Study guide: Earth’s Internal Structure and Dynamics

📋 Course Outline

1. Direct and Indirect Study Methods
2. Seismic Waves and Tomography
3. Geochemical Earth Model
4. Dynamic Earth Model
5. Asthenosphere Controversy
6. Deep Earth Convection and Volcanism

📖 1. Direct and Indirect Study Methods

🔑 Key Concepts & Definitions

• Gravimetric method: measures the actual value of gravity with gravimeters and compares it with the theoretical value expected from altitude and latitude
• Geothermal gradient: the increase in temperature as depth increases, and its values provide information about the physical state and chemical composition of rocks

★ Must-know

• Direct observation of Earth’s interior is limited to volcanic eruptions and drilling, which together reach only 3% of the planet’s depth.

📌 A positive gravimetric anomaly occurs when actual gravity exceeds theoretical gravity and indicates denser rocks, whereas a negative anomaly occurs when theoretical gravity exceeds actual gravity and indicates less dense rocks such as salt domes.

Further detail

• Meteorites are fragments of planetary bodies, mainly asteroids and planetesimals, formed at the same time as Earth from the same material, so their composition informs knowledge of Earth.

💡 Memory Hook

Direct observation reaches only the surface; indirect methods reveal the hidden interior.

📖 2. Seismic Waves and Tomography

🔑 Key Concepts & Definitions

• Seismic discontinuity: a boundary between layers of different composition and/or physical state, indicated by an abrupt change in wave speed and direction
• Seismic tomography: observes thousands of seismic waves from different positions and uses them to create a three-dimensional map of seismic velocities

★ Must-know

📌 P waves are longitudinal, arrive before S waves, and propagate through solids, liquids, and gases, whereas S waves are transverse and propagate only through solids.

• Wave speed increases in media with greater rigidity but decreases as density increases because the vibration of heavier atoms requires more energy.

Further detail

• The focus or hypocenter is the place inside Earth where an earthquake originates.

📌 Low seismic velocity indicates an increase in temperature, whereas increased velocity in a zone indicates lower temperatures.

💡 Memory Hook

P waves pass through all media, whereas S waves require solids.

📖 3. Geochemical Earth Model

🔑 Key Concepts & Definitions

• Geochemical model: divides Earth’s interior according to chemical composition into crust, mantle, and core, separated by seismic discontinuities

★ Must-know

• The continental crust is solid, rigid, has an average thickness of 50 km and an average density of 2.7 g/cm³, and contains igneous, metamorphic, and sedimentary rocks.

• The oceanic crust has an average thickness of 9 km, an average density of 3.0 g/cm³, consists of sediments over igneous rocks, and is no more than 180 million years old.

• The mantle is the intermediate and most voluminous layer, consists mainly of iron- and magnesium-rich peridotites, and is separated from the core by the Gutenberg discontinuity at 2,900 km depth.

📌 The external core is liquid and the internal core is solid because high pressure keeps its atoms together; both consist of an iron-nickel alloy.

Further detail

📌 The upper mantle is less rigid and has an average density of 3.5 g/cm³, whereas the lower mantle is more rigid and reaches an average density of 5.5 g/cm³.

• The core represents 16% of Earth’s volume and 31% of its mass, with an average density of 12 g/cm³, and its two parts are separated by the Lehmann discontinuity.

💡 Memory Hook

Crust → mantle → core, from exterior to interior.

📖 4. Dynamic Earth Model

🔑 Key Concepts & Definitions

• Dynamic model: divides Earth’s interior according to physical behavior into lithosphere, asthenosphere, mesosphere, level D, and endosphere
• Mesosphere: corresponds to the lower mantle of the geochemical model and, although solid, behaves like a fluid under high pressure and temperature, flowing more slowly than the asthenosphere
• Level D: a transition layer between the mesosphere and endosphere that can reach 200 km in thickness, where seismic velocities decrease considerably and partial melting and high fluidity are inferred
• Endosphere: includes the external and internal core; the external part flows rapidly, whereas the internal part flows much more slowly

📝 Essential Points

• The lithosphere is solid and rigid and consists of the crust plus part of the upper mantle; it is 60–100 km thick oceanically and 150–250 km thick continentally.

💡 Memory Hook

Lithosphere → asthenosphere → mesosphere → level D → endosphere.

📖 5. Asthenosphere Controversy

★ Must-know

• Kearey et al. (2009) and Braun (2010) considered the asthenosphere to be the entire sublithospheric upper mantle, extending to the lower-mantle boundary at 660 km, and this is the most accepted current definition.

Further detail

• Beno Gutenberg redefined the asthenosphere in 1926 as a seismic low-velocity zone beneath the lithosphere.

• Joseph Barrell defined the asthenosphere in 1914 as a mechanically weak sphere needed for isostatic compensation, with a detachment level at 100–200 km depth.

• Studies by Tackley (1993, 2000), Vidal (1994), and Kerr (1999) showed that the asthenosphere is not detected in every region and that the whole mantle flows.

💡 Memory Hook

Tomography challenged the localized asthenosphere and broadened it to the sublithospheric upper mantle.

📖 6. Deep Earth Convection and Volcanism

★ Must-know

• The level D produces thermal plumes, which are columns of hot material rising toward the surface.

• Deep subduction reaches the mesosphere–endosphere boundary, accumulates in level D, cools the upper endosphere, produces descending cold currents, and later rises again when heated.

📌 Continental volcanism results from the ascent of hot material, whereas oceanic volcanism results from melting of subducting oceanic lithosphere that may stop at the asthenosphere–mesosphere boundary.

Further detail

• Orogenic volcanism results from melting of subducting oceanic lithosphere that can reach great depth.

💡 Memory Hook

Heat and subduction drive ascending hot plumes and descending cold currents.

📊 Synthesis Tables

Geochemical layers

Layer	Main properties	Key boundary or value
Crust	Outer, solid, least dense; continental and oceanic types	Moho
Mantle	Solid peridotitic layer; upper and lower parts	Gutenberg discontinuity at 2,900 km
Core	Iron-nickel alloy; liquid external and solid internal	Lehmann discontinuity; 12 g/cm³ average density