Geophysics

Geophysics is the branch of physical science that investigates Earth and its surrounding space through quantitative observation and measurement. Its core subjects include Earth’s shape, its gravitational, magnetic, and electromagnetic fields, and the planet’s internal structure, composition, and movements—along with how these manifest at the surface as tectonics, volcanism, and rock formation. The field takes a broad view, covering the entire Earth system: oceans, atmosphere, cryosphere, ionosphere, and magnetosphere, as well as interactions between the Sun and Earth, and similar processes on the Moon, other planets, and their moons.

One of the oldest sciences, geophysics traces back to ancient times with early seismometers and magnetic compasses, later expanding through Newtonian studies of tides, precession, and Earth’s physical properties. Today, it serves both fundamental science and practical needs: exploring mineral and energy resources, assessing and reducing natural hazards, studying groundwater and the environment, conducting archaeological surveys, and monitoring environmental change.

The discipline combines theory, observation, and experiment to study Earth and planetary systems across many scales of space and time. Its methods include fieldwork, lab experiments, remote sensing, numerical modeling, and analysis of natural and instrumented records—including signal processing. As a unifying physical framework for Earth and planetary science, geophysics links phenomena from the deep interior to the upper atmosphere and near-space environment, forming the basis for understanding how planets evolve, behave dynamically, and become habitable.

**Physical phenomena**

Geophysics is highly interdisciplinary, with geophysicists contributing to all areas of Earth science, and some also working in planetary science. To clarify what geophysics involves, this section describes physical phenomena and how they relate to Earth and its surroundings. Geophysicists also study the physical processes and properties of Earth, its fluid layers, and magnetic field, along with the near-Earth environment in the solar system, including other planets.

**Gravity**

The gravitational pull of the Moon and Sun produces two high tides and two low tides each lunar day (24 hours and 50 minutes), so there is a 12-hour-25-minute gap between successive high tides and between successive low tides. Gravity compresses deeper rocks, increasing their density with depth. Measurements of gravitational acceleration and potential at and above Earth’s surface can locate mineral deposits (see gravity anomaly and gravimetry). The surface gravity field reveals information about tectonic plate dynamics. The geopotential surface known as the geoid defines one shape of Earth—it would be global mean sea level if the oceans were in equilibrium and could be extended through continents (for example, via very narrow canals).

**Vibrations**

Seismic waves are vibrations traveling through Earth’s interior or along its surface. The whole Earth can also oscillate in forms called normal modes or free oscillations. Ground motions from waves or normal modes are measured with seismographs. When waves come from a localized source like an earthquake or explosion, measurements at multiple locations can locate the source. Earthquake locations provide information on plate tectonics and mantle convection. Recording seismic waves from controlled sources reveals details about the regions they travel through. Changes in rock density or composition cause reflections; these reflections, recorded using reflection seismology, can show structure up to several kilometers deep and are used to understand geology and explore for oil and gas. Changes in travel direction (refraction) can infer Earth’s deep structure. Earthquakes pose risks to humans; understanding their mechanisms—depending on type (e.g., intraplate or deep focus)—can improve risk estimates and earthquake engineering.

**Electricity**

A downward electric field near Earth’s surface averages 120 volts per meter. Galactic cosmic rays penetrating the atmosphere ionize it relative to the solid Earth, leaving the atmosphere with a net positive charge. About 1800 amperes flow in the global circuit—downward from the ionosphere over most of Earth and back upward through thunderstorms. This flow appears as lightning below clouds and sprites above. Various electric methods are used in geophysical surveys. Some measure spontaneous potential arising from natural or human-made disturbances. Telluric currents flow in Earth and oceans, caused by two things: electromagnetic induction from the time-varying, external geomagnetic field, and the motion of conducting bodies (like seawater) across Earth’s permanent magnetic field. The distribution of telluric current density can detect variations in underground electrical resistivity. Geophysicists can also introduce electric currents themselves (see induced polarization and electrical resistivity tomography).

**Electromagnetic waves**

Electromagnetic waves occur in the ionosphere and magnetosphere, as well as in Earth’s outer core. Dawn chorus is thought to be caused by high-energy electrons trapped in the Van Allen radiation belts.