A Thin Layer of Clay and the Clue to the Dinosaurs' End

The child listens. The parent reads. This page helps you answer the questions your child may ask after the story. You can see the script with the script button while it plays.

What this story is about

"The dinosaurs died because an asteroid hit the Earth." Children already know this answer. This story follows it back to where the first evidence for it came from.

It did not start with dinosaurs. It started with one layer of clay, one centimeter thick, in a valley at Gubbio, Italy. The question was: "How long did this clay take to build up?" Walter Alvarez, a geologist, worked on it with his father Luis Alvarez, a physicist, and with Frank Asaro and Helen Michel. They tried to use material that falls from space at a steady rate as a clock. The clock failed. That failure became the asteroid impact theory (openalex:W274971555).

The set "People who measured the Earth" shares one idea: people who measured what they could not visit, by the traces it left. After a man who measured the Earth, a woman who found its core, and a man who saw that continents move, this story measures one day, sixty-six million years ago, with one layer of clay.

What really happened

• The rock at Gubbio is a record with no gaps. The Scaglia limestone there formed in deep water. It is made mostly of tiny plankton shells. It holds the history of the late Cretaceous and the Paleogene with no missing parts. In the 1970s, people found that this rock also records the flips of the Earth's magnetic field (DOI: 10.1111/j.1365-3091.2008.01010.x — Walter Alvarez, looking back).
• Walter went to Gubbio for the magnet. He came to Lamont in 1971. A couple of years later he began to study the magnetism of deep-sea limestones in Italy with a co-worker. They went to the small old town of Gubbio because a full set of rock layers was open to view in the Bottaccione Gorge, behind the town (DOI: 10.1029/eo067i035p00649). His wife was his field assistant for 56 years. For a time they lived in an old hill town in Italy (DOI: 10.1130/2022.2557(01 — her own account).
• The clay at the boundary is one centimeter thick. The iridium peak at the Cretaceous-Tertiary boundary sits in a clay layer one centimeter thick (PMID 11538083). In the Gubbio rock, the tiny plankton fossils suddenly vanish. This showed that the extinction was a sudden event (DOI: 10.1111/j.1365-3091.2008.01010.x).
• The first question was "how long?" The four authors wrote in a 1980 report: the clay was known to be about 65 million years old, from when many life forms died out, but no one knew how long it took to build up. To measure that time, they tried to use meteoritic material, which falls at a normal steady rate, as a clock. They measured iridium and many other elements. They looked for about 50 elements, found about 40, and measured about 30 well. Then they wrote: "We were not able to determine exactly how long the clay deposition took. Instead," the work led to the asteroid idea (openalex:W274971555).
• There was far more iridium than expected. Platinum metals are rare in the Earth's crust but more common in space. In deep-sea limestones from Italy, Denmark, and New Zealand, iridium rose to about 30, 160, and 20 times the normal level, right at the time of the extinction (PMID 17783054). Later, Walter wrote that the study "accidentally" found an "unexpectedly large" amount of iridium in the clay (DOI: 10.1111/j.1365-3091.2008.01010.x).
• The father was a physicist. The son was a geologist. A news story from 1980 called the work a study by Nobel Laureate Luis W. Alvarez, a physicist, and his son Walter, a geologist. The same story says that a year earlier, Luis had said that exploding stars (supernovae) might cause mass extinctions (DOI: 10.1029/eo061i019p00452-01). The 1980 paper gives reasons why the iridium came from space but not from a nearby supernova (PMID 17783054). In other words, they ruled out their own first idea.
• The 1980 idea. A large asteroid that crossed the Earth's path hit the Earth. It threw about 60 times its own mass of crushed rock into the air. Part of that dust stayed high in the sky for several years and spread around the world. The darkness stopped plants from making food from light. The expected results matched the extinctions in the fossil record quite closely. One thing they had guessed was already checked: the boundary clay is chemically very different from the clay mixed into the rock above and below. Four separate estimates of the asteroid's size all fell in the range of 10 ± 4 kilometers (PMID 17783054).
• Two years later, they fixed their own number. In 1982, the same four authors wrote that the darkness lasted "a few months, rather than our originally suggested few years." They gave four reasons, such as how fast the dust would spread and the plant record. The same report said two other ideas had been ruled out: that the iridium piled up because the clay built up very slowly, and that it came from a nearby supernova. It also said the impact site had not been found yet. It named the sea floor west of Portugal and the Deccan Traps of India as possible places. It gave a 20% chance that the site was on sea floor that has since sunk away (DOI: 10.1130/spe190-p305).
• At first, many had doubts. Most earth scientists were doubtful when they first heard the idea. Their main question was "Where is the crater?" But when the search began, several people, perhaps dozens, already knew where it was. Mexico's national oil company began a gravity survey in the Yucatán in 1947 and found hints of a round feature. Those people did not know a search was on, or they were not allowed to tell (DOI: 10.1093/oso/9780195101058.003.0005).
• 1991: Chicxulub. The impact site was found in 1991 at Chicxulub, on the Yucatán Peninsula of Mexico (DOI: 10.1111/j.1365-3091.2008.01010.x). The next year, a study dated melted rock from deep inside the 180-kilometer buried crater. Its age, about 64.98 million years, was almost the same as glass beads (tektites) from the boundary layer in Haiti and Mexico. Walter was one of the authors (DOI: 10.1126/science.257.5072.954). Later work showed the boundary, the extinction, and the impact happened within 32,000 years of each other (PMID 23393261).
• The father did not see it. The title of Luis Alvarez's obituary reads "1911–1988" (DOI: 10.1088/2058-7058/1/12/52). That was three years before the crater was found.

