The Cretaceous-Paleogene extinction was a mass extinction about 66 million years ago that wiped out roughly 75% of species, including non-avian dinosaurs. In Earth Science, it marks the end of the Mesozoic Era and the start of the Cenozoic.
The Cretaceous-Paleogene extinction is the mass extinction event that happened about 66 million years ago, when about three quarters of Earth’s species disappeared in a short geologic moment. In Earth Science, you see it as the boundary between the Mesozoic Era and the Cenozoic Era, so it is both a biological crash and a timeline marker.
The event did not just remove dinosaurs. It reshaped marine food webs, wiped out many plant and animal groups, and left surviving lineages to fill open niches afterward. Birds survived because they are the only dinosaur lineage that made it through, while mammals were small during the late Mesozoic but expanded in the aftermath.
Scientists think the extinction had multiple causes, not one simple trigger. The strongest evidence points to a large asteroid impact, which threw dust and aerosols into the atmosphere and blocked sunlight. That would have cooled the planet, reduced photosynthesis, and collapsed food chains from the bottom up. At the same time, massive volcanic activity in the Deccan Traps likely released gases that changed climate and ocean chemistry, adding more stress to already fragile ecosystems.
You can recognize this event in the geologic record through the K-Pg boundary, a thin layer that often contains elevated iridium, shocked quartz, soot, and abrupt fossil changes. Iridium is rare in Earth’s crust but more common in meteorites, which is why that clay layer became such a big clue for the impact hypothesis.
In practice, this extinction is one of the clearest examples of how Earth systems connect. A change in the atmosphere can affect oceans, plants, animals, and the fossil record all at once. That is why Earth Science treats it as a turning point, not just a sad chapter about dinosaurs disappearing.
The Cretaceous-Paleogene extinction matters because it is one of the best examples of a mass extinction reshaping life and the geologic record at the same time. In Earth Science, it connects geology, climate, and biology, since one event can leave a rock boundary, a fossil turnover, and a long-term change in ecosystems.
It also gives you a concrete way to read the geologic time scale. The K-Pg boundary is a named line in Earth history, so when you place it on a timeline, you are not just memorizing a date. You are showing how scientists divide deep time using major changes in fossils and rock layers.
This term also comes up when you study causes and effects in Earth systems. The extinction is a strong case study for how an asteroid impact or large-scale volcanism can trigger atmospheric change, climate disruption, and ecosystem collapse. If a question asks why one species group died out while another survived, the answer usually involves food webs, habitat loss, and environmental stress, not just the impact itself.
It matters too because the aftermath shaped the world you live in now. Mammals became more diverse after the extinction, and birds remained as the surviving dinosaurs. That makes the event a turning point for understanding why later Cenozoic ecosystems looked so different from those of the Mesozoic.
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Visual cheatsheet
view galleryImpact Hypothesis
The impact hypothesis explains one major cause of the extinction event. A large asteroid or comet impact would have blasted dust and aerosols into the atmosphere, blocking sunlight and disturbing climate. In Earth Science, this connection is often made through evidence like iridium-rich clay, shocked quartz, and the crater at Chicxulub.
Deccan Traps
The Deccan Traps were enormous volcanic eruptions that happened around the same time as the extinction. They likely released gases such as carbon dioxide and sulfur compounds, which could warm or cool climate and stress ecosystems. This makes them a good example of how volcanism can act as a long-term environmental pressure, even before the impact.
Mass Extinction
The Cretaceous-Paleogene extinction is one of the Big Five mass extinctions. That means it removed a large percentage of species in a relatively short geologic interval and reset the direction of evolution. Comparing it with other mass extinctions helps you see patterns, like environmental change, food-web collapse, and later recovery.
Mesozoic Era
This extinction marks the end of the Mesozoic Era, which is why it shows up in timelines as a major boundary. The shift into the Cenozoic matters because it helps you track major changes in dominant life forms. If you know what lived before and after the boundary, the geologic time scale makes a lot more sense.
A quiz item might show you a fossil layer, an iridium-rich clay band, or a geologic timeline and ask you to identify the K-Pg boundary. You might also need to explain why a sudden drop in dinosaur fossils lines up with an impact event or volcanic activity.
In a short response or class discussion, use the term to trace cause and effect: impact or volcanism, atmospheric change, reduced sunlight, ecosystem collapse, then extinction. If a question asks how Earth’s history is divided, you can place the event at the end of the Mesozoic Era and the start of the Cenozoic Era. When you see a graph, rock layer, or fossil diagram, look for the abrupt change in species and the chemical clue that points to this mass extinction.
The Cretaceous-Paleogene extinction is the event itself, while the Impact Hypothesis is one explanation for what caused it. They are related, but not the same. The event is the disappearance of species at the K-Pg boundary, and the hypothesis is the idea that a large asteroid impact helped trigger that crash.
The Cretaceous-Paleogene extinction happened about 66 million years ago and ended roughly 75% of life on Earth.
It marks the boundary between the Mesozoic Era and the Cenozoic Era in the geologic time scale.
The strongest evidence points to an asteroid impact, but massive volcanism from the Deccan Traps likely added stress too.
The K-Pg boundary is visible in the geologic record through fossils, iridium-rich clay, and other chemical clues.
After the extinction, surviving groups like birds and mammals diversified into newly open ecological niches.
It is the mass extinction event that happened about 66 million years ago, when around 75% of species died out. In Earth Science, it marks a major boundary in the geologic time scale and is famous for ending the reign of non-avian dinosaurs.
Iridium is rare in Earth’s crust but more common in meteorites, so an iridium-rich clay layer suggests extraterrestrial material. That clue supports the idea that a large impact helped trigger the extinction. It is one of the clearest pieces of evidence students see in this topic.
Most Earth Science explanations treat the asteroid as the main trigger, but not the only stress. The Deccan Traps may have already been changing climate and ocean conditions, which would have made ecosystems more vulnerable. The extinction is a good example of multiple causes working together.
Place it at the end of the Mesozoic Era and the start of the Cenozoic Era. If a question gives you fossils or rock layers, use the K-Pg boundary to identify a sudden biological change in Earth history. That is usually the connection teachers want you to make.