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30 08 2012 - Meteorite 18(3) - 2012

The Morasko

Meteorite and its Impact Craters



Meteorite August 2012

Discovery


The history of one of the most famous Polish meteorites began a few months after the outbreak of World War I, on November 12, 1914. The meteorite was discovered during the construction of military fortifications in the village of Morasko, near the city of Poznań (Fig.1 A, B). A German soldier, Sergeant Dr. Cobliner, found a mass of iron weighing 77.5 kg buried at a depth of about half a meter.


Today, Cobliner’s historic specimen can be seen in the Museum of the Institute of Geological Sciences in Cracow. The surface of the mass, though covered with rust, still showed visible regmaglypts characteristic of an iron meteorite. The specimen was sent to Berlin, where the iron’s meteoric origin was confirmed. Within a few years following the first discovery, additional meteorites weighing 4.2, 3.5, about 3.5 and about 3 kg were found.

Figure 2: A 560 gram Morasko meteorite specimen found 5 September 2012. Photo by the finder, Andrzej Owczarzak. The specimen is in the Grzegorz Składanek collection


It was not until after WWII, however, that real studies of the Morasko meteorite began. A search for more specimens turned up many masses, both smaller and larger than the previous discoveries. Still more have been found in recent decades (Fig.2). In 2006, K. Socha found the largest block of Morasko meteorite so far discovered, weighing 178 kg. After removal of the rust, the mass of the meteorite fell to 164 kg. The large number of masses that have been found in the Morasko strewnfield suggests a significant fall that rained many meteorites.


The Morasko Craters


Several of the Morasko impactors were large enough to excavate small craters surrounded by raised rims of earth. The first person to suspect the cosmic origin of the seven known impact pits near the village of Morasko was a Polish scientist, J. Pokrzywnicki (1892-1974).

Figure 3: The 2nd largest of the Morasko craters measures 63 meters in diameter and is filled with water hroughout the year. Photo by Tomasz Ogłaza.


The Morasko impact craters are located on a hill called Morasko Mountain. The hill was formed by glaciers during the Vistulian glaciation of the most recent ice age, which began about 115 thousand years ago, as a result of climate cooling in northern Europe. This glaciations event, one of many that make up the story of advancing and retreating glaciers in Europe, covered only the northern part of Poland. The southern edge of the glacial advance in the area of the strewnfield was only few miles south of Morasko. After about 100,000 years, the climate warmed enough that the glacier began to recede. 10,000 years ago, the glacier retreated past Morasko, and since then the ice sheets have not returned. For a long time, scientists did not recognize the meteoric origin of these craters, and believed that they were the result of glacier activity. More recent studies have shown that the glacial retreat constrains the age of crater formation to less than 10,000 years, since they would otherwise have been obliterated by the ice sheet. So far, seven meteorite craters have been identified within the fall area. A possible eighth crater, less than 20 meters in diameter, may exist on the opposite side of the village, but the structure has been nearly completely obliterated by agriculture, making it difficult to determine its origin with any certainty. As a result, the eighth location is not typically included in the list of known Morasko craters. On May 24, 1976, the Morasko strewnfield was established as a nature preserve (Fig.1B) that encompasses the 7 well defined craters within a protected area of 0.54 km2.

Characteristics of The Craters (See Fig.1C for location of craters)


The two largest of the Morasko craters (numbers 1 and 3 in Fig.1C) measure 85-95 m (4657 m2) and 63 m (1195 m2) in diameter, respectively. The walls of the largest slope inward at an angle of about 18 degrees to a depth of 11.5 m, and the 2nd largest, broad and shallow, has a depth of 2.2 m within a rim that slope at an angle of about 20 degrees. Both of the largest craters are permanently filled with water that varies in depth seasonally. The permanent pond in the second largest crater is distinguished by a small island in the center. The smallest of the Morasko craters (#2 on the map) is not dramatically different. It has a diameter of 18-24 m, a depth of 3 m, and covers an area of 284 sq m. The slope of the crater walls is about 21 degrees. This crater Romains dry throughout the year, unlike the second smallest crater (#6), which occasionally holds water in its shallower 2.2 m deep bowl.

This crater has a diameter of 22 to 24 m, covers an area of 415 m2, and has an inward slope of 17 degrees. Between these largest and smallest extremes lie 3 more craters between 30 and 57 m in diameter. #4 on the map contains a permanent pond, is 35 meters in diameter, and is a shallow bowl of only 2.3 m depth, with sharply sloped edges of 23 degrees. #5 is the shallowest, a peat-filled pan that slopes inward at about 6 degrees to a depth of 2.1 m and a diameter of 30-40 m. #7 on the map is the deepest, with steeply sloped (22 degrees) walls to a peat-filled bottom at 12 m, but it is only 30-57 m. in diameter.

The largest strewnfield in the world ?


Morasko is an iron octahedrite, type IAB–MG. Its chemical composition is identical with other Polish meteorites, including Przelazy (or Seeläsgen), irons found at Jankowo Dolne, and with the German iron, Tabarz. Some researchers suggest that these meteorites may come from a single fall, a hypothesis referred to as the Wielkopolska Bolide (Fig.4).

