Impacts, environmental change and extinction
About 66 million years ago an asteroid impact changed Earth's environment so fast that three quarters of species died out. This lesson follows the evidence and links it to how extinction happens.
- Describe the evidence that an impact occurred 66 million years ago.
- Explain how rapid environmental change can cause extinction.
- Evaluate how strong the evidence is.
Links to: KS3 biology, inheritance and variation: environmental change and extinction; working scientifically: evaluating evidence; maths: standard form and percentages.
The evidence piles up
In 1980 a team led by Luis and Walter Alvarez reported that a thin clay layer at the boundary between the Cretaceous and Paleogene periods, about 66 million years old, contained far more iridium than normal. Iridium is rare in Earth's crust but more abundant in many meteorites.
Other evidence followed. The same layer contains shocked quartz and tiny glassy spherules of melted rock, and is thickest around the Gulf of Mexico. In 1991 the Chicxulub structure, about 180 km across and buried beneath the Yucatán Peninsula, was confirmed as an impact crater of the right age. Each line of evidence on its own could be questioned; together they make a strong case.
From impact to extinction
The impact released vast amounts of dust, soot and sulfur-rich gases into the atmosphere. Sunlight was dimmed for months to years, and global temperatures fell. Photosynthesis collapsed on land and in the oceans, so food chains collapsed with it.
Species that were large, needed a lot of food or depended on particular conditions were least able to survive. Small animals that could eat many kinds of food, burrow or live on dead material did better. About three quarters of species became extinct, including all non-bird dinosaurs, the giant marine reptiles and the ammonites.
Not the only cause
Huge volcanic eruptions in India, called the Deccan Traps, were happening at about the same time and also changed the climate. Scientists still discuss how much each contributed, but most agree that the impact was the main trigger for the sudden extinction. This is a good example of how science weighs several hypotheses against the evidence.
Standard form and percentages
Write 66 million years in standard form. Then, if a fossil site records 240 species just below the boundary layer and 60 just above it, what percentage disappeared?
- 66 million = 66,000,000 = 6.6 × 10⁷ years.
- Species lost = 240 − 60 = 180.
- Percentage lost = (180 ÷ 240) × 100 = 75%.
Answer: 6.6 × 10⁷ years, and 75% of the species at that site disappeared, matching the global estimate of about three quarters.
Write a short evidence report as if you were a scientist in the 1990s. List each piece of evidence for an impact (iridium, shocked quartz, spherules, the crater, its age), say what it shows on its own, and then explain why all of them together are more convincing. Finish with one question scientists are still investigating.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1Why was a high level of iridium in the boundary clay important?
An iridium-rich layer around the world suggested material from a large extraterrestrial body.
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Question 2Where is the Chicxulub crater?
The crater, about 180 km across, is buried under the Yucatán Peninsula and the Gulf of Mexico.
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Question 3How did the impact cause extinctions?
Rapid environmental change left many species unable to find food or survive the cold.
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Question 4Which animals were more likely to survive?
Small size and a varied diet helped animals survive when food became scarce.
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Question 5Write 66 million in standard form.
In standard form the number before the power must be between 1 and 10: 66,000,000 = 6.6 × 10⁷.