The best reason is that he was a very smart astronomer/astrophysicist, who happened to have a lot of data about many asteroids in orbit around the Sun, with orbits between Mars and Jupiter.
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"...In 1980 Alvarez and his son, geologist Walter Alvarez, "uncovered a calamity that literally shook the Earth and is one of the great discoveries about Earth’s history"[1]
Walter Alvarez was doing geological research in central Italy during the 1970s on the walls of a gorge whose limestone layers included strata both above and below the so-called K–T boundary (sometimes called the K-Pg), the boundary between the Cretaceous and Tertiary periods corresponding to a time of 65.95 million years ago (Kuiper et al., 2008). Exactly at the boundary is a layer of clay about 1 cm thick. Walter removed a small piece of the rock containing both sections of limestone and the clay layer and later showed it to his father. Walter said "This layer marks where the dinosaurs and much else went extinct. Nobody knows why. Or what the clay is about. A big mystery!", and intended to discover why from then on.[1]
One of the first things Alvarez did was to try to figure out how long it had taken to lay down the centimeter of clay. It was not an easy question to solve considering the events happened 65 million years ago, but he thought he might be able to use the very slow deposition of iridium on earth from the cosmos as a kind of clock. Elements of the platinum group, including iridium, are rare in interplanetary space, but much more abundant than on Earth. There is very little iridium in the Earth's crust, but the planet is constantly bombarded with micrometeorites which, as they burn up in the atmosphere, lightly dust the surface with iridium at a constant and known rate. Alvarez reasoned that if there were a lot of iridium in the clay layer, then the clay layer had formed over a long time, and if there was very little, then the clay layer had formed in a short time.
Walter Alvarez was doing geological research in central Italy during the 1970s on the walls of a gorge whose limestone layers included strata both above and below the so-called K–T boundary (sometimes called the K-Pg), the boundary between the Cretaceous and Tertiary periods corresponding to a time of 65.95 million years ago (Kuiper et al., 2008). Exactly at the boundary is a layer of clay about 1 cm thick. Walter removed a small piece of the rock containing both sections of limestone and the clay layer and later showed it to his father. Walter said "This layer marks where the dinosaurs and much else went extinct. Nobody knows why. Or what the clay is about. A big mystery!", and intended to discover why from then on.[1]
One of the first things Alvarez did was to try to figure out how long it had taken to lay down the centimeter of clay. It was not an easy question to solve considering the events happened 65 million years ago, but he thought he might be able to use the very slow deposition of iridium on earth from the cosmos as a kind of clock. Elements of the platinum group, including iridium, are rare in interplanetary space, but much more abundant than on Earth. There is very little iridium in the Earth's crust, but the planet is constantly bombarded with micrometeorites which, as they burn up in the atmosphere, lightly dust the surface with iridium at a constant and known rate. Alvarez reasoned that if there were a lot of iridium in the clay layer, then the clay layer had formed over a long time, and if there was very little, then the clay layer had formed in a short time.
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