Holocene impact craters on Earth

Morasko is a strewn field located in central Poland (Szokaluk et al. 2019). It consists of seven recognized impact craters. The largest is 96 meters in diameter and about 12 meters in depth, and the smallest is about 30 meters in diameter (Wlodarski et al. 2017). Before entering the Earth’s atmosphere, the impactor weighed between 600 and 1100 tons (which corresponds to an asteroid of up to ~6 m in diameter); it moved with a velocity of 16–18 km/s and hit at an angle of 30–40° (Bronikowska et al. 2017). The Morasko meteorite associated with the craters belongs to the iron IAB group (Muszynski et al. 2012). The asteroid hit an edge of glaci-tectonically deformed terminal moraine from the Poznan phase (~18,500 years ago, the Frankfurt phase) of the last glaciation. The target rocks consist of Neogene clays in the base, and a couple of meters of glacial till overlaid by glacial and fluvioglacial sands; however, the distribution and thickness of those constituents is variable and patchy. The continuous ejecta blanket extends to about one crater-radius distance from the rim (Szokaluk et al. 2019). The age of the structures is estimated to be 3–6 ka, with the most probable age assumed to be about 5 ka (Szczuciński et al. 2016). Charcoal formed during the impact was found within the proximal ejecta (Losiak et al. 2022).

Kaali is a strewn field in western Estonia, on Saaremaa Island (Losiak et al. 2016). At least nine impact structures have been recognized there. The largest structure is 110 m in diameter and around 17 m deep to the current level of lake deposits (Plado 2012). It was formed by the impact of an IAB iron meteoroid weighing between 400 and 10,000 tons. The target rocks consist of horizontally layered Silurian dolomites covered by up to a few meters of glacial till. The ejecta blanket consists of an overturned sequence of those two types of materials, lying up to about a half crater radius of the rim, which are not mixed but separated into two distinctive layers. The age of the crater, based on charcoals formed during the impact (Losiak et al. 2022) and emplaced within its proximal ejecta, was determined to be shortly after 1530–1450 BCE (3237 ± 10 14C yr BP) (Losiak et al. 2016).

Kamil is a single impact crater, 45 m in diameter and 10 m deep, located in south-western Egypt (Folco et al. 2011). It is pristine: the satellite images show the ejecta rays that extend up to 300 m from the crater, a feature that is visible in only one other site among terrestrial impact craters. The continuous ejecta blanket extends up to 50 m from the rim. The crater was formed by the impact of an iron meteoroid: Ni-rich ataxite Kamil Gebel (D’Orazio et al. 2011). Target rocks are sub-horizontally layered sandstones overlaid by up to a few centimeters of loose unconsolidated aeolian sediments (Fazio et al. 2014). The crater has not been precisely and accurately dated; based on its excellent preservation and the paleoclimate in this area, its age is estimated to be 2000 BC–500 AD (Sighinolfi et al. 2015). It is unique among very small impact craters because it is associated with signs of shock metamorphism such as planar deformation features, shatter cones, and high-pressure polymorphs of SiO₂ (coesite and stishovite) as well as a significant number of different types of impact melt (Fazio et al. 2014). Interestingly, it was discovered thanks to the analysis of satellite images available through Google Earth.

Figure 2.

