The impact is physically recorded
About 66 million years ago a roughly 10-kilometre asteroid struck near today’s Yucatán Peninsula, forming the Chicxulub crater. Impact ejecta and a globally distributed boundary layer coincide with the disappearance of non-avian dinosaurs.
The killer was the aftermath
Dust, soot and sulfur-bearing aerosols injected into the atmosphere would have sharply reduced sunlight and cooled the planet. Climate and habitat modelling shows an impact winter capable of collapsing dinosaur habitat across the globe.
The oceans were hit too
Boron-isotope evidence from marine microfossils records rapid ocean acidification after the impact, consistent with a severe shock to marine food webs and carbon cycling.
Where the Deccan Traps fit
Massive eruptions in India were occurring around the same broad interval and affected climate and ecosystems. But modelling that isolates volcanic forcing does not reproduce the same global dinosaur habitat loss as the impact scenario.
Research record
Chiarenza et al. (2020) — asteroid impact, not volcanism, PNASPeer-reviewed · open sourceIODP Expedition 364 — Chicxulub impact crater drilling recordScientific programme · open sourceHenehan et al. (2019) — rapid ocean acidification after ChicxulubPeer-reviewed · open sourceWhat Really Killed the Dinosaurs?
Direct answer: The Chicxulub asteroid impact is the best-supported primary cause of the non-avian dinosaur extinction. Volcanism was an important environmental stressor, but the impact provides the clearest abrupt global kill mechanism.
Impact as primary trigger: strongly supported. Deccan volcanism remains part of the environmental context, not a reason to discard the impact explanation.