Glossary

The Chicxulub crater is a buried impact structure approximately 180 km in diameter located beneath the Yucatán Peninsula, Mexico, with its centre near the coastal town of Chicxulub Puerto. It was formed approximately 66 million years ago when an asteroid estimated at 10–15 km in diameter struck the Earth at a speed of roughly 20 km/s, releasing kinetic energy on the order of 72 teratonnes of TNT equivalent (approximately 300 zettajoules). The impact generated a transient cavity roughly 100 km wide and 30 km deep, which subsequently collapsed to form the final crater structure, including a prominent peak ring approximately 90 km in diameter. The impact is widely accepted as the primary cause of the Cretaceous–Paleogene (K–Pg) mass extinction, which eliminated approximately 75–80% of all species on Earth, including all non-avian dinosaurs. The collision injected vast quantities of dust, sulfate aerosols, and soot into the atmosphere, triggering a global impact winter that suppressed photosynthesis, disrupted food chains, and caused severe temperature fluctuations lasting years to decades. This event marks the boundary between the Mesozoic and Cenozoic eras, fundamentally reshaping the trajectory of life on Earth and enabling the subsequent radiation of mammals, birds, and flowering plants into ecological niches formerly occupied by dinosaurs and other Mesozoic fauna.

The Cretaceous–Paleogene (K-Pg) extinction event is a mass extinction that occurred approximately 66 million years ago at the boundary between the Cretaceous and Paleogene periods. It is the most recent of the geological 'Big Five' mass extinctions. The primary cause was the impact of an asteroid roughly 10 km in diameter that struck what is now the Yucatán Peninsula of Mexico, forming the approximately 180–200 km wide Chicxulub crater. The impact ejected vast quantities of dust, soot, and sulfate aerosols into the stratosphere, triggering an 'impact winter' that blocked sunlight, shut down photosynthesis, and collapsed food chains globally. Approximately 75% of all species on Earth perished, including all non-avian dinosaurs, pterosaurs, most marine reptiles, ammonites, and many groups of marine invertebrates. Simultaneously, the extinction created vast empty ecological niches that catalyzed the adaptive radiation of mammals and birds, ultimately establishing the ecological foundations of the Cenozoic Era.

The Deccan Traps are one of the largest continental flood basalt provinces on Earth, located in west-central India (approximately 17–24°N, 73–74°E). Composed of hundreds of tholeiitic basalt lava flows erupted primarily during the late Cretaceous to early Paleocene (~66 Ma), the province currently covers approximately 500,000 km² with a cumulative basalt thickness exceeding 2 km in the Western Ghats escarpment, and an estimated total eruptive volume of roughly 1 × 10⁶ km³. The original extent may have reached 1.5 million km² prior to erosion and tectonic fragmentation during India–Seychelles rifting. The Deccan Traps are widely attributed to the activity of the Réunion mantle plume, whose head is thought to have impinged on the Indian lithosphere as the subcontinent drifted northward following the breakup of Gondwana. High-precision ⁴⁰Ar/³⁹Ar and U-Pb geochronology has demonstrated that the main eruptive phase spanned approximately 700–800 kyr, straddling the Cretaceous–Paleogene boundary (KPB) at ~66.05 Ma. Voluminous degassing of CO₂, SO₂, and halogens during eruption is implicated as a significant environmental stressor, and the Deccan Traps constitute a central element in the ongoing scientific debate over whether the end-Cretaceous mass extinction was primarily driven by the Chicxulub asteroid impact, by Deccan volcanism, or by the synergistic effects of both events.

**Extinction** is the complete and permanent disappearance of a biological species, occurring when no living individuals of that species remain anywhere on Earth. Species become extinct due to a range of environmental and evolutionary factors, including habitat fragmentation, climate change, natural disasters, overexploitation, interspecific competition, genetic inbreeding, and declining reproductive success. An estimated 99% of all species that have ever existed are now extinct. Under normal conditions, species disappear at a low, continuous rate of roughly one to five species per year across the entire fossil record, a process termed **background extinction**. Periodically, however, extinction rates spike dramatically during **mass extinction** events, in which a substantial proportion of Earth's biodiversity — typically 75% or more of species — is lost within a geologically brief interval. These catastrophic events, driven by asteroid impacts, large-scale volcanism, rapid climate shifts, and other global-scale perturbations, fundamentally restructure ecosystems and open ecological niches for surviving lineages, thereby shaping the trajectory of evolution.

