Lecture 3 09/05/25 Earth's interior
Crust
Thin outer skin of Earth, rocky in composition
Two main types:
Oceanic crust: under oceans
Continental crust: under continents
Oceanic crust characteristics:
Composition: mainly igneous rock called basalt
Average thickness: 7 \text{--} 10\ \text{km}
Density: around 3\ \text{g cm}^{-3}
Continental crust characteristics:
Rock types vary; average is granite
Average thickness: 35--40\ \text{km} (can exceed 70\ \text{km} in high mountain belts)
Density: around 2.7\ \text{g cm}^{-3}
Why continental crust is thicker and higher than oceanic crust:
Crust is less dense than the mantle beneath, so it floats on top of the mantle
Regions with high elevation have thick crust; low-elevation regions have thinner crust
Oceanic crust is thinner and denser, so regions composed mostly of oceanic crust are well below sea level
Basalt vs Granite: basalt is typical for oceanic crust; granite is typical for continental crust
Visual analogy: Raft on a dense medium; crust floats on mantle
Mantle
Bulk of Earth’s interior; solid rock layer between crust and core
Thickness: 2{,}885\ \text{km}; volume ~ 82% of Earth
Composition: ultramafic rock called peridotite
Temperature/flow: rock is hot enough below ~100$-$150\ \text{km} to flow slowly
Convection: hot mantle rises, cold mantle sinks (slow process)
Subdivisions:
Upper mantle (a part of the lithosphere above the asthenosphere)
Transitional mantle
Lower mantle (denser and hotter)
Mantle convection illustrated: hot mantle rises, cools, moves laterally, sinks, warms, and rises again; plates form and diverge; convergence leads to subduction (cool plate sinks into mantle)
Mantle’s solid behavior over different timescales:
Short timescales: brittle, behaves like rock (earthquakes)
Long timescales: behaves like a viscous fluid (convection)
Silly Putty analogy to illustrate solid that flows over long timescales
Core
The core is an iron-rich sphere with a radius of r_{\text{core}} = 3471\ \text{km}
Divided into two components:
Inner core: radius r_{\text{inner}} = 1220\ \text{km}; solid due to immense pressures at Earth's center
Outer core: thickness t_{\text{outer}} = 2255\ \text{km}; liquid layer
Composition:
Iron–nickel alloy
Minor oxygen, silicon, and sulfur (react with iron)
Densities:
Inner core: \rho_{\text{inner}} \approx 13\ \text{g cm}^{-3}
Outer core: \rho_{\text{outer}} \approx 10{\text{--}}12\ \text{g cm}^{-3}
Surface rocks: ~3\ \text{g cm}^{-3} (for comparison)
Temperature: inner core temperature comparable to the surface of the Sun, ~6{,}000\ ^{\circ}\text{C}
Core material and evidence:
Very dense and hot; inner core remains solid under high pressure; outer core remains liquid due to high temperatures despite pressures
Additional note: iron meteorites provide evidence for core composition (iron-nickel alloy)
The Core: Rotation and Dynamo
Both inner core and outer core are rotating
Heat from the solid inner core drives convection in the liquid outer core
Convection in the outer core generates electric currents
These electric currents generate Earth’s magnetic field; field approximated as a magnetic dipole (bar magnet)
Magnetic Field and Geodynamo Evidence
Observable magnetic field exists; changes over time (secular variation) and occasional reversals
Magnetic reversals: North and South magnetic poles flip
Paleomagnetic evidence: rocks record past magnetic field states through remanent magnetization
Mechanisms by which rocks preserve ancient magnetism:
Thermoremanent magnetization: when certain rocks form, they lock in the magnetic conditions at that time
Depositional remanent magnetization: magnetization retained during rock deposition
Paleomagnetic time scale reveals the history of geomagnetic reversals; notable interval called the Cretaceous Quiet Zone (long period of normal polarity)
Time scales: magnetic chrons average about half a million years in duration, with irregularities in length but overall long sequences
Evidence for Earth’s Internal Structure: Direct Sampling and Indirect Methods
Direct sampling: deepest mine ~4\ \text{km}; deepest drill hole ~12\ \text{km}
