron atoms "lock" into position, pointing to the magnetic pole.
Normal Polarity: Our current state, where the north arrow on a compass aligns closely with the geographic North Pole.
Apparent Polar Wander: The perceived movement of the Earth’s paleomagnetic poles relative to a fixed continent.
Evidence for Plate Tectonics:
- Polar wandering curves show the migration of poles across the Earth’s surface over time.
- Curves for different continents do not agree on magnetic pole locations for the past, but all converge on the current pole location today.
- This disagreement provides evidence that the continents themselves have moved (drifted) while the pole remained relatively fixed.