Who was Luis Alvarez?

• A man who saw by traces. His obituary calls him "probably the most versatile man of science of his generation." It lists optics, cosmic rays, radar for landing planes in bad weather, the liquid hydrogen bubble chamber, the use of cosmic rays to search for hidden rooms in an Egyptian pyramid, and the asteroid theory, which he did with his son Walter (DOI: 10.1088/2058-7058/1/12/52).
• The bubble chamber. From the late 1950s through the 1960s, most discoveries in particle physics came from bubble chambers. Donald Glaser invented it. Luis and his group made it into a tool scientists could not do without (DOI: 10.1093/oso/9780199544691.003.0073). Luis built large bubble chambers filled with liquid hydrogen (DOI: 10.1063/pt.3.2156). Here is how it works. A liquid is suddenly taken past its boiling point. When a charged particle passes through, bubbles form along its path (DOI: 10.1063/pt.3.2156).
• The pyramid. In 1965 Luis put forward the idea of using cosmic-ray muons to look inside the pyramids at Giza. A joint project grew from that idea (DOI: 10.1063/1.3048477). Later, someone wrote that his team "found nothing." A reader corrected this: what the team found was that there were no hidden rooms (DOI: 10.1063/pt.3.1901). The line in the story, "But that was a finding too," comes from this.
• The Nobel Prize. He won the Nobel Prize in Physics in 1968 for developing the hydrogen bubble chamber (DOI: 10.1063/pt.3.2156). A 1980 news story also calls him a Nobel Laureate (DOI: 10.1029/eo061i019p00452-01). The story puts this in one line at the end of the bubble chamber part.

Who was Walter Alvarez?

• In 2013 he won the Barringer Award of the Meteoritical Society. In his speech he said that to learn how the world works, "we must never let our druthers — our emotional preferences — get in the way of what we observe, analyze, and conclude." He joked that because of this, scientists "write like accountants." He also spoke of field trips to Mexico in 1991 and 1992, and friendships that crossed borders even during the Cold War (DOI: 10.1111/maps.12200).
• The story's closing line, "A scientist must never let the answer he wants get in the way of what he sees," puts his words into a child's language.

Questions your child may ask

"So did a rock from space really kill the dinosaurs?" Most studies say yes. The Chicxulub impact is widely seen as the cause of the mass extinction that ended the non-bird dinosaurs (PMID 39819896). A 2010 review of boundary rock from around the world found that "the Chicxulub impact triggered the mass extinction" (PMID 20203042). A study that modeled where dinosaurs could still live also found the impact to be the main cause (PMID 32601204). But it is more correct not to tell your child that the story is fully closed. The next two questions show why.