Indirect evidence from density of Earth:
Global density of Earth ~5.5\ \text{g cm}^{-3} (Earth’s mass/volume)
Surface rocks have densities between 2.0\ --\ 2.7\ \text{g cm}^{-3}
This implies much denser material in the interior to achieve the overall higher density
Mantle rocks and xenoliths:
Mantle material occasionally samples reach the surface via volcanic magma as xenoliths
Xenoliths are pieces of mantle rock contained in erupted lava
Meteorites as clues to planetary composition:
Meteorites come in three broad types:
Stony meteorites: mantle-like material
Iron meteorites: core-like material
Stony-Iron meteorites: mix of rocky and metallic material
Meteorites support Earth’s layered composition (core/mantle/crust) and provide age/compositional evidence
Radiometric and density-based inferences complement direct sampling to characterize interior structure
Seismology: Probing the Interior with Seismic Waves
Seismic waves are energy waves radiated by earthquakes; used to infer Earth’s interior structure
Two main wave categories:
Body waves: travel through Earth’s interior
Surface waves: travel along Earth’s surface
Focus on Body Waves for interior structure
Body waves types:
P-waves (primary or compressional):
Particle motion in the direction of wave travel (compression/extension)
Can travel through solids and liquids
Fastest seismic waves
Typical speed ~v_P \approx 6\ \text{km s}^{-1}
S-waves (secondary or shear):
Particle motion perpendicular to wave travel direction
Cause shearing of material
Can only travel through solids (cannot travel through liquid)
Slower than P-waves; typical speed ~v_S \approx 4\ \text{km s}^{-1}
Seismographs record vibration as waves arrive; velocity differences reveal material properties and boundaries
Wave speeds depend on density and phase (solid vs liquid):
In general, higher density materials yield faster P-wave speeds
In a single material, P-waves are faster than S-waves; S-waves cannot propagate in liquids
Seismic wave paths are bent (refracted) when crossing boundaries where material properties change (density, phase)
The mantle is solid, allowing P- and S-waves; the outer core is liquid, causing S-waves to be blocked and P-waves to refract
Shadow Zones and Core Boundaries
Shadow zones are angular regions where certain seismic waves are not detected from a given earthquake due to core boundaries
P-wave shadow zone: not recorded at epicentral angles between roughly 103^{\circ} and 150^{\circ}
S-wave shadow zone: S-waves do not reach stations within epicentral angles greater than roughly 103^{\circ}
Interpretation:
S-waves do not travel through the liquid outer core; presence of S-wave shadow zones confirms a liquid layer
P-waves refract at the liquid/solid boundary, causing a drop in velocity when entering the outer core and bending paths downward
The two-layer core (outer liquid, inner solid) was inferred from refraction and reflection of P-waves at core boundaries
Seismic velocity changes with depth indicate phase changes and density variations; these changes map to different Earth layers
Technologies used: seismographs, seismograms, and global seismic networks (e.g., IRIS) to compile the seismic data across the globe
Seismic Wave Refraction and Boundary Crossing
Why waves bend: velocity depends on material properties; when crossing boundaries with different velocities, waves refract
Core–mantle boundary effects:
Mantle vs. core boundary causes significant velocity contrast
P-waves slow down when entering the outer core (from solid mantle to liquid outer core)
In the core, velocities behave differently due to different states of matter (solid inner core vs liquid outer core)
Velocities and materials:
P-wave velocity generally increases with depth due to increasing pressure and density
S-waves vanish in liquids; thus, their absence in the outer core is diagnostic of a liquid layer
How We Use Seismic Data to Infer Earth’s Interior
Relationship between velocity, density, and composition:
Seismic velocities reflect rock density and elastic properties
