Category: Geography

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…

Introduction — Passive margin A passive margin is the lithospheric transition between continental and oceanic domains that is not an active plate boundary; it records a former rifted edge of a continent that presently lacks plate‑boundary seismicity, active convergence, or subduction. These margins originate through a rifting‑to‑spreading evolution in which continental extension progresses to the…

Pancake domes — Introduction Pancake domes are a distinctive class of volcanic edifice found on Venus, representing a morphotype unique to Venusian volcanism. Individual examples, notably the Carmenta Farra group, illustrate their characteristic form. These domes are broadly scattered across the planet, typically occurring in clusters; however, such clusters contain fewer individual domes than the…

Introduction Paleomagnetism is the branch of geophysics that reconstructs Earth’s past magnetic field by analyzing magnetic signals locked into rocks, sediments and archaeological materials. Ferromagnetic minerals acquire a stable remanent magnetization during formation or early diagenesis that records the ambient geomagnetic vector (direction and intensity); these preserved signals serve both as archives of field behavior…

Introduction The Paleogene Period (66.00–23.04 Ma) is the opening interval of the Cenozoic Era and the tenth period of the Phanerozoic Eon, encompassing 43 million years of Earth history. It is formally divided into three successive epochs—Paleocene, Eocene, and Oligocene—each recording distinct phases of environmental and biotic change within the 66–23.04 Ma span. Nomenclatural variants…

Introduction — Pacific Plate The Pacific Plate is the largest oceanic tectonic plate, underlying the Pacific Ocean and encompassing roughly 103 million km2. It originated in the Early Jurassic, about 190 million years ago, as a small plate that formed at a triple junction between the Farallon, Phoenix, and Izanagi plates. Once established, the plate…

P wave — Introduction The P wave (primary or pressure wave) is the fastest seismic body wave and therefore normally constitutes the first clear phase recorded at a site following an earthquake. It is a compressional wave that propagates by alternately compressing and dilating material in the direction of travel, and unlike shear waves it…

Introduction From space the ozone layer is visible as a narrow blue band at Earth’s limb, lying within the lower edge of the broader stratospheric glow and commonly seen above the orange twilight of the troposphere, where cloud tops such as cumulonimbus appear in silhouette. Physically, the ozone layer denotes the region of the stratosphere…

Introduction Plate tectonics provides the overarching framework for understanding the behavior of the Earth’s outer rigid shell, the lithosphere, which is segmented into rigid plates that move horizontally over a weaker, ductile asthenosphere. This system of relative plate motions accounts for the global patterns of earthquakes, volcanism, mountain building, the opening and closing of ocean…

Introduction — Outline of Earth Sciences Earth science, often termed geoscience or the geosciences (and occasionally referenced as “Earthquake sciences”), comprises the collective scientific disciplines that investigate planet Earth. Situated within the physical sciences and the wider natural sciences, it employs empirical, physics-based and quantitative methods to observe, model and explain Earth’s systems. Earth’s unique…

Introduction Orogeny denotes the principal mountain‑building process that operates at convergent plate boundaries, where compressive plate motions deform, thicken and uplift crustal segments to form orogenic belts (orogens). Orogenesis encompasses a range of concurrent and successive geological processes: large‑scale structural deformation of pre‑existing continental crust (notably folding, thrusting, faulting and crustal thickening) together with syntectonic…

Introduction Oceans, which cover roughly 71% of Earth’s surface, dominate the planet’s geography and are fundamental to its capacity to support life. Geologic–biologic events from the Hadean through the Archean, Proterozoic and Phanerozoic eons (ca. 4,500–0 Ma) record the progressive emergence of water and life: formation of the planet and earliest evidence for water, the…

The Okinawa plate, often termed the Okinawa platelet, is a minor continental tectonic block located in the northern and eastern hemispheres. It extends along a north–south corridor from the northern extremity of Taiwan to the southern tip of Kyūshū and occupies the marine and insular domain between those landmasses, incorporating features such as Kikai Island…

Introduction The Okhotsk Plate is a minor tectonic block occupying northeastern Asia and adjacent seas, including the Kamchatka Peninsula, Magadan Oblast, Sakhalin Island, the Sea of Okhotsk, the Kuril Islands (disputed), and parts of northern Japan (Hokkaidō, Kantō and Tōhoku). Its position defines a discrete crustal entity whose boundaries coincide with major zones of deformation…

Introduction The contemporary account of India’s oil and gas industry begins with an explicit caveat: the source material (dated August 2025) incorporates text from a large language model and may contain hallucinated details or unverified references, so assertions and citations should be critically assessed and any copyright or unverifiable claims removed before reuse. Commercial petroleum…

Introduction — Oceanic crust Oceanic crust forms the outermost layer of Earth’s oceanic plates and, together with the immediately underlying rigid portion of the upper mantle, constitutes the oceanic lithosphere. It is markedly different from continental crust in composition, thickness and density: typically under 10 km thick and with a mean density near 3.0 g…

Introduction An oceanic (or ocean) basin can be understood from two complementary perspectives. Hydrologically, it denotes any part of the Earth’s surface covered by seawater; geologically, it refers to extensive subsiding basins—broad depressions below sea level—occupied by the oceans. Ocean basins are most commonly delineated on the basis of continental distribution, a convention that yields…

Introduction The ocean covers roughly 70.8% of Earth’s surface and is customarily partitioned into five principal basins—the Pacific, Atlantic, Indian, Southern (Antarctic) and Arctic—which are further subdivided into seas, gulfs and other named marine basins. Containing about 97% of the planet’s water, the ocean dominates the hydrosphere and functions as a vast thermal reservoir and…

Introduction — Ocean world An ocean world is a planet or natural satellite whose hydrosphere includes substantial liquid, either at the surface—potentially inundating all continental land—or as subsurface oceans beneath an outer shell. The broader concept of a thalassogen recognizes that such oceans need not be liquid water: other fluids (for example molten rock, eutectic…