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About Polana Stratovolcano
(CSTS Web Document Library)
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THE ORIGIN AND POLYGENETIC STRUCTURE OF POLANA
STRATOVOLCANO
(CSTS Document Library)
Mount Poľana is relatively well-preserved
stratovolcano when compared with several older and larger, but less preserved
similar stratovolcanos on the Slovak territory. All of them are andesitic, i. e.
featuring intermediate chemical composition.
Poľana is a polygenetic stratovolcano. Its
pre-volcanic basement is built by thrusted pre - Cainozoic (pre - Late -
Cretaceous) rock units. Poľana itself dates back to the Miocene age (Early and
Middle Sarmatian: 13,6 - 12,25 Ma).
The original altitude of Poľana volcano was
about 2500 m a. s. l. At least 1000 m were eroded and the current altitude of Poľana's
summit is 1458,7 m a. s. l.
In the stratovolcano evolution several stages
were distinguished:
- The oldest stage was the formation of the older volcanic cone, the Šútovka
stratovolcano (13,58 - 13,32 Ma BC). The stratovolcano was built by the
andesite lava flows alternated by layers of pyroclastic rocks.
- At the end of Šútovka stratovolcano formation, approximately 13,32 Ma
ago, that stratovolcano was mostly destroyed and a caldera developed.
- In the-time interval 13,32 – 13,03 Ma, i.e. during approximately 300 000
years the remnants of Šutovka volcano were deeply eroded.
- After a long break, approx. at 13,03 Ma powerful Plinian explosion
finished the Šutovka Volcano destruction. The paroximatic explosion was
followed by explosive volcanic activity producing the rhyodacite tuffs
deposited on the slopes of the Šutovka remnant rone and on surrounding
area. The Plinian explosions were accompanied by Peleean explosion of the
red-hot clouds (nues ardentes).
- The explosions of the rhyodacite tuffs caused the origin of a small
caldera named Bobrovo (Krakatoa type). In the area of newly formed caldera
the explosive breccias, tuffs of phreatomagmatic and phreatic explosions,
the lava flows, extrusions and dykes of the rhyodacite composition
accompanied by hydrothermal activity originated. This acide infill of
Bobrovo Caldera came to existence in relatively short time approx. 10 000
years.
- During the following time – interval approx. From 12,74 to 12,35 Ma the
new daughter stratovolcano – Veľka Detva started to rise, i.e. the upper
Poľana Stratovolcano cone originated. The lower part of Veľka Detva cone
is built by andesite agglomerates, rarely by lava flows and epiclastic
rocks. The upper part of the cone is mostly built by lava flows and in the
centre of volcano there are diorite and andesite - porphyric intrusions.
- The volcanic activity of Poľana Stratavolcano definitively stopped at the
end of the Middle Sarmatian 12,35 Ma BP. The following erosion lowered the
volcanic cone and made larger the crater on the volcano summit. The recent
caldera – like depression in the stratovolcano central part is not a real
explosive caldera but a false erosive one.
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