The day the dinosaurs disappeared

A clue in a thin layer of clay

In 1980 a team led by physicist Luis Alvarez and his son Walter Alvarez, a geologist, studied a thin layer of clay between Cretaceous rock and the rock above it near Gubbio, Italy, and found unusually high levels of iridium. This element is very rare at the Earth's surface but more common in meteorites, so they proposed that the Earth had been hit by an object from space about 10 kilometers across.^[2]

The same iridium layer was later found in many places around the world, along with "shocked quartz", which only forms under enormous pressure, and tiny glass beads made from molten rock that was thrown into the sky and cooled.^[2][4]

The boundary between the Cretaceous and the Paleogene (simplified, not to scale). In many places the clay between them is only about a centimeter thick.

The Chicxulub crater

The big question was where the crater was. The answer came in the early 1990s, when scientists confirmed that a buried ring structure about 200 kilometers wide under the Yucatán Peninsula in Mexico is an impact crater. It is named after Chicxulub, a community near its center.^[3]

Dating shows the crater formed about 66 million years ago, at the same time non-avian dinosaur fossils disappear from rock layers all over the world.^[1][3]

After the impact

Within minutes to hours, the impact caused huge earthquakes, giant tsunamis and hot debris falling back all over the planet, which may have started wildfires in many areas.^[1][6]

The worst came afterward. Huge amounts of dust, soot and sulfur droplets spread through the atmosphere, blocking sunlight so the Earth was dark and cold for years. Plants and plankton could not photosynthesize, and food chains on land and in the sea collapsed. About three-quarters of all species died out, including the non-avian dinosaurs, pterosaurs, large marine reptiles such as mosasaurs, and ammonites.^[1]

At about the same time, enormous volcanic eruptions in India, called the Deccan Traps, went on for hundreds of thousands of years and released gases that changed the climate. Scientists still debate how much the volcanoes contributed, but most agree the asteroid impact was the main cause.^[5]

What happened after the impact: orange is the disaster, green the recovery. (Spacing is not to scale; the recovery stops are based on fossils from Corral Bluffs, Colorado, USA.)

Who survived

Survivors were usually small, could eat many kinds of food, or had places to shelter, such as small burrowing mammals, crocodiles, turtles, frogs, snakes and lizards. Freshwater animals had an advantage, because food chains in water rely on dead plant and animal matter that builds up there, not directly on living plants.^[1]

Research suggests the birds that survived were probably ground-dwelling seed eaters, since seeds can last a long time in the soil without sunlight. These birds are the ancestors of every bird alive today. And once the large dinosaurs were gone, mammals had the chance to evolve into many new forms, including humans.^[1]

Nature recovered faster than many people expect. Fossils from Corral Bluffs in Colorado, USA, where the rock layers record the first million years after the impact without a break, show that ferns dominated the land for the first few thousand years. About 100,000 years after the impact, the variety of plants and mammals was close to what it had been before, and by about 700,000 years later there were mammals weighing around 45 kilograms, bigger than any mammal known from the age of dinosaurs.^[7]

In Thailand, every named dinosaur comes from rocks tens of millions of years older than this event, so we do not yet know what dinosaurs lived in what is now Thailand on that day.^[8]