Formation of the Crater

According to Reinwald (1933), the meteorite had pierced the dolomite massif by 2.5 m. The mighty impact was accompanied by heat which turned the water in the rocks instantaneously into steam and heated the gases, which had formed, so far that they caused burning of the rock not only at the level of explosion, but also at the depths they reached through a dense net of impact-generated cracks. At the same time, these huge quantities of instantaneously formed gases caused an acute explosion which shattered the surrounding rock thereby crushing it partly into rock-flour, and throwing it upwards out of the formed crater. The focus of the explosion was presumably situated in the place of the greatest pressure, i.e., directly under the solid body.

According to Reinwald, the meteorite that had produced the main crater had a diameter less than 3 m. His calculations showed that one of the small craters - crater 4 - with a diameter of 20 m was produced by a meteorite body the diameter of which was about 0.5 m, i.e. approximately 1/40 of the crater's diameter. In size, the meteorite generating the Kaali main crater is comparable to the Hoba meteorite in Namibia (length and width 2.7 m, height from the ground 0.9 m, mass ca 70 t) and Ahninghito meteorite (mass 34 t). However, compared to the Kaali meteorite, they hit the ground under a different angle and did not produce a crater.

With the death of Reinwald in 1941, the study of the Kaali craters was interrupted for a long time. It was resumed in 1955 on the initiative of Karl Orviku, head of the Commission on Meteoritics of the Estonian Academy of Sciences. The studies were carried out by researchers of the Institute of

Geology of the Academy of Sciences of the Estonian SSR, mainly by Ago Aaloe. Main attention focused on small craters, but in 1974 large-scale geophysical investigations were undertaken in the main crater in cooperation with researchers of Moscow State University (Aaloe et al. 1976). Using seismic and electrometric methods, the contours of the zone of shattered rocks surrounding the crater were determined. The zone surpasses the dimensions of the crater twice. The zone of shattered rocks has a specific contour, the axis symmetry of which is directed from east to west.

Opinions differ as to the direction from south to northeast and angle of incidence of the Kaali meteorite. On the basis of the size of the craters, Reinwald (1937a, 1937b) came to the conclusion that the direction of movement was from the east-southeast to the west-northwest. This idea was supported by Krinov (1962). It is known that a meteorite with a reasonable mass maintains its initial cosmic velocity for a longer time than smaller ones. Therefore, the larger one should be in front of the dispersal ellipse, and the smaller ones behind it (Fig. 1). This shows that the meteorite fell from the southeast (Raukas 2002). Based primarily on the study of impact traces at the bottom of craters 4 and 5, Aaloe (1958) maintained that the possible angle of incidence had been 35-400 relative to the horizon.

The energy needed for the formation of the Kaali main crater is estimated as 4 x 1012 J (or 1019 erg), and approximately two orders of magnitude less for the formation of small craters (Bronsten and Stanyukovich 1963). The initial velocity of the meteorite with an initial mass of 400-10,000 t (most probably ~1000 t upon entering the atmosphere) is estimated as 15-45 km/s. At the time of impact, its weight was probably 20-80 t and its velocity was 10-20 km/s (Bronsten and Stanyukovich 1963). According to Pokrovski (1963), the diameter of the Kaali meteorite was probably 4.8 m, its mass 450 t, and its impact velocity 21 km/s. Koval (1974) suggested a somewhat lower velocity (~13 km/s) of the Kaali meteorite at impact. His calculations showed that the mass of the meteorite, which produced the main crater, must have been about 40-50 t. According to Koval, the meteorites that formed the small craters may have weighed between 1 and 6 t. Thus, the force of the Kaali meteorite was too small to induce thus great environmental consequences as maintained by Veski et al. (2001a, 2001b). To our mind, its explosion did not cause any serious ecological catastrophe in the surroundings.

Isohypse Crater

Fig. 4. Morphology map of the Kaali main crater and the profiles of the crater in different directions. Abbreviations: R - bench mark (22.37 m a.s.l.); ABSO - absolute zero level of the Baltic System; K - digging; T - trench. Legend: 1- isohypses; 2 - old diggings; 3 -steps; 4 - steep slope; 5 - footpaths and small roads; 6 - stone wall; 7 - profiles; 8 -schoolhouse with bench mark (R); 9- old foundation; 10 - crossing point of profiles on top of the heap of stones; 11 - digholes made in 2001 (see Fig. 2); 12 - heap of stones in the centre of the lake; 13 - cross section of the stone wall; 14 - stones on the crater's slope. On the profiles: 15 - water; 16 - lake sediments; 17 - stones on lake bottom.

Fig. 4. Morphology map of the Kaali main crater and the profiles of the crater in different directions. Abbreviations: R - bench mark (22.37 m a.s.l.); ABSO - absolute zero level of the Baltic System; K - digging; T - trench. Legend: 1- isohypses; 2 - old diggings; 3 -steps; 4 - steep slope; 5 - footpaths and small roads; 6 - stone wall; 7 - profiles; 8 -schoolhouse with bench mark (R); 9- old foundation; 10 - crossing point of profiles on top of the heap of stones; 11 - digholes made in 2001 (see Fig. 2); 12 - heap of stones in the centre of the lake; 13 - cross section of the stone wall; 14 - stones on the crater's slope. On the profiles: 15 - water; 16 - lake sediments; 17 - stones on lake bottom.

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