In the Valley of Iridium

pics: The K-T site at Gubbio showing the Cretaceous (lower grey limestone), the thin 1 cm layer of iridium rich clay in the middle and the darker brown clay of the Tertiary period. The Valley of Iridium - the Bottaccione Gorge near Gubbio.



5 Jan 2010

It’s 2010 and another circuit around the Sun begins. It’s traditional to take this time to look back, and to look ahead. Looking back at 2009, one of the highlights for me was to visit the K-T boundary at Gubbio. Looking forward I'd like to learn as much as possible about this geological event.

I had the privilege last year to visit the world-famous K-T boundary site in Italy. It has been an ambition of mine to visit this site since reading about it in Time magazine in the early 1980’s. I was so intrigued by the site and the history of its discovery that I spent a part of my Italian holiday preparing a write-up on it. Back home in my study, in a moment of utter madness I decided to try and calculate the diameter of the meteor that had caused the K-T extinction and the energy released from the impact. I struggled through the calculations, and after a week of frustration and ludicrous answers, I finally succeeded in the math. Turns out that my knowledge of the metric system was pretty poor! The numbers show that the effects of an impact of this size were of global proportion as it produced enormous amounts of energy, molten rock and dust and caused almost incomprehensible destruction.

Here are the numbers:

Why would a meteor impact, even one as large as the Chicxulub feature, be enough to cause the extinction of the dinosaurs and a myriad of other species? We could perhaps get a better understanding of this by attempting to answer questions such as; how much material was ejected at the time of impact, how much iridium was distributed worldwide, how much did the meteor weigh and what was its diameter and how much energy was released during the impact? In an attempt to quantify these parameters I did a number of calculations using the estimated impact velocity, the average iridium content and mean thickness of the KT layer. The answers I got even with making several assumptions revealed staggering numbers. I give detail of the calculations and answers below:

(note: this crazy html text does not allow the superscript function, so I have added the up arrow ^ to denote that the number is a superscript i.e. 1 x 10^2 would be 1 times ten to the power of 2).

How much material was ejected from the site of impact?
The Iridium layer is globally present on average as a 1 cm thick layer. The surface area of the Earth is 1.3 x 10^8 km^2. Since, area x thickness = volume, then 1.3 x 10^8 km^2 x 1 x 10^-5 Km gives us the volume ejected in Km^3:

The volume of material ejected by the impact is 5.1 x 10^12 Km^3

How does this compare with other known events?

If we compare the amount of material ejected by some of the largest volcanic events in recent geological time with that of the K-T meteor impact, we see that the K-T event produced 2 million times more ejecta than the supervolcano at Yellowstone.

Krakatoa (1883) produced 21 km^3 of material
Yellowstone “Supervolcano” (100 000 years ago): 1000 km^3 of material
Yellowstone “Supervolcano”(2 million years ago): produced 2500 km^3 of material.
K-T Chicxulub impact produced: 5.10000000000 km^3

How much Iridium did the meteor contain?
Using the average density of crustal rock which is 3000 kg/m^3 and the total K-T clay mass of 1.53 x 10^16 kg dispersed around the earth and the mean iridium concentration of 0.3 parts per billion in KT layer we can calculate the metric tons of iridium in the meteor as:

4.59 x 10^6 Kg (4590 metric tons)


How much did meteor weigh?
From the average iridium abundance in chondritic meteorites, the mass of the meteor is calculated at:

4.83 x 10^14 kg (4.83 x 10^11 metric tons)


What was the diameter of meteor?

From the density of C1 Carbonaceous Chondrites (2110 Kg/m^3) the volume of the meteor is calculated at 2.30 x 10^11 m^3 (or 2.3 x 10^2 Km^3)

Assuming that the meteor was spherical, we can calculate its diameter:

Diameter = 2(3^√2.3 x 10^2 x 0.75/3.14)

Diameter = 7.6 Km


Alvarez et al used four independent methods to calculate the diameter of the impactor. The mean value they obtained was 10 Km with a standard deviation of ± 4 Km. This puts my calculation of the diameter of the meteor within the standard deviation obtained by Alvarez et al. As a size comparison Mount Everest is 8.84 Km in height.

How much energy was released on impact?

A 7.6 km diameter meteor travelling at 25 Km/second has a kinetic energy of:

Ke = ½ mv^2 = 3 x 10^23 Joules
(where Ke is Kinetic Energy, m = mass of meteor and v is the velocity of the meteor)

Energy released on impact = 3 x 10^23 Joules


Comparing the amount of energy released by the Chicxulub impactor with the Hiroshima A-bomb which produced 8.4 x 1013 Joules of energy, we can calculate that the K-T meteor impact was equivalent of 3.5 billion Hiroshima atomic bombs! That is, 0.5 atom bombs for every person living today. A megaton of TNT is 4.184 × 1015 joules, therefore the Chicxulub impactor would have produced energy equivalent to 71 million megaton of TNT.


The discovery of the K-T iridium anomaly at Gubbio is considered to be one of the most important discoveries in evolutionary science. It was also central to the recognition by scientists that occasional catastrophic events like great impacts require a rejection of strict uniformitarianism in geology.

While the impact hypothesis is now one of the strongest and most widely-accepted theories about the extinction of the dinosaurs, other ideas remain valid, and it is highly unlikely the question will be definitively answered in the near future. Vulcanism could be a source of some of the iridium, but to have provided enough of this element to give the concentrations found uniformly worldwide in boundary clays, would have involved a degree of volcanic activity far beyond human comprehension. Even the basalts of the Deccan Traps in India, proposed by some to have been a source of such iridium, contain only 0.005 ppb of this element.

What is important to note, however, is that the death of the dinosaurs was the direct cause for the rise of mammals. Dinosaurs had their chance, and lost out to a group of small, furry animals that would evolve into the forms that now have dominion over the Earth. But that doesn't mean we're exempt from the same dangers. From research of craters on the Moon and Mercury, it is estimated that the time to collision is proportional to the square of the diameter of the object . Therefore, meteors of 10 kilometers diameter will collide with Earth on average once every 100 million years, meteors of 1 kilometer in diameter every 1 million years and 100 meter diameter meteor every 10 000 years.

So pause for a moment and look up—you never know what's coming.