Earth's Internal Structure Overview

Overview

This lecture covers Earth's internal structure, detailing the properties, composition, and dynamics of the crust, mantle, and core, and how these influence geological phenomena.

Earth's Crust

• Earth's crust is the planet's thin outermost layer, divided into continental and oceanic types.
• Continental crust is felsic (silica-rich), 20–70 km thick, and less dense.
• Oceanic crust is mafic (iron- and magnesium-rich), 5–10 km thick, and more dense.
• Oceanic crust subducts beneath continental crust at convergent boundaries due to higher density.
• Subducted oceanic crust is recycled and rarely older than 340 million years, while continental cratons can be over 4 billion years old.

The Mantle

• The mantle lies below the crust, from 70 to 2900 km deep, made mostly of peridotite.
• The mantle is solid but flows slowly (convects) due to internal heat.
• Mantle convection drives plate tectonics, with hot rising plumes and cold sinking slabs.
• The D’’ layer, a mysterious zone at the mantle's base, may collect subducted slabs ("slab graveyard") and spawn mantle plumes.
• Diamonds form at depths over 150 km within mantle-derived rocks called xenoliths.

Plate Tectonics & Forces

• Subducting oceanic slabs create a "slab pull," the main driver of plate motion.
• Buoyancy changes as oceanic crust ages and increases in density, enabling subduction.

Core Structure and Dynamics

• Below the mantle is the liquid outer core (mainly iron and nickel), 2900–5100 km deep, with temperatures up to 5500°C.
• Outer core convection, influenced by Earth's rotation, generates Earth's magnetic field (geodynamo).
• The solid inner core, starting at 5100 km (Bullen discontinuity), stabilizes and anchors magnetic field lines.
• The inner core rotates faster than Earth's surface ("super-rotation").

Magnetic Field & Reversals

• Earth's magnetic field is generated by movements in the outer core.
• Magnetic reversals periodically flip the field; last reversal was 780,000 years ago.
• Sudden changes in D’’ layer dynamics may trigger reversals.

Earth's Formation & Differentiation

• Earth began as a hot, homogeneous body, later melting and differentiating by density.
• Heavy metals like iron and nickel sank to form the core; lighter materials formed the mantle and crust.
• The inner core is slowly growing as the planet cools and the liquid core solidifies.
• If Earth's cooling continues, the core will eventually solidify completely, ending the magnetic field.

Key Terms & Definitions

• Crust — Earth's outermost solid layer, includes continental and oceanic types.
• Mantle — Layer beneath the crust, solid but convects over geologic time.
• Peridotite — Ultramafic igneous rock, main component of the mantle.
• Subduction — Process in which denser oceanic crust sinks beneath lighter continental crust.
• D’’ Layer — Transition zone between mantle and outer core, site of complex dynamics.
• Outer Core — Liquid layer of iron/nickel, source of Earth's magnetic field.
• Inner Core — Solid iron/nickel sphere at Earth's center, stabilizes magnetic field.
• Geodynamo — Mechanism generating Earth's magnetic field via core convection.
• Cratons — Ancient, stable regions of continental crust.
• Xenolith — Foreign rock fragment from deep mantle, brought up by volcanic eruptions.

Action Items / Next Steps

• Review the physical properties of each Earth layer.
• Prepare to learn how seismic waves reveal the structure and properties of Earth's interior.