Volcanism of st. Anna mountain

In the Tertiary period, in the turn of Palaeogene and Neogen (27 -15 million years ago), throughout the Silesia hundreds of volcanoes were created as a result of the global, so-called Alpine orogenetic movements (in Europe initialized by pushing the African tectonic plate by the European), which caused stresses and cracks inside the Earth, and the change of pressure resulted in liquidation of deep rock layers and lifting of the magma flowing inside the cracks towards the surface. Within today's St. Anne Mountain area the volcano was created 27 million years ago (end of the early Tertiary - Palaeogene), it was proved by radiometric dating of local Palaeogene basalts. Over this period Triassic rocks were covered with thick, measuring a few meters, layer of Cretaceous rocks, sandstones and marls which formed the base for the new volcano. Magma was created in the process of rock liquidation at the depth of approximately 50 km, then it migrated up to the magma chamber, located shallowly under the volcano, from where it was delivered in the form of lava, to the crater. Outflows of lava were accompanied substantial ejections of pyroclastic material: tuffs and breccias (containing large pyroclasts, e.g. volcanic bombs and sharp-edge pieces of basalt taken out by an explosion from the volcano walls). The volcano cone thus consisted of alternate layers of lava and tuff, just like Vesuvius. After thousands of years of activity, the rapid paroxysm thrown out the rest of the smooth magma from the magma chamber on the surface of the volcano, and the empty chamber collapsed under the pressure of the surrounding rocks. As a consequence, a considerable part of the volcano, standing over the chamber collapsed and the caldera was created. During the disastrous collapse, large blocks of basalt, tuff, but also sedimentary rocks (Triassic and Cretaceous) were detached from the rocks on which the volcano was located, moved a few dozen meters inland, below the ground level. After this explosion, the volcanism was slowly disappearing and the remains of the conical shape along with the caldera started gradually eroding (by airing, maybe aided by the activity of streams etc.). For some time afterwards, hot solutions being the remnants of volcanic activity circulated around the rocks. Sometimes they reacted with surrounding rocks (this is how the jasper of St. Anne mountain was created), on other occasions they helped to crystallize minerals. After a few million years the whole volcanic mountain was destroyed and after subsequent several million years the progressive erosion removed also the cover of Cretaceous rocks, uncovering the Triassic rocks which are slowly eroding until today. Hence today, although when looking at the Franciscan monastery, we see it standing on the top of the steep mountain, resembling volcanic cone and is made of Palaeogene basalts, it is absolutely not a real volcanic mountain, because millions of years ago the mountain was removed along with thick sedimentary rocks on which it was standing. Today we can see only the fragment of volcanic chimney, namely the canal, filled with frozen lava, linking the former with the chamber inside the Earth. Its conical form is an effect of erosion. Basalt was significantly more resistant to the erosion than surrounding limestones and volcanic tuffs, so the surrounding rocks eroded more quickly than the harder chimney shaped in the form of the hill. Such forms of are called monadnocks. The question arises: why to the south of the chimney, in an old quarry blocks of Cretaceous rocks, tuffs and lava breccias of the former cone are preserved, even though the cone and chalk itself were destroyed? These were simply large parts of the former volcano which at the time when it was collapsing, slid down deeply under the surface, and thus only now were exposed by the erosion. 

 

 

Information comes from: "Zanim Góra Św. Anny wynurzyła się z morza" ("Before St. Anne Mountain emerged from the sea"). "Skamieniałości, jaskinie i drogie kamienie wokół sanktuarium św. Anny" (Fossils, caves and valuable stones around the St. Anne sanctuary)

Robert Niedźwiedzki, Marek Zarankiewicz

The selected bibliography (only Polish)

* Thanks to the courtesy of Gazeta Wyborcza, Opole, the fragments of text by Robert Niedźwiedzki, published in Gazeta Wyborcza 3 XI and 30 XII 2006 were used in this study. Also the archaeological consultation provided by Dr Andrzej Wiśniewski from the University of Wrocław was used here.

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