Within geophysics, two major branches are distinguished: internal geophysics and external geophysics.

The gravitational attraction of the Moon and the Sun gives rise to two high tides and two low tides each lunar day, or every 24 hours and 50 minutes. Therefore, there is an interval of 12 hours and 25 minutes between each high tide and between each low tide.

Gravitational forces cause rocks to press against deeper ones, increasing their density with depth. Measurements of gravitational acceleration and gravitational potential on and above the Earth’s surface can be used to search for mineral deposits (see gravity anomaly and gravimetry). The surface gravitational field provides information about plate tectonics dynamics. The geopotential surface called the geoid is a definition of the shape of the Earth. The geoid would be the global mean sea level if the oceans were in equilibrium and could extend across the continents (as with very narrow channels).

Heat Flow

The Earth is cooling, and the resulting heat flow generates the Earth’s magnetic field through the geodynamo and plate tectonics through mantle convection. The main sources of heat are primordial heat and radioactivity, although phase transitions also contribute. Heat is carried mainly to the surface by thermal convection, although there are two thermal boundary layers: the core-mantle boundary and the lithosphere, where heat is transported by conduction. Some heat is carried from the bottom of the mantle by mantle plumes.

Vibrations

Seismic waves are vibrations that travel through the Earth’s interior or along its surface. The entire Earth can also oscillate in ways called normal modes or free oscillations of the Earth. Ground motions from waves or normal modes are measured with seismographs. If the waves come from a localized source, such as an earthquake or an explosion, measurements at more than one location can be used to locate the source. Earthquake location provides information about plate tectonics and mantle convection.

The recording of seismic waves from controlled sources provides information about the region the waves travel through. If the density or composition of the rock changes, the waves are reflected. Reflections recorded by reflection seismology can provide a great deal of information about the structure of the Earth up to several kilometers deep and are used to improve our understanding of geology as well as to explore for oil and gas. Changes in travel direction, called seismic refraction, can be used to infer the deep structure of the Earth.

Earthquakes pose a risk to humans. Understanding their mechanisms, which depend on the type of earthquake (e.g., intraplate or deep-focus), can lead to better estimates of earthquake risk and improvements in seismic engineering.

Electricity

Although we notice electricity mainly during thunderstorms, there is always a downward electric field near the surface averaging 120 volts per meter. Relative to the solid Earth, the atmosphere has a net positive charge due to cosmic ray bombardment. In the global circuit, a current of about 1800 amperes flows. It flows downward from the ionosphere over most of the Earth and back upward through thunderstorms. The flow manifests as lightning below the clouds and sprites above.

Various electrical methods are used in geophysical studies. Some measure spontaneous potential, a potential that arises in the ground due to natural or man-made disturbances. Telluric currents flow in the Earth and in the oceans. They have two causes: electromagnetic induction by the Earth’s magnetic field of external and time-varying origin, and the movement of conducting bodies (such as seawater) through the Earth’s permanent magnetic field. The distribution of telluric current density can be used to detect variations in the electrical resistivity of underground structures. Geophysicists can also provide the electric current themselves.