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Popular Science: When volcanic ash flew over the Bohemian Massif

Czechs know huge volcano eruptions primarily from disaster movies, but definitely not from their own country. Yet, even here, history was quite explosive. One of the geological mysteries of the Bohemian Massif is a widespread layer of ash fall tuff, called the “bělka” in Czech, preserved in the carboniferous basins of Central and Western Bohemia. But, how did it get there, when exactly, and what kind of volcano could have caused this? A team of Czech specialists from Charles University, the Czech Academy of Sciences, and the Czech Geological Survey decided to finally unravel this mystery, led by Filip Tomek from the Institute of Geology and Paleontology.

Carboniferous basins in the Bohemian Massif were formed in the final stages of the relaxation of the Variscan orogen (around 320–300 million years ago = ~320–300 Ma). They were filled with various sedimentary material from the surrounding mountains and, from time to time, they were also intercalated by volcanic layers. One of them is the bělka tuff, a layer of fallen volcanic ash deposits present in many coal-bearing sequences and bore holes across a distance of 100 km. Its presence makes it a quite unique chronostratigraphic marker of these late-Variscan basins. However, the problem with the bělka tuff was, where to set its episode in the big picture of the complex Bohemian Massif history. The reasons for that are obvious, as various erosion and younger sedimentary cover obscured the continuity of the layer. Also, further deformation and chemical alteration may have modified the rocks and the older the deposits are, the less accurate their radiometric dating is. Those are regular difficulties stratigraphers and sedimentologists usually struggle with.

Schematic geological map of the Altenberg-Teplice Caldera (modified from the original article).

Another big question concerning the bělka tuff was: where exactly did it all come from? One large volcano (with impressive dimensions of 35 km × 18 km) similar to the bělka tuff in age is situated on the border between Czechia and Germany and is known as the Altenberg-Teplice Caldera. Caldera is a type of volcanic structure that forms when a significant amount of magma is erupted, and the overlying surface can lose its stability and collapse into an emptied magma chamber. Calderas and their origins are often associated with huge and dangerous eruptions (some of the most destructive examples include Yellowstone in the USA and Toba in Indonesia). The elliptical shape of the Altenberg-Teplice Caldera is well delineated in present day erosional level by porphyritic microgranite ring dikes, which intruded along the caldera bounding faults during the caldera floor collapse.

However, the radiometric dating of the Altenberg-Teplice Caldera rocks was not constrained precisely enough. Hence the scientific team conducted extensive research into the previously published geochronology and also introduced new U/Pb dating of zircon. All this was done in order to better constrain and compare the absolute ages of both the caldera and the bělka tuff horizon. In addition, the team also focused on variations in the bělka tuff thickness and grain size across the carboniferous basins and its minimum volume estimates.

Reconstruction of the bělka tuff thickness isolines (original article).

The results showed that the entire period of volcanic activity of the Altenberg-Teplice Caldera was within the range of ~318–313 Ma, and the bělka tuff fits in the period of the principal caldera-forming activity at ~314 Ma. Based on the grain size and thickness distribution, a reconstruction reveals that a tuff layer 10 cm thick could have been found at distances of up to 220 km from the caldera near the city of Regensburg (wind was probably blowing to the southwest). Deposits of pyroclastic density currents (a rapidly moving mixture of pyroclastic material and gases) could have reached as far as 50 km from the caldera center. With the minimum total volume of erupted material of more than 100 km3, this major eruption reached the 7th degree (of maximum 8) of the volcanic explosivity index (VEI). The total erupted material of the whole Altenberg-Teplice Caldera volcanic activity is estimated to be >350 km3. It can thus be considered to be a moderately big caldera such as other calderas around the world, for example the Valles Caldera in New Mexico.

Filip Tomek, Stanislav Opluštil, Martin Svojtka, Václav Špillar, Vladislav Rapprich & Jitka Míková (2021) Altenberg-Teplice Caldera sourced Westphalian fall tuffs in the central and western Bohemian Carboniferous basins (eastern Variscan belt), International Geology Review, DOI: 10.1080/00206814.2020.1858357

Petr Vitouš


Published: Jul 19, 2021 04:50 PM

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