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Introduction Skyquakes are atmospheric acoustic events characterized by sudden, loud noises perceived as originating overhead rather than from obvious terrestrial sources. Observers typically describe an abrupt, forceful sound—most often a sharp “bang” or a brassy, horn‑like blast—that stands out clearly from ambient noise. The acoustic energy of these events can couple into nearby buildings, producing…

Introduction A sinkhole is a natural topographic depression formed when surface material subsides or collapses into voids in the subsurface, and it often marks a direct connection between surface and underground systems. Various terms are used to emphasize different aspects of these features: doline or shakehole for enclosed depressions; ponor, swallow hole or swallet for…

Introduction The Shetland (South Shetland) plate is a small tectonic microplate located off the tip of the Antarctic Peninsula that encompasses the South Shetland Islands and constitutes a distinct crustal block within the Southern Ocean. Bordered on three sides by the Antarctic plate and on the fourth by the Scotia plate, it occupies a relatively…

Introduction Seismology is the branch of geophysics devoted to the study of earthquakes and the elastic waves they generate as they propagate through planetary interiors and across surfaces. The term derives from the Ancient Greek σεισμός (seismós, “earthquake”) combined with -λογία (-logía, “study of”) and is pronounced /saɪzˈmɒlədʒi/ or /saɪs-/. Its scope includes characterization of…

Seismic waves are transient elastic disturbances that transport vibrational energy through Earth and other planetary bodies, generated by abrupt mechanical events such as fault rupture, volcanic activity, magma migration, large landslides, and anthropogenic explosions. They are distinct from the continuous, low-amplitude background vibrations produced by natural and human activity and are characterized by measurable waveforms,…

Seismic velocity structure denotes the spatial distribution of seismic-wave speeds within the subsurface of Earth and other planetary bodies, providing a depth- and laterally varying description of how seismic energy propagates rather than a single bulk value. The principal observables in such profiles are compressional (P-) velocity, shear (S-) velocity, and bulk density; in the…

Introduction Seismic noise denotes the persistent ground vibrations recorded by seismometers that are generally generated at or near the Earth’s surface and are therefore strongly controlled by elastic surface-wave propagation. By convention the ambient field is partitioned by frequency: signals below about 1 Hz are usually labeled microseisms, while those above 1 Hz are called…

Seiche — Introduction A seiche (pronounced /seɪʃ/) is a standing oscillation that occurs in an enclosed or partially enclosed body of water—lakes, reservoirs, swimming pools, bays, harbours, caves and even semi‑enclosed seas—when reflections from basin limits sustain a resonant wave. The phenomenon requires at least partial bounding so that incident and reflected motions interact persistently…

Introduction to sedimentary rocks Sedimentary rocks originate at Earth’s surface through the accumulation, burial and lithification of particulate material derived either from the breakdown of older rocks or from the remains and products of organisms. Sedimentation encompasses any mechanism that leads particles or chemical precipitates to settle and become incorporated into the sedimentary record, whether…

Sedimentary basins are region-scale crustal depressions produced by long-term subsidence that generate accommodation space for accumulation of thick, three-dimensional packages of sedimentary rock. Infill occurs over millions to hundreds of millions of years, largely by gravity-driven transport of eroded material and marine deposition; progressive burial imposes increasing pressure that compacts and lithifies the deposits. Basins…

Introduction A sea is a large body of saline water; in general usage “the sea” refers to the world ocean—the contiguous body of seawater that covers the majority of Earth’s surface—while individual seas include open-ocean marginal seas (for example, the Mediterranean) and large, nearly landlocked basins. In coastal settings such as the Faroe Islands in…

The Scotia Plate is a small tectonic plate occupying the Scotia Sea at the southern margin of the South Atlantic. Its name derives from the steam yacht Scotia, used by the early 20th-century Scottish National Antarctic Expedition that produced the first bathymetric data for the region. Plate genesis is linked to late Eocene tectonics: the…

