Passage IV

Walter Alvarez’s Impact

[1]

While studying a canyon in Italy during the 1970s, a mystery caught the attention of geologist Walter Alvarez. At the K-Pg boundary, a band of rock that marks the end of the Cretaceous period and the start of the Paleogene, Alvarez recognized evidence of a large extinction. Fossils of many species of foraminifera (single-celled marine organisms) was abounding in the Cretaceous rock layer; however, in the Paleogene rock layer above, only one species remained. Alvarez later learned that the formation of the Paleogene fossils correlated to the mass extinction that wiped out over 70 percent of plants and animals, including the dinosaurs. [A]

[2]

Most geologists in the 1970s believed at the time that this mass extinction occurred over millions of years. Alvarez thought the foraminifera fossils pointed to a more rapid extinction. They decided to test the clay between the Cretaceous and Paleogene layers to determine how long it took to form. They looked for iridium, a rare element in Earth’s crust that can also fall from space in trace amounts (52). The clay had an iridium level of nine parts per billion—six hundred times greater than that of the rock around it.

[3]

Alvarez thought that such a staggering amount of iridium must have come from space. Luis Alvarez, a Nobel scientist and Walter’s father, joined his son in developing a hypothesis about the extinction and the iridium. [B] In 1980, Alvarez’s team published its theory: a huge meteorite had slammed into Earth. The dust and ash thrown into the atmosphere and blocking sunlight for months.

[4]

For years, Walter Alvarez futilely searched in vain for an impact crater large enough to substantiate his theory. Then, in 1990, geoscientist Alan Hildebrand found evidence of an ancient asteroid impact that had occurred near Texas. [C] This find led Hildebrand to two geophysicists who had discovered the Chicxulub Crater, a ninety-two-mile-wide crater off the coast of Mexico. [D] The discovery encouraged Alvarez’s theory. A meteorite caused the K-Pg extinction.