Field of Science

The Alvarez and the Crater of Doom

"It was nothing of this earth, but a piece of the great outside; and as such dowered with outside properties and obedient to outside laws."
"The Colour Out of Space", by H.P. Lovecraft (1927)

Until 50 years ago, it seemed that a characteristic peculiarity of the Cretaceous-Palaeogene (or K–Pg) ) transition, famous for the mass extinction event that "killed" off the dinosaurs, was the apparent lack of a complete stratigraphic record. However, in the decade 1960-1970 the American geologist Walter Alvarez discovered a homogeneous and complete succession of bedded limestone- and marl-layers in the gorge of Gubbio (also Gola del Bottaccione, located in the far north-eastern part of the Italian province of Perugia, Umbria), called the "Scaglia rossa"-formation.

Alvarez attempted to calculate the rate of deposition of this formation by analyzing the concentration of rare earth metals found in the sediments. A constant rain of micrometeorites, enriched in such metals, coming from outer space causes a constant concentration of rare earth metals in the sediments. A sudden change, therefore, can indicate that the rate of sediment deposition also suddenly changed.

It was during this research that Alvarez discovered the today well-known Iridium anomaly. At first, the origin of this anomalous concentration remained unclear. In 1980, Walter Alvarez and his father, the nobel-prize physicist Luis W. Alvarez (1911-1988), proposed two possible explanations - a very slow sedimentation rate of the Scaglia Rossa, resulting in an apparent concentration of micrometeorites, or the impact of a large mass of extraterrestrial material at once.

But Alvarez could not provide further evidence to confirm or disprove both hypotheses.


In 1981, geologists Antonio Camargo-Zanoguera and Glen Penfield presented during a geophysical conference their research on a geological mystery discovered 30 years earlier, during surveys on the Yucatan peninsula (south-eastern Mexico). The two researchers proposed a new interpretation of a circular structure revealed by seismic investigations and buried under 300 to 1.000 m of sediments, considered until then of volcanic origin. They suggested that the circular structure was a weathered and buried crater, formed by the impact of a large meteorite.

Only ten years later, some researchers from the University of Arizona started to study the crater and obtained the first absolute age - 65-million-years. Research in 1993 revealed that the structure was in fact an impact crater with a total diameter of 180 km. The crater was named after the nearby town of Chicxulub - meaning "the devil's tail".

Alvarez, meanwhile, continued his research on the impact hypothesis and noted the temporal coincidence of the Iridium anomaly, the Chicxulub-impact and the mass extinction at the end of the Cretaceous. The impact could explain the observed rare earth elements anomaly, was an important time marker, and also appeared to be the main culprit to blame for the extinction of the dinosaurs. In 1995 the limit between the Cretaceous and the Palaeogene was therefore defined at the stratotype (the GSSP) of El Kef (Tunisia), coinciding with the peak of Iridium and the mass extinction of foraminifera at the base of a clay layer (also referred as K-Pg boundary clay) deposited after the impact.

Today more than 350 sites worldwide are known to record the Cretaceous-Palaeogene sedimentary transition. Within a radius of 500 km the debris layer of the impact is very thick; around the crater it reaches a thickness of 100 to 80 m. In a radius of 500 to 1.000 km the sediments are typical tsunami deposits - layers containing debris and spherules (molten rocks that subsequently cooled to form droplets) transported by the waves from the impact site. With increasing distance, the layer thins out to form the known Iridium rich clay overlying a layer with small spherules. In a distance over 5.000 km the impact is represented by a single layer of red clay that still contains traces of the material ejected from the crater.

The geological evidence supports the hypothesis that a large extraterrestrial mass collided with earth.

According to the most popular scenario, the mass extinction event at the end of the Cretaceous marking the end of dinosaurs, large marine reptiles and ammonites, was caused by the consequences of the impact. Shock wave and fire storms were soon followed by the release of large quantities of gas from the vaporized rocks, rich in carbonates and sulphates. The gases reacted with the water vapor to form acid rain, and the dust in the atmosphere blocked the sun causing a many years-long nuclear winter. Depending on the locality and the ecological niche that a species occupied - and a good dose of luck - these changes decided a species' fate - to survive or to be doomed.

De La Beche's Awful Changes

Caricatures are exaggerated sketches of a person or human behavior. However, such cartoons appear only at a superficial glance as simple drawings, as they contain deep and complex insight in our culture and society. This consideration is also true for scientific caricatures, dealing with subjects or persons involved in science and research.

For a long time, the caricature by British geologist Henry De la Beche (1796-1855) "Awful Changes. Man Found only in a Fossil State - Reappearance of Ichthyosauri" was considered a caricature of fellow geologist and paleontologist William Buckland (1784-1856). The sketch was widely publicized in Francis Buckland's (1826 - 1880, son of William) book-series "Curiosities of Natural History" (1857-72), including a biography of his father.