"I heard something about volcanoes." At the same time, huge eruptions in India were building the Deccan Traps (PMID 20203042). A study of sulfur in Deccan lava suggests the volcanoes stressed living things with short cold periods long before the impact, and the impact gave the final blow (PMID 37792933). A record of past heat and cold from the western United States found that a volcano-made cold period ended before the extinction. From the land side, it says, volcanic climate change was not the main cause (PMID 39693422). Another study raised the idea that the impact may have set off the largest Deccan eruptions (DOI: 10.1126/science.aac7549). A review on dinosaurs says the suddenness points to a key role for the impact, but the fossil record is too rough to test the effect of the volcanoes (PMID 25065505). A 2022 look back from the impact side says that opposition continues to the present day. The theory is stronger than ever, it says, but many questions remain (DOI: 10.1130/2022.2557(19).

"Was it a meteorite or an asteroid?" The 1980 paper said asteroid (PMID 17783054). The 1982 report found that the metal ratios in the clay matched a kind of early meteorite called a carbonaceous chondrite (DOI: 10.1130/spe190-p305). But a 1997 study, using the crater's size and the amount of space material in the boundary layer, found a long-period comet most likely (PMID 11543120). Still, the recent studies we checked mostly say "asteroid impact" (PMID 39819896, PMID 35197634, PMID 32601204). The comet view is closer to a minority view. That is why the story does not firmly call it "a rock." It says "something that fell from the sky" or "the impact" instead. (The comet study pictures a comet made mostly of ice and similar matter.) A rock from space that lands on the ground is called a meteorite, so "a meteorite fell" is not badly wrong in everyday speech.

"So we never learned how long the clay took?" In 1980 they wrote that they could not find out exactly (openalex:W274971555). Later, scientists drilled into the Chicxulub crater. The rock core shows the first moments in minutes, hours, and one day. Within minutes, the central peak collapsed. Within an hour, the sea rushed back in. Within a day, a returning tsunami reached the crater. Bits of charcoal point to fires caused by the impact (DOI: 10.1073/pnas.1909479116).

"Did all the dinosaurs die?" The sources say that all non-bird dinosaurs died out. Flying reptiles, ammonites, and most sea reptiles also vanished. About 76% of species were lost (PMID 35197634). Why the words "non-bird" are used (how birds and dinosaurs are related) is not covered in the sources we collected, so the story does not go into it.

Ideas inside the story

• Measuring by traces. No one ever saw the asteroid. Yet four separate methods gave sizes in the same range (PMID 17783054). When different paths give the same answer, we can trust it more.
• Using steadiness as a clock. Anything that builds up at a steady rate can be a clock. And when the clock does not work, that tells you the time was not normal.
• Writing down a failure. "We were not able to determine exactly how long" is in their own report (openalex:W274971555).
• Fixing your own work. A few years became a few months (DOI: 10.1130/spe190-p305). Changing your first idea is not a weakness. It is part of the work.
• Rare disasters. Gubbio was central to the view that rare disasters, like great impacts, do happen. So the strict idea that the Earth always changes slowly, at the same speed, had to be dropped (DOI: 10.1111/j.1365-3091.2008.01010.x).

What is backed by sources, and what is only told

What the sources clearly state — the first question (how long the clay took), the clock idea, and its failure (openalex:W274971555); the one-centimeter clay layer (PMID 11538083); 30, 160, and 20 times; 10 ± 4 kilometers; ruling out a supernova; darkness and plants (PMID 17783054); Luis as a Nobel-winning physicist, Walter as his geologist son, and the supernova idea (DOI: 10.1029/eo061i019p00452-01); years changed to months (DOI: 10.1130/spe190-p305); the oil company's hints from 1947 (DOI: 10.1093/oso/9780195101058.003.0005); Chicxulub found in 1991 (DOI: 10.1111/j.1365-3091.2008.01010.x); Luis's death in 1988 (DOI: 10.1088/2058-7058/1/12/52).

