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In the 1980researchers Luis And Walter Álvarez – physical father and geologist son – they published a scientific article in which they hypothesized that the extinction of most dinosaurs, which occurred 66 million years ago, was triggered by the impact of an asteroid. The genesis of what went down in history as Hypothesis of Álvarez took place in Italy, in Bottaccione gorgeAt Gubbio, where the two researchers organized an expedition to study a small layer of dark grafted between the rocks. Right there, in this harmless layer of rock, lies the story of one of the greatest cataclysms that our planet has gone through in geological history.
The limit between two eras
What is lower is older, what is higher is more recent: it is one of the basic principles of geology, the so-called superposition principle. In the course of geological eras, in fact, rocks settle to form layers in subsequent events, such as volcanic ones or through the deposition of material from wind and water. In these layers the geological history of our planet remains immortalized, complete with fossils of the living beings that populated it and the traces of the cataclysms that led to their death. That thin layer of dark rock studied by the Álvarezes corresponded precisely to one of these cataclysms. In the older underlying layers, those of the Cretaceousthere were many fossils that in the overlying layers, those of the Paleogene, they were gone. Indeed that dark line – which marks the so-called KT limit – appears at a huge mass extinctionin which they disappeared beyond the 75% species on Earth including most of those of dinosaurs.
The asteroid hides in the details
By studying the KT limit, the Álvarezes discovered that this thin layer of rock was very rich in iridium, a chemical element that is very present in the nuclei of the planets, but which is very rare to find on the earth’s surface. In fact, iridium is a heavy element that when a planet is formed, ends up inwards, towards the nucleus, while the less dense compounds rise in the outermost layers (a magic that planetologists call differentiation). It is found, in small parts, due to the deposition of cosmic dust, that dust that wanders almost everywhere in interplanetary space and that continually ends up on the earth’s surface – yes, even in that dust that accumulates there, on the bedside table. However the Álvarezes measured an extremely high amount of iridium in the KT limit: 20 to 160 times higher compared to what you would have with cosmic dust alone. And considering that the clay layer is very thin, and therefore settled in a short time, the possibility seems to be only one: that iridium was brought on by a planetary impact. There was the smoking gun, all that remained was to find the bullet hole.
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