E&E 151 S Introductory Geology: Chapter 17 (Earth's Interior) Definitive Study

Geophysics

The branch of geology that applies physical laws (seismic waves, gravity, magnetic fields, heat flow) to indirectly study Earth's interior.

Seismic Reflection

The bouncing of seismic wave energy off a subsurface boundary between rock layers of differing densities back up to the surface.

Seismic Refraction

The bending of seismic waves as they cross a boundary between rock layers with different densities or seismic velocities.

Seismic Tomography

A computer-modeling technique using earthquake wave travel times along thousands of paths to build 3D cross-sections of the mantle.

Crust

Earth's thin, outermost rock shell, accounting for less than 1% of Earth's total volume.

Oceanic Crust

Thin (~5-8 km), dense (~3.0 g/cm³), mafic crust composed of basalt underlain by gabbro, with P-wave speeds of ~7 km/s.

Continental Crust

Thick (~20-70 km), less dense (~2.7 g/cm³), felsic crust composed mainly of granite, schist, and gneiss, with P-wave speeds of ~6 km/s.

Mohorovičić Discontinuity (Moho)

The distinct seismic boundary separating the base of the crust from the top of the mantle, marked by a sharp increase in P-wave velocity.

Mantle

A thick shell of solid ultramafic rock (peridotite composed of olivine and pyroxene) making up about 84% of Earth's volume.

Lithosphere

The strong, rigid outer layer consisting of the entire crust and uppermost solid mantle, broken into tectonic plates (~70 km thick beneath oceans, 125-250 km beneath continents).

Asthenosphere

The hot, ductile layer of upper mantle directly below the lithosphere containing a low-velocity zone where rocks are near their melting point and flow slowly.

D'' Layer & Ultralow-Velocity Zones (ULVZ)

A ~200 km transition zone at the base of the mantle where seismic velocities drop sharply due to partial melting or chemical reactions with the core.

Core

The central metallic sphere (15% of Earth's volume), composed primarily of an iron-nickel alloy.

Outer Core

The liquid metallic outer portion of the core.

Inner Core

The solid metallic center of Earth.

P-wave Shadow Zone

The region between 103° and 142° from an earthquake epicenter where direct P waves disappear because they are bent sharply inward by Earth's core.

S-wave Shadow Zone

The region beyond 103° from an epicenter where direct S waves are completely absent, proving that Earth's outer core is liquid.

Isostasy

The state of gravitational balance or equilibrium where rigid lithospheric blocks float on the denser, ductile asthenosphere.

Mountain Root

A deep extension of low-density continental crust projecting downward into the mantle beneath high mountain ranges.

Isostatic Adjustment (Crustal Rebound)

Vertical movement of the lithosphere to regain equilibrium, such as crust depressing under ice mass or rising up (glacial rebound) after ice melts.

Gravity Meter (Gravimeter)

An instrument measuring local gravitational attraction to detect rock density contrasts.

Positive Gravity Anomaly

A gravity reading higher than normal, indicating dense underground rock or crust held above equilibrium by tectonic forces.

Negative Gravity Anomaly

A gravity reading lower than normal, indicating low-density rock/sediments or crust held down below equilibrium.

Geoid

The hypothetical equipotential surface of Earth's gravity field that best fits global mean sea level.

Geodynamo & Magnetic Field

Earth's magnetic field, generated by electric currents in the rapidly convective liquid iron outer core.

Magnetic Reversal

Periodic flips in Earth's magnetic polarity (normal vs. reversed) preserved in cooling igneous rocks.

Curie Point

The temperature threshold (~580°C for magnetite) below which magnetic minerals lock in Earth's magnetic field orientation.

Paleomagnetism

The study of ancient magnetic fields preserved in iron-bearing rocks.

Geothermal Gradient

The rate of temperature increase with depth inside Earth, averaging ~25°C/km near the surface.

Heat Flow

Gradual loss of internal heat through Earth's surface, driven by radioactive isotope decay and primordial heat.