Figure 4: Image of hypothetical strewnfield for the ‘Wielkopolskad Bolide,’ that would explain similarities between the Tabarz, Przelazy (Seeläsgen), and Morasko (incl. Jankowo Dolne) iron meteorites. Modified from: Czajka, http://astroblemy.pl/pdf/tabarz.pdf.


If this hypothesis is ever confirmed, this will be the largest strewnfield in the world, spanning an area of 14 by 22 km. The 102 kg Przelazy meteorite was found before Morasko, in 1847, by a farmer from the Przelazy village (German: Seeläsgen). It was quickly recognized as a meteorite and was taken to Wroclaw, where it was cut into many pieces. Pieces can be found in several collections around the world. In Poland, the largest fragments of the Przelazy meteorite are located in the collections of the Institute of Geology at Adam Mickiewicz University in Poznań (580 g), in the Mineralogical Museum in Wrocław (278 g) and at the Geological Museum of the Jagiellonian University in Krakow (239 g). Verbal reports suggest that a few subsequent specimens of this meteorite may have been found, but precise locations have not been reported.

Mineralogy of the Morasko Meteorite


The Morasko meteorite is composed primarily of iron and nickel, which represent about 99% of the meteorite’s mass. The average content of nickel is 6.75% and ranges from 6.6 to 6.9%. The most important minerals are kamacite, about 90% by volume, and taenite, about 0.5% by volume. Inclusions of other minerals, in the form of nodules comprising about 10% of the volume of the meteorite, are present within the nickel-iron (Fig.5).

Figure 5: A 699 gram specimen of the Morasko meteorite with a hole in it. Photo by Andrzej Owczarzak. pecimen in Grzegorz Składanek collection.


These nodules are made up of troilite-graphite. Troilite is quite common, and is paragenetic (forms in conjunction) with graphite, schreibersyte, cohenite and sphalerite. After polishing and etching, the surface reveals Widmanstätten figures (Fig.6) and Neumann lines.

Figure 6: The etched surface of a 117.9 gram section of a Morasko meteorite shows graphite and troilite nclusions. Photo and collection of Piotr Górski



A Visit by the “Meteorite Men”


In the summer of 2011, the fall area was visited by the television broadcast team of the Discovery Science series, “Meteorite Men.” Geoffrey Notkin and Steve Arnold arrived with a film crew with the intention of making two episodes for the third season of the show while hunting the Morasko and Pultusk meteorites. The result of their exploration in the reserve, near Poznan was to find a Morasko meteorite weighing 34 kg. Analysis was performed by Professor Łukasz Karwowski from the Department of Earth Sciences at the University of Silesia in Sosnowiec. Using scanning electron microscopy, the professor detected chukanovite, which had previously been identified only in the Dronino meteorite.
The authors wish to thank people who helped to create the article: Andrzej Owczarzak, Tomasz Ogłaza and Grzegorz Składanek.

References


-Bartoschweitz, R. and Spettel, B. (2001). Tabarz – A fragment of the Morasko strewfield? Meteoritics & Planetary Science, v. 36, no 9, p. 15 – 16.

- Czajka, W. Obraz hipotetycznej trajektorii Tabarz-Przełazy- Morasko-Jankowo Dolne na geodezyjnej powierzchni odniesienia (The picture of the hypothetic trajectory Tabarz- Przełazy-Morasko-Jankowo Dolne on the Earth’s ellipsoid), http://astroblemy.pl/pdf/tabarz.pdf. (in Polish).

- Karwowski, Ł., Pilski, A. S., Muszyński, A., Arnold, S., Notkin, G., and Gurdziel, A. (2011). New finds in the Morasko mete orite preserve, Poland. Meteorites, v. 1, no 1, p. 21-28.

- Pokrzywnicki, J. (1964). Meteorites of Poland, Studia Geologica Polonica, v. 15, p. 49 – 70. (in Polish).

- Wasson, J.T. and Kallemeyn, G.W. (2002). The IAB iron meteorite complex: A group, five subgroups, numerous grouplets, closely related, mainly formed by crystal segregation in rapidly cooling melts, Geochem. et Cosmochim. Acta, v. 66, no.13, p. 2445 – 2473.

- Pilski, A. S. (2011). The Morasko iron shower – a few scientific and ethical questions, IMCA Insights, http://imca.cc/index. php?option=com_wrapper&Itemid=190

http://www.ptmet.org.pl/
http://www.morasko2.republika.pl/
http://wiki.meteoritica.pl/index.php5/Morasko
http://www.meteoryty.org
About the Authors

-Tomasz Brachaniec is a master of geology at the University of Silesia, Faculty of Earth Sciences, Department of Stratigraphy & Paleontology, Poland. His work focuses on iridium anomalies at the K/T boundary in eastern Poland. Member of the Polish Meteoritical Society. Contact: tribal216@gmail.com

-Adam Broszkiewicz is a member of the Polish Meteoritical Society and has collected meteorites for many years. He is interested in astronomy, ancient art and paleontology.

-Piotr Górski is a member of the Polish Meteoritical Society. He runs a website about meteorites, where his point of view is not necessarily scientific: http://meteoryty.npx.pl.


Brachaniec Tomasz, Broszkiewicz Adam, Górski Piotr, (2012), The Morasko Meteorite and its Impact Craters. Meteorite, 18(3), 2012, s. 41-43
 
Dodane przez gural100 dnia sierpień 30 2012 14:55:18 3509 Czytań · Drukuj
 
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