The geological evidence for the K–Pg extinction

The end-Cretaceous extinction is reconstructed from rocks scattered across the world. Its central evidence is not one dramatic fossil bed but a set of signals at the same geological boundary: an iridium-rich layer, minerals deformed by intense shock, glassy ejecta and a large impact structure at Chicxulub. Together, these observations connect an asteroid impact to the Cretaceous–Paleogene (K–Pg) boundary. Other parts of the story, especially the precise environmental sequence and the contribution of Deccan volcanism, require additional interpretation.

A thin layer with an unusual chemistry

At many boundary sections, researchers find elevated iridium and other platinum-group elements. These elements are scarce in much of Earth's crust but more common in meteorites. Their concentration in a thin layer at widely separated sites supports a global source delivered around the boundary. The exact thickness and preservation vary: some sites have erosion, gaps in deposition, burrowing or later chemical movement that blurred the signal.

Iridium alone would not identify the impact site. The case becomes stronger because the boundary layer also contains shocked quartz and other minerals whose crystal structures record extreme pressure. Spherules and altered glass droplets represent material that melted or vaporised during impact and later fell out of the atmosphere. Their size and abundance generally decline with distance from the Gulf of Mexico region, consistent with a source near Chicxulub.

Connecting boundary deposits to Chicxulub

The Chicxulub structure is a buried impact crater roughly 180 kilometres across. Geophysical surveys mapped its ring-shaped structure beneath younger rocks, and drilling recovered impact breccias, melt rocks and sediments from within it. Radiometric dating places crater formation at about the same time as the K–Pg boundary within the precision of the methods.

Core samples and distant boundary sections provide complementary records. A borehole through the crater captures the impact structure and its early deposits. A marine or continental section far away captures the fallout and biological change in another setting. Matching mineral chemistry, ages and stratigraphic position helps connect them, while also exposing local differences in sedimentation.

From impact to ecosystem disruption

The crater and boundary minerals are direct geological evidence. The global consequences are reconstructed using those deposits together with climate models, fossil ranges, charcoal, plant spores and changes in marine microfossils. Dust, sulfate aerosols and soot can reduce sunlight and alter temperature and precipitation, but their duration and distribution depend on what entered the atmosphere and how it interacted with clouds and the ocean.

Fossils near the boundary show an abrupt and selective biological crisis. Many marine plankton groups declined sharply, ammonites disappeared, and no non-avian dinosaurs are known above the boundary. Birds survived and are living dinosaurs. Freshwater food webs show different patterns of survival from many terrestrial and marine communities. A boundary section records what was deposited at one location; it does not by itself represent every ecosystem.

What Deccan volcanism adds

The Deccan Traps in India are enormous flood-basalt deposits whose eruptions began before and continued across the K–Pg boundary. Volcanic carbon dioxide and sulfur gases could alter climate and ocean chemistry, and researchers test their timing against fossil and isotope records. Some evidence indicates environmental stress before the impact; other studies emphasise that the most severe extinction is tightly aligned with the impact boundary. The relative contribution of volcanism remains a research question, but it does not erase the global impact evidence.

Geology distinguishes the observed event from the causal explanation. Lava flows, dated ash, isotope shifts and fossil changes are measured; links among them are hypotheses tested through their timing and mechanisms. Current evidence identifies Chicxulub as the principal trigger of the K–Pg mass extinction, while the scale and timing of Deccan effects remain important to refine.

Read the broader narrative of the end of the Mesozoic, compare it with why non-avian dinosaurs did not survive, and see the period that ended at the boundary in the Cretaceous overview.