Sand dunes, whether on coasts or in deserts, are mobile aeolian landforms that serve both geomorphic and ecological functions. They act as dynamic reservoirs of wind-blown sand, provide habitat for specialized—and sometimes rare or endangered—plants and animals, and buffer adjacent shorelines by storing and releasing sediment, thereby reducing coastal erosion. Dune morphology and position strongly…

Introduction Shear or S waves are the transverse elastic body waves that carry information about the Earth’s internal shear response. Particle motion in an S wave is perpendicular to propagation and is restored by shear stresses, in contrast to compressional (P) waves whose motion is parallel to propagation and whose restoring force is volumetric compression….

Rubidium–strontium (Rb–Sr) dating is a radiometric chronometer that exploits the beta decay of 87Rb to 87Sr (half‑life 49.23 billion years) to determine rock and mineral ages. Strontium occurs naturally as four stable isotopes; 87Sr is unique among them because its abundance can increase in a closed system by in‑situ decay of 87Rb while the non‑radiogenic…

Introduction The rock cycle is a central conceptual framework for interpreting how the three principal rock types—igneous, sedimentary and metamorphic—transform into one another over geologic time. It frames these transformations as responses to disequilibrium: rocks are carried into new physical, chemical or biological environments by internal and external forces (for example, tectonic transport, burial, uplift,…

Rocks are naturally forming solid aggregates of minerals or mineraloids that compose Earth’s outer solid shell (the crust) and the greater part of the planet’s solid interior, aside from the liquid outer core and transient magma bodies within the asthenosphere. They are characterized and classified by their mineral constitution, chemical makeup, textures and by the…

The Pacific “Ring of Fire” (also Rim or Circum‑Pacific belt) is an arcuate, subduction‑dominated zone of volcanic and seismic activity that encircles much of the Pacific Ocean. Extending roughly 40,000 km and locally up to about 500 km wide, the belt hosts between 750 and 915 active or dormant volcanoes — roughly two‑thirds of the…

Reverse weathering describes authigenic clay‑forming reactions in aquatic sediments and pore waters whereby new clay minerals precipitate from dissolved or particulate precursors, effectively consuming dissolved cations and alkalinity by pathways distinct from continental silicate breakdown. Two principal mechanistic routes have been identified: neoformation via interaction of biogenic silica (SiO2) with aqueous cations or cation‑bearing oxides…

Remotely triggered earthquakes — Introduction Remotely triggered earthquakes are seismic events that occur outside the immediate aftershock zone of a large earthquake, separated in space and/or time from the initiating rupture such that causal linkage becomes increasingly uncertain with greater distance and elapsed time. Their study rests on two complementary physical mechanisms. First, static stress…

Introduction — Reflection Seismology Reflection seismology is an exploration geophysics technique that infers subsurface stratigraphy and physical properties by recording seismic waves that return to the surface after reflecting from underground interfaces. Data consist principally of the travel times and amplitudes of reflected arrivals, acquired with controlled energy sources and receiver arrays (e.g., land geophones…

Introduction Rare‑earth minerals are those in which one or more rare‑earth elements (REEs) constitute primary metallic components of the mineral formula rather than trace impurities. They are most commonly associated with silica‑undersaturated, highly alkaline to peralkaline magmatic systems and are frequently enriched in pegmatitic fractions or within carbonatite intrusions, reflecting a magmatic affinity for alkaline…

The Rare Earth hypothesis, articulated most prominently by Peter Ward and Donald E. Brownlee in their 2000 book Rare Earth: Why Complex Life Is Uncommon in the Universe, contends that the emergence of biologically complex life—specifically sexually reproducing, multicellular organisms and the evolutionary trajectories that can yield advanced intelligence—depends on a highly improbable conjunction of…

At Water Canyon on the Isle of Arran, a marine terrace now situated about 4 m above present high tide — implicated in the development of King’s Cave — underlies an older terrace near 30 m, with associated relict sea‑cliffs exposed along the island’s south‑west coast. Such examples illustrate the defining characteristics of raised beaches…

