Impacts in Earth's history
Impacts have shaped Earth from its formation to the present. This lesson looks at craters, the end of the dinosaurs, modern airbursts and how the risk is managed.
- Describe how impact craters are recognised.
- Summarise the evidence for the Chicxulub impact.
- Put the modern impact hazard in proportion.
Links to: Big questions: impacts and planetary defence.
Recognising craters
About 200 impact structures have been confirmed on Earth. Many are eroded or buried, so proof comes from features that only impacts create: shocked quartz with microscopic planar features, high-pressure minerals such as coesite, shatter cones and impact melt rocks. The largest known, Vredefort in South Africa, was originally perhaps 250 to 300 km across and is about 2 billion years old.
Chicxulub
About 66 million years ago an impactor roughly 10 km across struck what is now the Yucatán Peninsula of Mexico, creating the Chicxulub crater, about 180 km across. A worldwide layer rich in iridium, shocked quartz and glassy spherules marks the moment. Dust, soot and sulfur aerosols darkened and cooled the planet, and about three quarters of species died out, including all non-bird dinosaurs. Huge volcanic eruptions in India at about the same time also stressed the climate, but most scientists see the impact as the decisive trigger.
The modern hazard
Small bodies arrive often and usually explode harmlessly high up. Larger events are rare: the 2013 Chelyabinsk airburst, from an asteroid about 18 m across, injured about 1,500 people, mostly by broken glass, and the 1908 Tunguska airburst flattened about 2,000 km² of Siberian forest. Survey telescopes have found nearly all near-Earth asteroids larger than 1 km, and none threatens Earth in the foreseeable future. The search for smaller ones continues, and NASA's DART mission showed in 2022 that an impactor can change an asteroid's orbit.
Why speed matters
Kinetic energy = ½ × mass × speed². Compare a 1,000 kg rock at 20 km/s with the same rock at 10 km/s.
- At 20 km/s (20,000 m/s): ½ × 1,000 × 20,000² = 500 × 400,000,000 = 2 × 10¹¹ J.
- At 10 km/s (10,000 m/s): ½ × 1,000 × 10,000² = 500 × 100,000,000 = 5 × 10¹⁰ J.
- Ratio: 2 × 10¹¹ ÷ 5 × 10¹⁰ = 4.
Answer: Doubling the speed quadruples the energy. At 20 km/s, a one-tonne rock carries about 50 tonnes of TNT equivalent, most of it released high in the atmosphere.
Use the impact calculator on this site to compare three bodies: 5 m, 50 m and 500 m across, all stony and at 18 km/s. For each, note whether it bursts in the air or reaches the ground, and the energy. Then read the Chelyabinsk and Tunguska pages and decide which of your three runs each most resembles.
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 1Which evidence proves a crater was made by an impact?
Only hypervelocity impacts produce shocked quartz, shatter cones and high-pressure minerals in nature.
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Question 2How big was the Chicxulub impactor?
An impactor roughly 10 km across made the 180 km crater.
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Question 3What injured most people at Chelyabinsk in 2013?
The airburst's shock wave shattered windows across the city.
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Question 4What did DART demonstrate?
In 2022 DART shortened the orbit of Dimorphos around Didymos by about 33 minutes.
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Question 5What happens to kinetic energy if speed is tripled?
Energy depends on speed squared: 3² = 9.