What we left out on purpose, or could not check

• The scene where the son shows the clay to his father. This is a popular story, but it does not appear in any source we found. The sources say only that the four worked together to measure the clay's time. They also do not say who first had the iridium clock idea. So the story only says "they."
• Why they wanted to know the time. The sources do not say, so we did not make it up.
• Different numbers. The 1980 paper placed the boundary at 65 million years ago (PMID 17783054). Recent work says about 66 million (PMID 35197634). The story uses the newer number. The crater's size is not the same in all sources: 180 kilometers in the 1992 dating paper (DOI: 10.1126/science.257.5072.954), and a basin of about 300 kilometers in a 1997 study (PMID 11543120). The story says "about 180." The iridium numbers also change with the method. A better 1990 study measured an average of 3,000 parts per trillion in the Gubbio clay, against a normal level of 12 to 13 (PMID 11538083). The story uses the original paper's "thirty times."
• Luis's work on nuclear weapons. His obituary lists implosion methods for nuclear weapons and ways to measure shock-wave pressure (DOI: 10.1088/2058-7058/1/12/52). This is far from the story's main thread, and a child is not likely to hear of it, so we left it out of the story.
• Were dinosaurs already in decline before the impact? The sources disagree. One study finds a decline from about 76 million years ago (PMID 34188028). A review finds little support for a long, worldwide decline (PMID 25065505). We left this out of the story.

Sources and what came after

The original paper — Luis W. Alvarez, Walter Alvarez, Frank Asaro, Helen V. Michel, "Extraterrestrial Cause for the Cretaceous-Tertiary Extinction," Science (1980). PMID 17783054 · DOI: 10.1126/science.208.4448.1095

Main sources

• Results of a Dating Attempt (Asaro, Michel, L. Alvarez, W. Alvarez, 1980 report — the first question). openalex:W274971555
• Current status of the impact theory for the terminal Cretaceous extinction (the same four, 1982). DOI: 10.1130/spe190-p305
• Meteorites and geoextinction (news story, 1980). DOI: 10.1029/eo061i019p00452-01
• The historical record in the Scaglia limestone at Gubbio (Walter Alvarez, 2008). DOI: 10.1111/j.1365-3091.2008.01010.x
• Toward a theory of impact crises (Walter Alvarez, 1986). DOI: 10.1029/eo067i035p00649
• Iridium profile for 10 million years across the Cretaceous-Tertiary boundary at Gubbio (1990). PMID 11538083
• The Saga of the Chicxulub Crater (1996). DOI: 10.1093/oso/9780195101058.003.0005
• Coeval 40Ar/39Ar Ages of 65.0 Million Years Ago from Chicxulub Crater Melt Rock (1992). DOI: 10.1126/science.257.5072.954
• Obituary: Luis Alvarez 1911–1988. DOI: 10.1088/2058-7058/1/12/52
• Acceptance speech for the Barringer Award (Walter Alvarez, 2013). DOI: 10.1111/maps.12200

What came after

• We can now read the first day of the impact. A drill core from the crater's central ring holds about 130 meters of rock laid down on the first day of the Cenozoic (DOI: 10.1073/pnas.1909479116).
• The question moved from "what" to "how." One study finds the impact may have released about five times less sulfur than older estimates (PMID 39819896). A model finds that darkness and body size decided which sea plankton lived or died (PMID 42203866). Today's research asks how the impact chose which living things would vanish.

Things to try together

• Find layers. Dig a little in a flower bed or a sandbox and look for layers of different colors. Then ask, "How long did this take to build up?" This links to the last line of the story.
• Break an hourglass clock. Turn over a sand timer. Halfway through, pour in a spoon of extra sand. Now the timer "lies" and says more time has passed. This is how the iridium clock broke when there was too much iridium.
• Measure in different ways. Measure your room in steps, with a tape measure, and by counting floor tiles. If the three answers are close, you can trust them more. That is why the Alvarez team estimated the asteroid's size in four ways.