Introduction A pyroclastic flow (also called a pyroclastic density current or pyroclastic cloud) is a ground-constrained, rapidly moving mixture of hot volcanic gas and fragmented rock (tephra) that travels outward from a vent or volcanic edifice. Typical translational speeds are on the order of 100 km/h (≈30 m/s; ≈60 mph), although the most extreme flows…

Precipitation denotes any condensed atmospheric water that falls from clouds under gravity; it requires air to reach saturation so that droplets or ice crystals grow sufficiently to overcome updrafts and descend. Observable categories range from light drizzle and freezing drizzle through rain, rain–snow mixtures (sleet in Commonwealth usage), snow, ice pellets, graupel and hail, to…

Introduction to Plate Tectonics Plate tectonics is the unifying theory that describes Earth’s rigid outer shell—the lithosphere, comprising crust plus the uppermost mantle—as fractured into discrete, mobile plates. Evolving from the continental‑drift hypothesis, the theory achieved broad acceptance after mid‑20th century confirmation of seafloor spreading; geological evidence indicates plate behavior has operated for much of…

Plate reconstruction systematically recovers the past positions and motions of tectonic plates to rebuild former continental and oceanic configurations and to provide the spatial framework for paleogeographic interpretation. Reconstructions may be carried out in relative terms (plate-to-plate fits and plate circuits) or referenced to absolute frames; common absolute frameworks derive from paleomagnetism, chains of age-progressive…

Introduction Star and planet formation begins within the diffuse interstellar medium, where gas and dust concentrate into cold, dense molecular clouds. Within these clouds compact, optically thick condensations such as Bok globules and dark nebulae shield their interiors from external radiation, permitting gravitational fragmentation and collapse. Collapsing fragments accrete material from surrounding envelopes and disks…

Planetary habitability is an evaluative construct in astrobiology that integrates geological, atmospheric, orbital and stellar parameters to estimate a planet’s or satellite’s capacity to develop and sustain environments favorable to life. Earth serves as the principal empirical benchmark—many criteria are framed by terrestrial physical and chemical regimes—while recognizing that alternative biochemistries or environmental thresholds remain…

Planet-hosting stars constitute the dominant mass, energy source, and dynamical centre of planetary systems; their fundamental properties set the initial conditions for planet formation and govern subsequent orbital and atmospheric evolution. Stellar mass and spectral type largely determine disk mass and temperature structure, producing systematic differences in planetary architectures: early-type (A–F) stars, with higher masses…

Introduction Plagioclase denotes a continuous solid-solution series of framework silicates within the feldspar group rather than a single mineral species. Composition varies progressively between the albite and anorthite endmembers, whose ideal stoichiometries are NaAlSi3O8 and CaAl2Si2O8, respectively. Variations arise from coupled substitution of Na+ and Ca2+ on equivalent lattice sites in the tetrahedral framework; this…

The Assam Valley and its environs occupy a clearly delimited sector of northeastern India between 89°42′–96°00′E and 24°08′–28°02′N, encompassing 78,438 km2—an extent comparable to Ireland or Austria. This longitudinal span of ~6.3° and latitudinal span of ~3.9° frame the region’s insolation regime, exposure to prevailing winds and the broad bioclimatic gradients that influence its hydroclimate…

The Phoenix plate (alternatively Aluk or Drake) was an oceanic plate that operated from the early Paleozoic into the late Cenozoic and played a long‑lived role in plate interactions across Panthalassa and the southwestern Pacific. Its early history included establishment of a triple junction with the Izanagi and Farallon plates by at least ~410 Ma…

Philippine Sea Plate — Introduction The Philippine Sea plate (also termed the Philippine plate) is an oceanic lithospheric plate underlying the Philippine Sea to the east of the Philippine archipelago; it is distinct from the Philippine Mobile Belt, which comprises most of the Philippine islands (including northern Luzon). Its margins are dominated by active convergent…