Heavy, dense materials yield higher velocities
Typical seismic velocities (approximate values):
Continental crust: density ~ \rho{crust}\approx 2.6\ \text{g cm}^{-3}, P-wave velocity ~ vP \approx 6\ \text{km s}^{-1}
Oceanic crust: density ~ 2.9\ \text{g cm}^{-3}, P-wave velocity ~ v_P \approx 7\ \text{km s}^{-1}
Mantle (average): density ~ 3.3\ \text{g cm}^{-3}, P-wave velocity ~ v_P \approx 8\ \text{km s}^{-1}
Core composition and state explain seismic observations:
Outer core is liquid (S-waves do not propagate; P-waves slow and refract into the core)
Inner core is solid (S-waves can propagate in the inner core, but their paths are affected by the solid–liquid boundaries)
Radiometric and seismic data together support a layered Earth model with distinct velocity and density profiles
Key Concepts and Connections to Foundational Principles
Layered structure of Earth by composition and by behavior
Lithosphere vs. Asthenosphere vs. Deep Mantle: mechanical vs. compositional definitions
Isostasy and buoyancy: crust floats on mantle due to density differences
Geodynamo: convection-driven magnetic field generation in the outer core
Paleomagnetism: rocks recording historical magnetic fields, enabling reconstruction of geomagnetic reversals
Seismology as a probe: inverse problem of inferring interior from wave travel times and paths
Evidence from meteorites corroborates Earth’s differentiated structure and age
Real-world relevance: plate tectonics, volcanic activity, earthquakes, and magnetic field shielding
Concept Questions (from lecture prompts)
Lithosphere vs. crust: which statements are accurate and why?
Student 1: Lithosphere and crust are the same; lithosphere is another name for the crust
Student 2: Lithosphere and crust describe different aspects; terms have different meanings
Student 3: Lithosphere lies beneath the crust; they are different layers
Which Earth layers are liquid? Which are solid?
Which crust type is thicker and which is denser?
What is special about mantle below 100–150 km depth (asthenosphere and deep mantle)?
How do P- and S-waves help map Earth’s interior?
Concept sketch: label major layers and approximate thicknesses, plus P- and S-wave paths and where they terminate or bend
Historical Perspective: How the Earth’s Layers Were Discovered
Seismic data from earthquakes and global networks revealed discontinuities in velocity and wave behavior
Early analogies: ultrasound-like techniques for Earth
Key evidence came from shadow zones, refractions, and reflections of seismic waves across core boundaries
Modern sources include global networks (e.g., IRIS) and earthquake catalogs to map Earth’s interior
Practical Activity References
Mystery Box activity (group exercise) to explore interior structure through deduction about internal features and observations
Reflect on how seismic data and physical properties infer unseen interiors
Summary of Layered Structure (Quick Reference)
Crust: outermost layer; two types (oceanic and continental); thicknesses and densities as above
Mantle: thick, ultramafic peridotite; convection drives plate tectonics; three subdivisions (upper, transitional, lower)
Core: deepest layer; outer core liquid (Fe–Ni with minor light elements); inner core solid; extreme densities and high temperatures
Lithosphere vs. Asthenosphere: mechanical behavior-based definitions
Seismic waves: P-waves (compressional, through solids and liquids, fastest) and S-waves (shear, only through solids, slower); surface waves also exist but are not the focus here
Shadow zones: critical evidence for liquid outer core and core–mantle boundaries
Evidence: density, xenoliths, meteorites, and seismology together build a consistent picture of Earth’s interior
vP \approx 6\ \text{km s}^{-1},\qquad vS \approx 4\ \text{km s}^{-1}
r{\text{inner}} = 1220\ \text{km},\quad r{\text{core}} = 3471\ \text{km},\quad t{\text{outer}} = 2255\ \text{km} \rho{\text{inner}} \approx 13\ \text{g cm}^{-3},\quad \rho{\text{outer}} \approx 10\text{--}12\ \text{g cm}^{-3} T{\text{inner}} \approx 6{,}000\,^{\circ}\text{C}