Why did the dinosaurs disappear?
The key clue is a huge crater near Mexico.The leading explanation for why the dinosaurs (other than birds) disappeared is that a giant asteroid hit Earth about 66 million years ago. Soot thrown into the sky reduced sunlight, plants struggled to grow, and food chains broke down. Birds, however, survived.
The clue: a crater in Mexico
In 1980, the physicist Luis Alvarez and his son Walter, a geologist, proposed that a giant asteroid had struck Earth, because rock layers from the same time all over the world contained high levels of iridium, a rare metal. Later, the Chicxulub crater was found buried under the seafloor off the Yucatán Peninsula in Mexico, and it turned out to be exactly the same age as the dinosaur extinction. The idea faced a lot of opposition at first, but it is now the most widely accepted explanation.
What happened after the impact
The asteroid is thought to have been about 10-15 km wide, but because it hit at tremendous speed, it made a crater 150 km across. A huge blast and heat spread out, vast amounts of material were thrown into the sky, and giant tsunamis struck. The soot that spread around the world did not block out the Sun completely, but it reduced the light reaching the ground, so plants struggled to grow. With fewer plants, plant-eating dinosaurs ran short of food, and so did the meat-eating dinosaurs that ate them. In this way, food chains are thought to have broken down.
Other causes and open questions
Some scientists think the asteroid was not the only cause. At that time, in what is now central India, huge volcanic eruptions went on for about 2 million years and had a major effect on the climate. Some researchers have also suggested that dinosaurs were already declining, but a study published in 2020 found that many groups of dinosaurs were still doing well right up to the end. Much about exactly how the extinction happened is still unknown.
Birds are surviving dinosaurs
At this time, about 75% of Earth's animals disappeared at once, including the flying reptiles called pterosaurs, large sea reptiles, and ammonites. But not all dinosaurs died out. There is plenty of evidence that birds evolved from a group of small meat-eating dinosaurs, and researchers even call birds "avian dinosaurs." Some kinds of birds went extinct too, but the lines that led to today's birds survived.
Want to know more?
- The extinction used to be dated to 65 million years ago, but after the rocks were dated again, it is now placed at 66 million years ago.
- After this mass extinction, all land animals weighing more than 25 kg are thought to have disappeared.
- One study suggests that if the asteroid had arrived just a few minutes later and hit deeper water, a mass extinction might have been less likely.
Quick quiz: test yourself
What is the leading explanation for why the dinosaurs disappeared?
Show the answer
- Cold weather stopped eggs hatching
- A giant asteroid impact
- Mammals ate their eggs
A giant asteroid impact About 66 million years ago, a giant asteroid hit Earth. Soot reduced sunlight and plants struggled to grow. Birds survived.
About 66 million years ago, a giant asteroid hit Earth. Soot reduced sunlight and plants struggled to grow. Birds survived.
Next quiz ›Sources
- How an asteroid ended the age of the dinosaursNatural History Museum（ロンドン自然史博物館）
- Dinosaurs were not in decline before the asteroid wiped them outNatural History Museum（ロンドン自然史博物館。Royal Society Open Science 掲載の2020年の論文の紹介）
- Dinobuzz: Are Birds Really Dinosaurs?University of California Museum of Paleontology（カリフォルニア大学古生物学博物館）
This article was written after checking the sources above as of 2026-10-03. Explanations may change as research progresses. Photo credits are shown below each photo.
