The Sirente crater field: The first impact crater in Italy

In the Prati del Sirente plain in the heart of the Abruzzo mountains, a small circular lake is clearly visible. A prominent ridge encircles the lake. The peculiar appearance of the lake drew the attention of this papers first author (Jens Ormö) when working in Pescara between 1999 and 2002. Jens did at that time a European Union Marie Curie post-doc as an impact geologist at the International Research School of Planetary Sciences (IRSPS), Universita d’Annunzio. After the years of research that followed upon the discovery, and which we will describe in this article, the lake in Prati del Sirente is now proposed to be the first known impact crater in Italy.

The discovery of the Sirente crater was a pure coincidence, but nevertheless shows the importance of mobility of researchers with different experiences. People with different backgrounds look at things with different eyes.

While preparing for a weekend hike in the mountains Jens saw a photo in a guide book of the Sirente Regional Park. It showed a high plain in the Sirente massif with a small circular lake at the centre. Small lakes are common in the Abruzzo mountains, but this one is encircled by an elevated rim, which makes it exceptional. A study group was formed with the colleagues Angelo and Goro. A first reconnaissance trip to the lake gave that the lake was larger than first expected and that it showed a striking resemblance to small impact craters known from other areas in the world (sse: T.S. number 4: about Barringer crater, or T.S. number 16). Due to the geological setting of the lake and its voluminous elevated rim it was soon clear it is not a karstic feature.

After ruling out some other alternative causes of formation based on the local geology (e.g. volcanic), only three plausible alternatives remained: periglacial feature (i.e. Pingo), a man-made water reservoir, or an impact crater.

Due to fact that the rimmed lake is formed entirely in soft sediments, no breccias or other rocks typical for impacts into solid targets occur. The work had to be focused primarily on a detailed description of the morphology and the age of formation.

The morphology was studied by detailed levelling of the elevated rim and close surroundings. This gave that the elevated rim rises about 2.2 m above the surrounding plain and that it is 15 m wide. It was also evident that the lake was slightly oval with a maximum rim-to-rim diameter of 140 meter. The studies continued with core drillings, one near the centre of the lake (7 meters in length) and two on the elevated rim (3 meters and 5 meters in length). The core drillings confirmed that the lake depression is developed entirely in soft sediments. Radiometric carbon dating of material in one of the cores from the rim gave that the elevated rim is formed by overturned material in a way typical for impact crater rims, and that the age of formation could be set to the 4th or early 5th century AD.

At this point the periglacial alternative was ruled out, mainly due to the young age of formation. We also discovered that the morphology of the crater rim corresponded exactly to what can be expected at an impact crater in this specific target material. At a crater formed in solid rock, the ejected material at the rim rests on a surface that has been uplifted due to brecciation of the rock (volume expansion) and displaced material below the rim. In a target consisting of wet clays, however, the shock will cause a compaction of the target (volume decrease). This has been noted at both experimental craters and impact craters. At Sirente this is visible as a depression along sections of the crater rim as well as in the drill cores.

Theoretically, an impact crater of the size of the Sirente lake depression cannot exist alone. It must be surrounded by a field of small craters. This is due to the fact that small cosmic objects that approach the Earth would fragment during the passage through the atmosphere. A common velocity of such objects (e.g. asteroids and comets) when they impact the Earth is about 20 km/second. It is more than 20 times the velocity of a bullet from a high-velocity rifle. The collision with the atmosphere generates so high stresses in the object that it breaks apart. Only objects forming craters larger than about 1 km in diameter can survive the passage through the atmosphere as reasonable coherent bodies. Hence, the study at Sirente continued with a survey of the surrounding plain. It was soon clear that the lake was the largest crater in a field of craters. The crater field covers an area somewhat less than a square kilometre, which is the correct dimensions compared to what has been calculated for impact crater fields based on the dynamics of atmospheric break-up. The probability for the remaining alternative formation, a man-made construction, decreased drastically with the discovery of the crater field.

It is known that when an object strikes the ground with such a high velocity that it generates a crater of the size of the Sirente lake depression, most of the object would melt and vaporise. However, the broken off fragments may get their velocities so reduced by atmospheric friction that they merely penetrate into the ground without disintegrating. The process can be compared with the impact of a high-velocity rifle bullet, although the larger size of the fragments and the still high velocity (about 5-7 km/second) generate craters a few meters in diameter. These fragments would be responsible for the smaller craters in the Sirente crater field. Meteorites often have a magnetic signature that is different from the present Earth magnetic field. This can generate distinctive magnetic anomalies. To investigate if any of the smaller craters contains meteoritic fragments, a magnetic survey of the crater field was performed together with colleagues from the Istituto Nazionale di Geofisica e Vulcanologia in Rome. Preliminary results from this survey indicate sources of magnetic anomalies at some meters depth in most of the small craters, but the evaluation of the data is not yet concluded.

Two adjacent small craters in the crater field were excavated to about 9 meters deep in order to investigate the source of the magnetic anomalies and to reveal the subsurface morphology. The excavation did not retrieve any macroscopic meteoritic fragments, possibly due to that it did not reach deep enough. It is estimated that any meteoritic fragments would be at least 12 meters below the surface. Instead soil samples were obtained for geochemical analyses. These analyses are not yet finished. In addition, several other discoveries were made during the excavation, which gave further support to the formation by a meteoritic impact.

It is very unlikely that any meteoritic material is exposed on the ground surface at the Sirente crater field. Due to the soft target, any meteoritic fragments would have penetrated too deep to be recoverable. In the large crater that contains the small lake, the meteorite exploded and vaporised at impact. Thus, there is no large meteorite hidden below the lake.

Detailed results from the studies of the Sirente main crater and the crater field will be published in the November issue of the international scientific journal Meteoritics and Planetary Science. The studies of the Sirente crater field are a continuous project and previous results are constantly being updated by new data. The results from the latest excavations mentioned above are not yet published.

The morphology of the main crater and its relation to a crater field strongly supports a meteoritic impact origin. The research is now focused on finding geochemical traces of the meteorite, which would provide unequivocal evidence for a meteoritic impact.

The Sirente crater field is of great value for the researchers, and to the Sirente-Velino Regional Park. We would like to express our gratitude to the former park director Edoardo Alonzo who gave a great support to our studies in the park. Due to the soft material of the crater it is very sensitive for erosion, especially the elevated rim. Only the this grass cover protects the craters from being destroyed by rain wash and wind. We would like to advice any visitors to be very careful not to damage the grass cover.

There are about 155 known impact craters in the world. Many of them are millions of years old and very eroded. Due to the geologically young age of Italy no impact craters have been found until today. The Sirente main crater and its adjacent smaller craters are very small compared to most the other impact craters in the world. However, due to the young age, the craters are very well preserved. They provide a rare, and valuable, example of an impact into a soft target.

For more information please contact:

Dr. Jens Ormö (email: ormo@inta.es), Principal Investigator of the Sirente impact crater
Angelo Pio Rossi (email: arossi@irsps.unich.it) and Dr. Goro Komatsu (email: goro@irsps.unich.it)