An impact winter is a hypothesized period of prolonged global cooling and darkness triggered by the injection of massive quantities of dust, sulfate aerosols, and soot into the stratosphere following the collision of a large asteroid or comet with Earth. In the specific context of the Cretaceous–Paleogene (K–Pg) mass extinction approximately 66 million years ago, the Chicxulub impactor—an asteroid roughly 10–12 km in diameter—struck the Yucatán carbonate platform in present-day Mexico, ejecting enormous volumes of fine silicate dust from pulverized bedrock, sulfate aerosols from vaporized anhydrite target rock, and soot from both the combustion of sedimentary organic carbon within the crater and subsequent global wildfires. These atmospheric contaminants partially to almost completely blocked incoming solar radiation, reducing surface sunlight to levels insufficient for photosynthesis. The resulting impact winter produced severe global surface cooling—modeled estimates range from approximately 15 °C to over 26 °C below pre-impact temperatures—and persisted on timescales of years to decades. The collapse of photosynthesis disrupted both marine and terrestrial food webs at every trophic level, making the impact winter the primary proximate killing mechanism in the K–Pg mass extinction that eliminated approximately 75% of all species, including all non-avian dinosaurs.

The iridium layer is a globally distributed thin stratum of clay, typically a few millimeters to centimeters thick, found at the Cretaceous-Paleogene (K-Pg) boundary approximately 66 million years ago, containing anomalously high concentrations of the platinum-group element iridium (Ir). At the K-Pg boundary, iridium concentrations reach levels up to several tens of parts per billion (ppb) — enriched by two to four orders of magnitude above the continental crustal background of roughly 0.05 ppb — because iridium, a highly siderophile element, is extremely scarce in Earth's crust but relatively abundant in primitive meteoritic material such as carbonaceous chondrites (approximately 450–550 ppb). The anomaly was first measured in 1979–1980 by Luis W. Alvarez, Walter Alvarez, Frank Asaro, and Helen V. Michel in boundary clay samples from Gubbio, Italy, and Stevns Klint, Denmark, and was interpreted as evidence that a large asteroid impact had occurred at the end of the Cretaceous. The iridium layer, now identified in more than 350 marine and terrestrial K-Pg boundary sections worldwide, serves as the single most iconic geochemical marker linking the Chicxulub impact event to the end-Cretaceous mass extinction and is formally recognized as part of the criteria defining the base of the Danian Stage in the Geological Time Scale, with the Global Stratotype Section and Point (GSSP) located at El Kef, Tunisia.

The Permian–Triassic extinction event is the most severe mass extinction in Earth's history, occurring approximately 251.9 million years ago at the boundary between the Permian and Triassic periods. High-precision U-Pb geochronology from the Global Stratotype Section and Point (GSSP) at Meishan, China, constrains the main extinction interval to just 61 ± 48 thousand years, between 251.941 ± 0.037 Ma and 251.880 ± 0.031 Ma. The event eliminated an estimated 57% of biological families, 81% of marine species, and approximately 70% of terrestrial vertebrate species. Several major taxonomic groups were driven to complete extinction, including trilobites, rugose and tabulate corals, fusulinid foraminifers, blastoid echinoderms, and eurypterids. The primary cause is widely attributed to the eruption of the Siberian Traps Large Igneous Province, specifically the initial pulse of widespread sill emplacement into the volatile-rich Tunguska sedimentary basin, which liberated massive volumes of greenhouse gases through contact metamorphism of organic-rich sediments. The resulting cascade of environmental disruptions included rapid global warming of approximately 10°C in sea surface temperatures, widespread ocean anoxia and euxinia, carbon cycle disruption evidenced by a sharp negative δ¹³C excursion, and possible ocean acidification. The extinction marks the boundary between the Paleozoic and Mesozoic eras and fundamentally restructured both marine and terrestrial ecosystems. Ecological recovery was protracted, with marine ecosystems requiring at least 5–10 million years and terrestrial vertebrate community diversity not being fully restored for approximately 30 million years, well into the Late Triassic.