Ilumetsa is a set of two rimmed crater structures in southeast Estonia (Plado 2012). The larger structure is 80 m in diameter and 8 m deep, while the smaller one is 50 m wide and 3.5 m deep; they are located 725 m from each other. The geology of this area consists of a couple of meters of Quaternary sedimentary cover (glacial tills and sands) overlaying unconsolidated Middle Devonian sandstones (Losiak et al. 2020). Ilumetsa is not considered a confirmed impact site because neither remnant of the projectile nor other identification criteria, such as planar deformation features (PDFs), have yet been found. Only circumstantial evidence exists to support the extraterrestrial origin of these structures: (1) the presence of sedimentary beds deformed in a way consistent with being part of proximal ejecta (Aaloe 1963, Losiak et al. 2020); (2) a small thickness of the glacial sediments around the craters; (3) the presence of charcoal (Losiak et al. 2020), within what can be interpreted as a proximal ejecta blanket in a geomorphic setting, which is similar to charcoals found in other impact structures (Losiak et al. 2022); (4) 14C dating of those charcoals, showing that both craters formed simultaneously between 7170 and 7000 cal. years BP, about 7 ka after deglaciation of this area (Losiak et al. 2020). Further research is needed to test the impact origin hypothesis of Ilumetsa and numerous similar features because most of the roundish depressions in the ground are not impact craters, especially in a young glacial landscape (Plado et al. 2022).

The largest known impact crater on Earth, Vredefort in South Africa, which has a diameter of over 200 km, released two magnitudes more energy (~1E+23 Jules) in a nearly single point in space and time (couple of minutes) than the entire Earth releases over a year – as heat flow, seismic and volcanic energy (~1E+21 Jules table 2.1, French 1998). No other geological process on Earth’s surface induces such high temperatures (>> 1000°C) and pressures (>>5 GPa), and those atypical conditions leave characteristic signs within rocks, called shock metamorphism. Finding characteristic features formed as a result of a shock wave passage allows us to unequivocally identify the site of an asteroid impact. Those features include shatter cones, planar deformation features, high-pressure (diaplectic) mineral glasses, high-temperature, whole-rock impact melts, and various high-pressure mineral phases such as coesite; these are commonly found in large impact sites (recent reviews in French & Koeberl 2010 and Osinski et al. 2022).

The smallest known impact craters are formed by more than ten magnitudes of energy less than the largest ones; for example, a crater of about 100 m in diameter such as the largest Morasko in Poland (Szokaluk et al. 2019) was formed with a similar amount of energy as that released during the explosion of the atomic bomb in Hiroshima (~1E+13 Jules: French 1998). As a result, during their formation, only a limited volume of target rocks experience high enough pressures and temperatures to be recognizably shock metamorphosed. Additionally, because very small impact craters tend to develop in unconsolidated Quaternary materials, the shock wave passage affects material differently than in consolidated rocks and distributes modified material over a large area. For instance, in the Morasko strewn field in Poland, the volume of sediment shocked to pressures greater than 10 GPa (the minimal pressure necessary for producing planar deformation features in a portion of quartz grains experiencing those conditions) is very small – around 200 m³ compared with >7000 m³ total volume of ejected material – that is, less than 3% (Bronikowska et al. 2017). As a result, the search for shocked grains and impact melt has been unsuccessful. Within the 15 known recent craters (Table 1), planar deformation features and coesite (high pressure polymorph of quartz) were found only in Kamil and Wabar, while impact melt particles are also present in the Henbury crater field. Therefore, the primary criterion used to identify small impact craters in unconsolidated sedimentary materials is not the shock wave passage indicators but the discovery of meteorite fragments.

Impactors that are one kilometer in diameter or larger are usually not substantially influenced by the atmosphere during their transition. However, smaller bodies, especially those within the size range of up to 50 m in diameter, as is the case for all known Holocene craters (<160 m in diameter; Table 1), interact considerably with the atmosphere (Artemieva & Shuvalov 2019). Air compression and friction induced during the atmospheric passage heats up an asteroid, which leads to its ablation, fragmentation, and deceleration. The most important parameters that influence the way an asteroid interacts with the atmosphere are: (1) the meteoroid’s physical properties (size, density, strength, pre-existing zones of weakness); (2) its entry velocity and angle; and (3) the properties of the atmosphere (particularly important when considering different planets such as Mars or Venus).

