share “Understanding just how we decipher a an excellent historical occasion written in the book of rocksmay be as interesting as the event itself.”—Walter…By Sean B. Carroll

“Understanding exactly how we decipher a good historical event written in the publication of rocksmay it is in as exciting as the event itself.”—Walter Alvarez

Built upon the slopes of mount Ingino in Umbria, the ancient town the Gubbio boasts many well-preserved frameworks that paper its glorious history. Established by the Etruscans in between the second and an initial centuries B.C., its roman inn theater, Consuls Palace, and also various churches and fountains are spectacular monuments come the Roman, Medieval, and Renaissance periods. It is just one of those special destinations that draws tourists to this famous component of Italy.

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It was no the ancient architecture however the much longer natural history preserved in the absent formations external the city walls that carried Walter Alvarez, a young American geologist, to Gubbio. Just exterior the town lay a geologist’s dream—one that the many extensive, constant limestone absent sequences all over on the world (See Father and Son). The “Scaglia rossa” is the regional name because that the attractive pink outcrops discovered along the mountainsides and gorges the the area (“Scaglia” means scale or flake and refers to how the absent is quickly chipped right into the square blocks used for buildings, such as the roman theater. “Rossa” refers to the pink color). The substantial formation is composed of countless layers the span around 400 meters in total. Once an ancient seabed, the rocks represent some 50 million year of earth history.


Geologists have actually long offered fossils to help identify components of the rock record from approximately the world and also Walter employed this strategy in researching the formations around Gubbio. Throughout the limestone he uncovered fossilized shells of small creatures, called foraminifera or “forams” for short, a team of single-celled protists that deserve to only be seen with a magnifying lens. Yet in one centimeter the clay the separated 2 limestone layers, he uncovered no fossils in ~ all. Furthermore, in the larger layer below the clay, the forams were much more diverse and much larger than in the younger layer above the clay (See Foraminifera). Anywhere he looked about Gubbio, he found that thin layer of clay and also the exact same difference between the forams listed below and above it.

Father and also Son: louis (left) and Walter Alvarez in ~ a limestone outcrop near Gubbio, Italy. Walter’s best hand is touching the peak of the Cretaceous limestone, in ~ the K-T boundary.Courtesy that Lawrence Berkeley nationwide Laboratory

Walter to be puzzled. What had actually happened to cause such a change in the forams? How fast did it happen? just how long a period of time did that thin layer without forams represent?

These questions around seemingly mundane microscope creatures and also one centimeter the clay in a 1,300-foot-thick absent bed in Italy might appear to be trivial. However their pursuit led Walter to a important Earth-shattering discovery around one the the most vital days in the history of life.

Foraminifera : Walter Alvarez was confused by the rapid, dramatic adjust in foram size in between the end of the Cretaceous (pictured at the bottom here) and also the start of the Tertiary (top) periods, which is seen worldwide. This specimens are from a different location (not Gubbio). Photos courtesy that Brian Huber, Smithsonian Museum that Natural background

The K-T Boundary

From the circulation of fossils and other geological data, the was recognized that the Gubbio development spanned components of both the Cretaceous and Tertiary periods. The surname of these and other geological time periods come from at an early stage geologists’ ideas about the major intervals in planet history, and also from few of the functions that mark certain times. In one scheme, the history of life is separated into three eras—the Paleozoic (“ancient life,” the first animals), the Mesozoic (“middle life,” the period of dinosaurs), and the Cenozoic (“recent life,” the age of mammals). The Cretaceous period, named after properties chalky deposits, develops the last 3rd of the Mesozoic era. The Tertiary duration (which has been renamed and also subdivided right into the Paleogene and Neogene) begins at the end of the Cretaceous 65 million years earlier and ends at the start of the Quaternary period 2.6 million years ago.

Geologic time scale: Geologists to organize Earth’s history into eras and also periods. The KT boundary drops right in ~ the border the the Cretaceous period and the Tertiary period, approximately 65 million years ago.

Walter and his colleague bill Lowrie spent number of years examining the Gubbio formation, sampling increase from the Tertiary and down with the Cretaceous. Castle were first interested in trying come correlate reversals in the earth’s magnetic ar with the fossil document as a method of deciphering the time-scale of earth history. They learned to figure out wherein they to be in the rock development by the forams properties of certain deposits, and also by finding out to recognize the boundary in between the Cretaceous and also Tertiary rocks. That boundary was always right wherein the dramatic reduction in foram diversity dimension occurred. The rocks below were Cretaceous and also the rocks over were Tertiary, and the slim layer the clay remained in the gap in between (See The K-T border at Gubbio). The border is described as the K-T boundary.

One thousands kilometers from Gubbio, in ~ Caravaca on the southeast shore of Spain, a dutch geologist, january Smit, had actually noticed a comparable pattern of changes in forams in rocks around the K-T boundary. Smit knew the the K-T boundary significant the most renowned extinction of all—the dinosaurs. As soon as a colleague pointed out that truth to Walter, he became even an ext interested in those small forams and the K-T boundary.

The K-T border at Gubbio: The white Cretaceous limestone is separated indigenous the red Tertiary limestone by a thin clay class (marked with coin). Courtesy of frank Schonian, Museum of organic History, Berlin

Walter was relatively brand-new to scholastic geology. After he received his Ph.D. He had functioned for the expedition arm that a multinational oil firm in Libya, till Colonel Qaddafi expelled every one of the Americans the end of the country. His work on magnetic reversals had gone well yet he realized the the abrupt readjust in the Gubbio forams and also the K-T extinction gift a much bigger an enig that he came to be determined to solve.

One the the first questions Walter want to answer, naturally, was how long it took for that thin clay layer to form? come answer this that would require some help. It is an extremely common for youngsters to get assist from your parents with their science projects. However, it is exceptionally unusual, together it was in Walter’s case, that the “child” is in their late 30s. But few children of any type of age had a Dad prefer Walter’s.

From A-Bombs to Cosmic Rays

Luis Alvarez knew very tiny about geology or paleontology yet he knew a lot around physics. He was a main figure in the birth and growth of nuclear physics. He received his Ph.D. In physics in 1936 from the university of Chicago and also worked in ~ the university of California, Berkeley under Ernest Lawrence, the receiver of the 1939 Nobel prize in Physics for the creation of the cyclotron.

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His early work in physics to be interrupted by the onset of world War II. During the an initial years the the war, Luis worked on the development of radar and also systems that would help airplanes floor safely in bad visibility. He got the Collier Trophy, the highest honor in aviation, for arising the ground Controlled strategy (GCA) device for bad weather landings.