The element Iridium (Ir) is a dense, silver-white metal that is extremely rare in the rocks of Earth’s surface. A highly unusual concentration of this element exists within a thin, worldwide layer of sediment marking the boundary between the Cretaceous and Paleogene periods, known as the K-Pg boundary. This striking geological feature, the iridium anomaly, serves as the primary evidence supporting the hypothesis that a massive asteroid impact caused the global environmental catastrophe 66 million years ago. The elevated Iridium level provided a direct link between an extraterrestrial object and the sudden end of the Mesozoic Era.
Why Iridium Is Scarce on Earth’s Surface
The scarcity of Iridium in Earth’s crust is a direct consequence of planetary differentiation, the planet’s formation process. Early in Earth’s history, the planet was largely molten, allowing materials to separate based on density. Iridium is classified as a highly siderophile element, meaning it has a strong chemical affinity for iron.
As the planet cooled, dense iron metal sank toward the center to form the core, dragging the majority of siderophile elements, including Iridium, with it. This process effectively stripped the upper mantle and crust of Iridium, concentrating it deep within the core. The average concentration of Iridium in the crust is exceptionally low, measured in parts per trillion, or about 0.0004 parts per million (ppm).
Any Iridium found on the surface today is attributed to minor, continuous deposition from micrometeorites or occasional volcanic activity. This extremely low baseline concentration makes any significant surface spike of the element remarkable. This depletion establishes a context where a sudden, global increase in Iridium cannot be explained by terrestrial processes alone.
The Asteroidal Source of Iridium
The contrasting abundance of Iridium in space rocks provides a compelling explanation for the anomaly found on Earth. Unlike Earth, smaller celestial bodies such as asteroids and comets did not undergo intense planetary differentiation. Their Iridium content remained distributed throughout their entire structure, mixed with the bulk material.
Primitive, undifferentiated meteorites, such as chondrites, contain Iridium at concentrations significantly higher than Earth’s crust, around 0.5 ppm. This concentration is orders of magnitude greater than the background level found in surface rocks. An impactor originating from the asteroid belt would naturally carry this high concentration of Iridium and other platinum-group metals.
The sheer volume of Iridium contained within a large impactor, estimated to be 10 to 14 kilometers in diameter, would be enormous. When such an object collided with Earth, it delivered a massive pulse of this extraterrestrial metal to the surface environment. The Iridium acts as a distinct chemical fingerprint of a foreign object.
The Discovery of the Iridium Anomaly
The anomalous Iridium layer was first identified in the late 1970s by a research team led by physicist Luis Alvarez and his son, geologist Walter Alvarez. They were studying a thin layer of clay at the Cretaceous-Paleogene (K-Pg) boundary near Gubbio, Italy, representing the time of the mass extinction event. Chemical analysis of this specific layer revealed Iridium concentrations hundreds of times higher than normal crustal rock.
The initial finding showed Iridium levels peaking at about 10 parts per billion (ppb) within the clay, compared to an expected background of less than 0.1 ppb. This dramatic spike was subsequently confirmed in K-Pg boundary sections worldwide, including sites in Denmark and New Zealand. The widespread nature of this anomaly, confirmed by multiple independent laboratories, suggested a single, global event rather than a localized phenomenon.
The thin, distinct nature of the clay layer indicated that the Iridium deposition occurred suddenly and relatively quickly across the globe. This finding was the foundation of the Alvarez hypothesis, which proposed that the extinction event was caused by the impact of an extraterrestrial object. The Iridium anomaly remains a powerful chronological marker for the K-Pg event, approximately 66 million years ago.
How the Impact Spread Iridium Globally
The presence of the Iridium anomaly across marine and terrestrial sites worldwide required a mechanism capable of global distribution. The hypervelocity impact provided the necessary energy for this widespread dispersal. Upon striking Earth, the immense kinetic energy of the roughly 11-kilometer-wide object instantly vaporized both the asteroid and a substantial volume of the surrounding target rock.
This vaporization created a colossal plume of superheated material, known as ejecta, which contained the Iridium-rich dust from the shattered impactor. The force of the explosion launched this fine-grained material high above the troposphere, pushing it into the stratosphere. Once suspended at this altitude, global atmospheric circulation patterns efficiently distributed the fine Iridium dust around the entire planet.
This stratospheric dust slowly settled back to Earth’s surface over a period estimated to be years to decades, coating the planet in a thin, uniform blanket. The resulting layer of Iridium-enriched clay represents the fallout from the impact. This layer essentially marks the precise moment of the collision in the geological record.
Corroborating Geological Evidence
The Iridium anomaly is accompanied by a suite of other physical markers locked within the same K-Pg boundary layer. These co-located materials provide independent evidence of an immense, singular impact event.
Shocked Quartz and Microtektites
The layer contains shocked quartz grains, which display a specific deformation pattern created only by the extreme pressures of a hypervelocity impact. Volcanic activity cannot produce this deformation. The layer also contains microtektites, which are tiny, spherical glass beads formed when molten rock from the impact site solidifies during its fall back to Earth.
Soot and the Chicxulub Crater
High concentrations of soot are often found in the boundary layer, suggesting massive, global wildfires ignited by the thermal pulse of the incoming ejecta. The ultimate corroboration came with the discovery of the Chicxulub impact structure, a vast, buried crater over 180 kilometers in diameter beneath the Yucatán Peninsula in Mexico. The crater’s age aligns precisely with the K-Pg boundary, confirming it as the source of the Iridium and the other physical debris.