A significant portion of the initial kinetic energy of an asteroid of less than 100 m in diameter is deposited within the atmosphere (Artemieva & Shuvalov 2019). On the one hand, this slows down the impactors, limiting the amount of energy delivered to the surface that might produce an impact crater; on the other hand, shock waves formed in the air can be strong enough to cause significant damage to the environment without forming an impact crater (e.g., Chelyabinsk bolide: Popova et al. 2013; Tunguska: Florenskiy 1963). However, they are unrecognizable in the geologic record after just a few decades, so their frequency can only be estimated.

If at least one fragment of an asteroid survives the atmospheric passage with a velocity above about 1 km/s, an impact crater can be formed within unconsolidated sediments (Schmalen et al. 2022). Depending on the altitude and degree of meteoroid fragmentation, different patterns of craters can be formed. If just a single particle with sufficient velocity survives, a single small impact crater is formed; for example, Whitecourt in Canada (Herd et al. 2008). If an asteroid is separated into multiple particles that preserve a sufficient portion of their kinetic energy, then a strewn field is formed; for example, Kaali in Estonia (Losiak et al. 2016). Based on the spatial and size distribution of the craters within the group, it is possible to estimate the direction and impact angle of an asteroid (Bronikowska et al. 2017).

The older and larger the crater, the rarer the meteorite fragments associated with it (Melosh 1989). This is because, during the largest impacts, the asteroid experiences such high pressures and temperatures that it is fully melted or vaporized. However, even in this case, the geochemical and isotopic fingerprints of this extraterrestrial material may be detectable (e.g., Alvarez et al. 1980, Koeberl et al. 2007). In contrast with larger structures, all confirmed Holocene craters are associated with meteorite fragments because they are the only indicator widely applied, as other shock metamorphic indicators (French & Koeberl 2010) are usually missing or very hard to find.

There are two types of meteorites that are found around Holocene impact craters (e.g., for Whitecourt, see Newman & Herd 2015). The “primaries” are derived from the fragments of the initial body, that were caused to decelerate during the atmospheric passage, so that their velocity at impact was not sufficient to produce an impact crater. These can be recognized by signs of atmospheric ablation preserved on their surface in the form of a fusion crust (a thin layer of melted material). They are similar in properties to “normal” meteorites that fall in non-crater-forming events. A special sub-type of “primaries” are fragments that hit the Earth with a velocity below the ~1 km/s necessary for the formation of an impact crater, but above the terminal velocity of 300–600 m/s; as a result, they produce terminal funnels (Schmalen et al. 2022). These are typically holes of up to twenty to thirty meters in diameter with a large meteorite inside (e.g., multiple sites at the Campo del Cielo strewn field in Argentina: Vesconi et al. 2011). The second type of meteorites are called “shrapnels” and are derived from the disruption upon impact of the crater-forming asteroid fragment. They are characterized by angular and irregular edges and smoother, concave interior surfaces. The primary meteorites are usually larger and found farther away from the impact site than shrapnels (Newman & Herd 2015).

Interestingly, nearly all confirmed Holocene impact structures are associated with iron or stony-iron meteorites (12 out of 13 cases: Table 1), even though iron meteorites form only 5% of witnessed falls. This may be caused by the fact that the rocky impactors are not capable of surviving the atmospheric passage in large enough and fast enough pieces to produce craters (Bland & Artemieva 2006, Artemieva & Shuvalov 2019). However, the formation of Carancas crater by a stony meteorite (H4-5 ordinary chondrite) in 2007 (Tancredi et al. 2009) may suggest an alternative hypothesis. If this event had not been witnessed, it is very probable that Carancas would not be recognized as having been formed by an extraterrestrial collision because the impactor was very fragmented and hard to find even after only a year. This suggests that all confirmed Holocene small impact craters are associated with iron meteorites because fragments of iron meteorites are much easier to find than small fragments of stony meteorites, especially after a few thousand years of weathering. Two impact sites (Table 1), Ilumetsa in Estonia (Losiak et al. 2020) and Sobolev in Russia (Khryanina 1981) are considered probable, but not confirmed, impact sites because no meteorites were found nearby.