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 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.

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.