K-Pg境界絶滅に関するアルバレス仮説

The Alvarez hypothesis, published in the journal Science in 1980, provided a compelling, testable explanation for one of the most profound mysteries in paleontology: the sudden disappearance of dinosaurs. Geologist Walter Alvarez was studying a thin layer of clay at the K-Pg boundary in Gubbio, Italy, which marked the dramatic shift in the fossil record. To determine the rate of deposition of this clay, he consulted his father, Nobel laureate physicist Luis Alvarez. They decided to measure the concentration of iridium in the layer. Iridium is a siderophile (iron-loving) element, and most of Earth’s supply sank to the core during planetary differentiation. However, it is much more abundant in asteroids and comets. They expected a very low concentration but found levels hundreds of times higher than the background level in Earth’s crust. This iridium spike was subsequently found at the same geological boundary all over the world.

The team calculated that an asteroid approximately 10 to 15 kilometers in diameter would be required to deposit this amount of iridium globally. Such an impact would release energy equivalent to billions of atomic bombs, triggering a cascade of devastating environmental effects. These would include a massive heat pulse from re-entering ejecta, global wildfires, and an “impact winter” caused by dust and aerosols blocking sunlight, leading to the collapse of photosynthesis and food chains. The hypothesis was initially met with skepticism from many paleontologists who favored more gradual causes like volcanism or climate change. However, the discovery of other impact evidence, such as shocked quartz and tektites, and ultimately the identification of the massive Chicxulub crater off the coast of Mexico in the early 1990s, provided overwhelming confirmation and cemented the hypothesis as the leading explanation for the K-Pg extinction.