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# Extraterrestrial Cause for the Cretaceous-Tertiary Extinction

## Details

**Authors** Luis W. Alvarez, Walter Alvarez, Frank Asaro, Helen V. Michel

**Year** 1980

**Publisher** Science

**Kind of work** article

**Discipline** Geology

**Secondary disciplines** Evolutionary Biology

**Applied** false

[Read it at the publisher](https://doi.org/10.1126/science.208.4448.1095) 
10.1126/science.208.4448.1095

## In authors' words

### Abstract

Platinum metals are depleted in the earth's crust relative to their cosmic abundance; concentrations of these elements in deep-sea sediments may thus indicate influxes of extraterrestrial material. Deep-sea limestones exposed in Italy, Denmark, and New Zealand show iridium increases of about 30, 160, and 20 times, respectively, above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago. Reasons are given to indicate that this iridium is of extraterrestrial origin, but did not come from a nearby supernova. A hypothesis is suggested which accounts for the extinctions and the iridium observations. Impact of a large earth-crossing asteroid would inject about 60 times the object's mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record. One prediction of this hypothesis has been verified: the chemical composition of the boundary clay, which is thought to come from the stratospheric dust, is markedly different from that of clay mixed with the Cretaceous and Tertiary limestones, which are chemically similar to each other. Four different independent estimates of the diameter of the asteroid give values that lie in the range 10 ± 4 kilometers.

### What they set out to do (purpose)

To determine the cause of the mass extinctions marking the Cretaceous-Tertiary boundary by analyzing trace element concentrations in the boundary clay.

### Who or what was studied (sample)

Deep-sea limestone sections spanning the K-T boundary at three sites: Gubbio (Italy), Stevns Klint (Denmark), and Woodside Creek (New Zealand).

### How they did it (methods)

Neutron activation analysis of platinum-group element (iridium) concentrations across the boundary clay layer at each site, compared to background levels in the surrounding Cretaceous and Tertiary limestone.

### What they found (results)

Iridium concentrations spike 20 to 160 times above background exactly at the Cretaceous-Tertiary boundary clay at all three sites, a pattern the authors attribute to debris from a roughly 10-km asteroid impact whose stratospheric dust cloud is proposed to have suppressed photosynthesis and driven the mass extinction.

## Commentary

### In short

A sharp, worldwide chemical marker sets a hard boundary between two geologic periods, and that same marker supports a causal chain linking an extraterrestrial impact event to a global biological extinction.

**Patterns it shows** D, R

**Added** 2026-09-21

**How to cite this** Luis W. Alvarez, Walter Alvarez, Frank Asaro, Helen V. Michel (1980). Extraterrestrial Cause for the Cretaceous-Tertiary Extinction. Science.