Dating the K–Pg Extinction: Impact, Volcanism and the Timeline

The Cretaceous–Paleogene (K–Pg) extinction occurred about 66 million years ago. Its timing is studied using fossil succession, rock layers, radiometric dates and a worldwide boundary layer enriched in impact-related material. Those records establish a close relationship between the Chicxulub impact and the abrupt loss of many species, including all non-avian dinosaurs. They also provide a way to test how Deccan volcanism in India related to the crisis.

What the boundary layer records

At many sites, the boundary is marked by a thin layer enriched in iridium and containing shocked minerals or glassy spherules. These materials are consistent with a large impact that spread debris around the planet. The Chicxulub structure on Mexico’s Yucatán Peninsula provides the corresponding crater. Together, the global layer and crater connect a sudden geological event with a sharp change in the fossil record.

Not every site preserves the boundary equally well. Erosion, gaps in sedimentation, burrowing and later geological alteration can blur or remove layers. Geologists correlate sections by combining fossils, chemistry and dates rather than assuming every cliff exposes a complete record. The fossil sequence shows that many groups declined abruptly at or near the boundary, although local preservation and extinction timing differ.

Dating Deccan eruptions

The Deccan Traps are an immense volcanic province formed by repeated eruptions of lava flows. Radiometric dating of minerals and volcanic units estimates when eruptions occurred. Researchers also use magnetostratigraphy and stratigraphic relationships to compare those dates with the boundary. Each method has uncertainty, and a single date should not be mistaken for the start or end of the entire volcanic episode.

The eruption history began before the impact and continued across the boundary. High-resolution studies have examined whether the main pulse came before or after the extinction and how much sulfur dioxide and carbon dioxide entered the atmosphere. These gases can cool or warm climate through different processes and timescales. Their estimated effects depend on eruption rate, gas release, atmospheric chemistry and climate response.

Why simple timelines fail

Older popular explanations sometimes state that volcanism began only a few centuries before the impact and lasted a few centuries afterward. Such precision is not warranted by the geological record. Lava flows and ash deposits are dated with uncertainty far larger than a few hundred years, and multiple pulses occurred over a much longer interval. The evidence supports comparing sequences at geological timescales, not a day-by-day countdown.

Chronology can still distinguish plausible models. If a major climate perturbation preceded the impact, it may have altered ecosystems before the final crisis. If intense eruptions overlapped with recovery, they may have shaped post-impact conditions. These possibilities require dated records and climate evidence; they cannot be settled by describing volcanoes and impact as two interchangeable catastrophes.

What caused the extinction?

The impact has the strongest evidence as the immediate trigger of the mass extinction: crater, ejecta and an abrupt global boundary coincide with major biological turnover. Deccan volcanism affected climate and ecosystems before, during and after that event, and its contribution remains an active research question. Current investigations refine its scale and timing rather than replacing the impact with a volcanic event at an arbitrary date.

Scientists therefore keep three questions separate: when each event occurred, what environmental changes it produced, and which biological losses followed. That method avoids false precision and lets new dates revise the timeline. For the broader evidence, compare the geology of the K–Pg boundary with the relative timing of Chicxulub and Deccan volcanism.