Earth's Layers and Tectonics

Overview

This lecture covers the structure and composition of Earth's layers, their formation, and the processes driving plate tectonics and the magnetic field.

Earth's Crust

• Earth's crust is divided into continental (felsic, silica-rich, 20–70 km thick) and oceanic (mafic, iron/magnesium-rich, 5–10 km thick) types.
• Oceanic crust is denser than continental crust and subducts below it during collisions, a process called subduction.
• Continental crust contains ancient regions called cratons (up to 4 billion years old); oldest oceanic crust is only 340 million years old.

The Mantle

• The mantle is solid but slowly convects over millions of years, transferring heat from Earth's interior to the surface.
• Composed mostly of ultramafic peridotite (rich in olivine), temperature can reach up to 3500°C.
• Slab pull, caused by the increasing density of aging oceanic crust, drives plate tectonics as subducting slabs sink into the mantle.
• The D’’ (D double-prime) layer at the base of the mantle may be where subducted slabs accumulate and mantle plumes originate.

The Core

• Below the mantle, the liquid outer core (mainly iron and nickel) convects, creating Earth's magnetic field via the geodynamo effect.
• The inner core is solid iron, hotter than the sun’s surface, and rotates faster than the rest of Earth (super-rotation).
• The Bullen discontinuity marks the boundary between the liquid outer and solid inner core.

Earth's Magnetic Field and Reversals

• The geodynamo is the process in which the movement of conductive liquid iron in the outer core generates Earth's magnetic field.
• Periodic magnetic field reversals occur, sometimes failing, with the last reversal about 780,000 years ago.
• Collapses of cold, subducted slabs into the D’’ layer may reorganize convection and trigger reversals.

Differentiation and Evolution of Earth's Layers

• Earth formed hot and homogeneous from chondrite meteorites, then differentiated as heavy materials sank and lighter ones rose.
• The mantle and crust crystallized as Earth cooled, while the core stayed molten longer.
• The inner core is currently growing as the outer core’s liquid iron solidifies.
• Eventually, if Earth survived long enough, the core would fully solidify and the magnetic field would disappear.

Key Terms & Definitions

• Crust — Outermost, thin rocky layer of Earth, divided into continental and oceanic types.
• Subduction — Process where denser oceanic crust sinks beneath less dense continental crust.
• Mantle — Thick, solid, yet slowly convecting layer beneath the crust.
• Peridotite — Ultramafic, olivine-rich rock making up most of the mantle.
• D’’ Layer — Boundary at mantle’s base, possibly a graveyard for subducted slabs and source of mantle plumes.
• Outer Core — Liquid, iron-nickel layer generating Earth's magnetic field.
• Inner Core — Solid, iron-rich center of Earth, rotates faster than the rest of the planet.
• Geodynamo — Mechanism where convection in the liquid outer core generates a magnetic field.
• Magnetic Reversal — Event where Earth’s magnetic field poles flip.

Action Items / Next Steps

• Review the physical properties of each Earth layer.
• Prepare for discussion on how seismic waves reveal Earth's internal structure.