Naslov
Order and disorder in columnar joints of Rupnica Geosite (Papuk Geopark)

Autori
Balen, Dražen ; Radonić, Goran ; Pavić, Goran

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Knjiga sažetaka / Abstracts Book / Horvat, Marija - Zagreb : Hrvatski geološki institut, 2010, 137-138

ISBN
978-953-6907-23-6

Skup
4. Hrvatski geološki kongres s međunarodnim sudjelovanjem

Mjesto i datum
Šibenik, Hrvatska, 14.10.2010. - 15.10.2010

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
columnar joints; rhyolite; geopark; Rupnica; Papuk

Sažetak
The Rupnica geosite is one of the key-localities of the Papuk Geopark. It is under protection as a Geological Monument of Nature since 1948. and is famous due to a characteristic exposure of the columnar joints developed in the albite rhyolite. Locality is a part of a larger volcanic body known also as the Voćin volcanic mass. K-Ar dating of volcanic rocks (interstratified basalts) near the locality gave Cretaceous age while the alternative Miocene age has been inferred base on broader geological/stratigraphic relations. Position of the Rupnica geosite inside the volcanic body, its depth i.e. pressure of solidification and cooling rate, which has a significant bearing on the development of columnar joints, has not been estimated in detail so far. Fundamentally, simple and universal laws of physics govern the behavior of cracks and development of columnar joints in volcanic rocks, but also in analogous materials like mud, ice and starch slurry. Simplified, the fast cooling rate of magmatic materials near or close to surface conditions will cause the formation of cracks which will reorganize themselves in the form of “T-junctions”, producing mostly four-sided columns, while on greater depths and with slow cooling rates cracks will propagate into depth, reorganizing themselves into “Y-junctions”, in ideal case giving six-sided columns. Statistical analysis performed on the Rupnica columnar jointing (Table 1) shows the development and reorganization of propagating joints and formation of n-sided columns. The values are expressed as n-gonality (hexagonality, pentagonality, ... ) indices, which represent a measure of discrepancy in the number of joints between the real sample and an ideal (theoretical) case. Statistically significant are those results approaching value 1. Differences in the total number of counted columns in 2008 (300-400) and 2009 (450-550) are due to mud avalanche (landslide) which covered a significant part of the locality. The outcrop cleaning using pumping water „cannon“ (fall 2008) made it possible to access and counts a greater number of columns in 2009. Four-sided (0.98) and five-sided (0.88) columns show the greatest resemblance to the idealized case. Such results are in good agreement with results expected for shallow placed and relatively rapidly cooled volcanic mass. If such measurements are correlated with other features visible at or near the Rupnica locality, like relatively narrow columns, tightly packed striae (propagating cracks inside columns), curved sets of columns and occurrence of pyroclastic material in the vicinity (Omerovac site), than we can depict the Rupnica columnar jointing as the upper part (entablature) of the volcanic body. Consequently, order/disorder analysis of cracking pattern responsible for the origin of columnar joints at the Rupnica geosite shows that this genetic process is related to a shallow, i.e. low-pressure, fast cooling environment.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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