Geophysics

Geophysics applies physics and mathematics to the structure, composition, and evolution of planets, including Earth. It connects measurements with mechanisms: plate tectonics explains moving crust, the geodynamo explains a planetary magnetic field, and albedo connects reflected sunlight to a planetary surface energy balance. Even a small model can reveal a useful organizing principle, such as the competition between ice-ocean albedo feedback and stabilizing thermal radiation.

A climate statistic calculated over a stated long reference period, conventionally often 30 years. Specify the variable, site, period and averaging procedure; a changing climate makes the choice of baseline consequential.^[ref]

The study of how rock composition, structure, pressure, temperature, fluids and deformation determine measurable physical properties such as density, elastic wave speeds, electrical conductivity and attenuation. These relations are needed to interpret seismic tomography and gravity anomalies; one measured property rarely determines a unique material state.

British geological survey organization publishing geological and earthquake observations and explanatory material. www.bgs.ac.uk

An electromagnetic method using naturally varying fields to infer subsurface electrical resistivity; conductive anomalies require geological interpretation.

Failure and widening of a borehole wall under concentrated stress; its orientation helps constrain in situ principal stress directions.

The spatial distribution of temperature in a region, such as a slab and its surrounding mantle wedge.

Institutional teaching source for applied geophysical survey methods.

Instrument manufacturer and issuer of manuals for field magnetometers.

Measurement of a magnetic field over a region to locate contrasts in subsurface magnetization. Temporal background changes and cultural interference must be distinguished from spatial magnetic anomalies.

A stationary magnetometer recording time variation of the background magnetic field while a moving instrument measures spatial variation. Subtracting the synchronized base changes reduces temporal contamination.

An absolute scalar magnetometer that infers $|\mathbf{B}|$ from the free Larmor precession frequency of polarized protons. It measures field strength rather than field direction.

Study of ancient magnetic-field records preserved in rocks. Primary magnetizations can constrain former latitude and rotation, subject to age and alteration tests.

Acquisition of a younger remanent magnetization that partly or completely replaces an older magnetic record. Heating, chemical alteration or later mineral growth can cause it. A successful continental reconstruction must connect the age of the measured magnetic component to the geological event being reconstructed.

Test whether normal- and reversed-polarity remanent magnetization directions are statistically antipodal after suitable structural correction. Agreement supports recording of opposite polarities of the same large-scale magnetic field; failure can reveal overprints or other complications. Passing is not, by itself, proof of primary acquisition.

Compare the scatter of remanent magnetization directions before and after correcting for folding. Improved agreement after restoring bedding supports acquisition before folding; agreement only in present coordinates supports a younger overprint. Partial unfolding can indicate acquisition during deformation or more complicated histories. The test constrains relative timing, rather than supplying an absolute age.

A time-ordered sequence of ancient magnetic pole positions inferred from dated remanent magnetization in a continental block. The apparent motion includes the block's movement relative to the rotation axis and can include true polar wander. Consistent paths after a fixed relative rotation support an interval of shared motion; conventional paleomagnetism does not independently determine absolute longitude.

Quantitative simulation of evolving planetary deformation, flow and thermal or compositional structure under specified physical laws and constitutive assumptions. Feasibility of a regime does not supply its historical onset date.

An air parcel is a small material volume followed through an atmospheric flow. The elementary parcel model assumes rapid pressure adjustment to its environment but negligible heat and material exchange during the displacement. Its potential temperature is conserved during reversible dry adiabatic process at fixed composition.

Loose fragmented material above coherent bedrock. On the Moon its fractures and debris strongly scatter seismic waves, complicating detection of weak deep-interior phases.

The gravitational potential energy per unit mass, including the centrifugal contribution when using the effective gravity of a rotating planet. In a local atmosphere with approximately constant effective gravitational acceleration, $\Phi =gz$ up to a constant. Its horizontal gradient on an isobaric surface drives the balanced wind.

A surface on which geopotential is constant. Effective gravity is perpendicular to it. In a local constant-gravity approximation these surfaces are horizontal; comparing their slopes with isopycnals identifies the geometry permitting an adiabatic parcel exchange to release available potential energy.

The statistical properties of atmospheric and related oceanic conditions over an extended period, including means, variability and extremes. Radiation budgets, ocean circulation and atmospheric transport help determine its large-scale structure.

The climate of an earlier geological interval, inferred from dated geological, geochemical or biological indicators. Spatially coherent glacial deposits, striations and climate-sensitive rocks can test a continental reconstruction, provided their ages and original tectonic contexts are established.

A region of reduced precipitation on the leeward side of a topographic barrier.

The energy entering and leaving a system through electromagnetic radiation. A top-of-atmosphere radiation budget compares absorbed sunlight with outgoing thermal radiation. A latitude-dependent imbalance can be balanced by atmospheric and ocean heat fluxes even when the global mean is zero.

Publisher of IERS Conventions (2010) by International Earth Rotation and Reference Systems Service.

The geothermal gradient is the rate at which temperature varies with depth. For steady one-dimensional thermal conduction with constant thermal conductivity $k_T$, the magnitude of upward heat flux is $q=k_{T}dT/dz$.

For a crustal layer of thickness $h_c$ with constant thermal conductivity $k$, no internal heating and a linear conductive profile, the upward heat flux magnitude $q$ obeys Fourier's law, $T(z)=T_s+qz/k$ with depth positive downward. Depth averaging givesThe basal excess temperature is $q{h}_c/k$, twice the mean excess. The formula requires a thickness assumption and a conductive interpretation of the measured flux. Hydrothermal advection, sedimentary thermal effects, internal production and nonlinear conductivity can invalidate a linear extrapolation.

The planetary mantle is the predominantly silicate rock region beneath a rocky planet's crust. On geological timescales, solid rock can undergo material creep and support mantle convection without wholesale melting.