"A lecture, - 'You will at once perceive,' continued Professor Ichthyosaurus, 'that the skull before us belonged to some of the lower order of animals; the teeth are very insignificant, the power of the jaws trifling, and altogether it seems wonderful how the creature could have procured food."

However, geologist and earth-science historian Martin J.S. Rudwick realized the connection of this scene with some drawings produced before 1831 by De la Beche in his diary, where he ridiculed the approach adopted by Charles Lyell. In the unpublished drawings, a lawyer (the reference to Lyell, who actually was a lawyer, seems obvious) is carrying a bag with "his" theory around the world, or he is shown wearing particular glasses (like Professor Ichthyosaurus), and offering his "view" and the resulting "theoretical approach" to a geologist carrying a hammer and collecting bag, a reference to the geologist actually working in the field. De la Beche never completed the sketch, because he abandoned this design in order to try out others, including the now famous "Awful Changes."

De la Beche believed that Lyell injected too much of his lawyer profession into the emerging field of geology, focusing too much on theories than real research. "Awful Changes" does lampoon one crucial part of Lyell's uniformitarianism - theory, the concept of time repeating itself, as prehistoric animals are behaving much like Victorian scholars. In a second cartoon De la Beche is mocking another idea of Lyell, the effects of present causes operating at the same slow magnitude throughout geologic history. The cartoon shows a vast U-shaped valley, in the foreground a nurse with a child, presumably the son of William Buckland, can be spotted. The child is peeing into the huge valley and in the caption De la Beche has his nurse exclaiming, "Bless the baby! What a valley he have made!!!"

The caricature was inspired by the ongoing debate of river-erosion at the time. The glacial theory wasn't still accepted to explain the formation of large valleys and the shape of many valleys in Europe was hard to explain only based on, as proposed by Lyell's uniformitarianism, slow fluvial erosion.

Carl Friedrich Christian Mohs's Mineralogical Legacy

Carl Friedrich Christian Mohs, lithography by Joseph Kriehuber (1832).

Talc – Gypsum – Calcite – Fluorite – Apatite – Feldspar – Quartz – Topaz – Corundum – Diamond - the Mohs Scale of Mineral Hardness is familiar to rock-hounds and earth-science students alike. The ten-point hardness scales lists common minerals in the order of the relative hardness, with talc being the softest and diamond the hardest mineral found in nature.

The Mohs scale is named after German mineralogist Carl Friedrich Christian Mohs, born January 29, 1773, in the town of Genrode, at the time part of the principality of Anhalt-Bernburgs. After attending school, he worked in his father's business as a merchant, but in 1796 he went to the University of Halle to study there mathematics, physics and chemistry. He continued his studies at the famous Royal Saxon Mining Academy of Freiberg, where he studied under the renowned geognost Abraham Gottlob Werner. Werner published in 1787 a »Kurze Klassifikation und Beschreibung der verschiedenen Gesteinsarten« - Short classification and description of the various rock types - as a guide for identifying and classifying rocks and minerals. Unlike other mineralogists at the time, mostly using chemical analysis, Werner uses easily recognizable features, like color or crystal shape, to classify minerals and rocks. Mohs is impressed by Werner's approach. In 1804, he publishes himself a “student-friendly” classification chart for minerals, based on his experience in the mining district of the Harz mountain and as a consultant for wealthy mineral-collectors. In his book »Über die oryktognostische Classification nebst Versuchen eines auf blossen äußeren Kennzeichen gegründeten Mineraliensystems« - The genetic-geological classification and an attempt to introduce a mineral-system based on outer properties - Mohs combines various physical properties of minerals, like color, hardness and density, with six classes of crystal shapes, to identify 183 different minerals.

Mohs scale of hardness sets from the 19th century, Mohs's geological hammer, and a letter to his wife.

Mohs continues to travel, collect material and improve his mineral classification system. He visits Štiavnica in Slovakia, famous for the local Mining Academy, and the mining district of Bleiberg in Carinthia. He visits and studies mines in Hungary, Transylvania and Scotland, and quarries in Germany and Austria.

In 1812, now a professor in the Austrian city of Graz, he creates a preliminary hardness scale and continues to publish guidelines for mineral identification. In 1818 he returns to Freiberg and between 1822-1824 Mohs publishes his final version of the hardness scale in the book »Grund-Riß der Mineralogie« - Essentials of Mineralogy.

The Mohs scale of mineral hardness is based on the ability of one natural sample to scratch another sample visibly. The samples of matter used by Mohs are readily available to a student or miner. Minerals with a hardness of 1 or 2 can be scratched with a fingernail. A coin will scratch minerals with a hardness of 3, the blade of a pocket knife scratches minerals of the hardness 5 and 6. Glass will scratch minerals with a hardness of 7, and harder minerals scratch each other.

Calcite crystals, example of a common mineral with hardness 3.