An extensive, low hill on the southern edge of the village of Mikołajowice in the Legnickie Pole municipality reveals one of the most intriguing geological mysteries at the foothills of the Sudetes. At the hillside, in the currently inactive open-pit mine, valuable rock material – basalt, was extracted, which proved significant for various industrial sectors. The dark-colored rock, visually unappealing, represents the record of one of the most spectacular and tumultuous geological processes—volcanism.
Basalts are the result of solidified lava emerging from volcanoes, and their source is the magma located at significant depths within the Earth. It is believed that the majority of basaltic lavas in Lower Silesia were associated with magmas formed in the upper mantle of the Earth, at depths of approximately 75 – 90 km, while a smaller portion is linked to magmas formed at depths of 30 – 45 km. In Lower Silesia, 314 occurrences of basaltic rocks have been identified so far, in the form of veins within other rocks, lava streams, and volcanic chimneys. Horizontally lying basaltic lava covers can reach thicknesses of up to 100 metres. In the terrain, these are marked by flat hills. On the other hand, volcanic chimneys form the highest elevations in the region – they are remnants of eroded volcanic cones.
The basalts occurring in the foothills of the Sudetes represent the so-called basalt formation of Lower Silesia, which is part of the Central European volcanic province, stretching approximately 700 km from the Moravian Gate in the east to the Rhine Valley in the west. Based on the dating of minerals from basalts (using the decay of the radioactive isotope potassium-40K to the stable isotope argon-40Ar), it has been determined that the basaltic lava eruptions occurred in two time intervals: 33 – 18 million years ago (corresponding to the geological epochs of Eocene/Oligocene to early Miocene) and 5 – 4 million years ago (Pliocene). In these two time intervals, the landscape of the Kaczawskie Mountains and Foothills regions was heavily dominated by the presence of volcanoes and shaped by associated phenomena, including lava flows and eruptions of pyroclastic material. The isotopic age of the basalt from Mikołajowice has been determined to be nearly 31 million years, indicating that it originated during the first of the two aforementioned periods of volcanic activity.
The basalt exposed in the quarry is a rock with a black colouration, a characteristic derived from its mineral composition. In most cases, they exhibit an aphanitic structure (without visible crystals); however, locally they also show a porphyritic structure, where larger crystals, such as green olivines, are present in the black so-called groundmass.
In basalt, well-developed columns can be observed, which are the result of thermal shock, i.e., fractures that develop during the cooling stage of the lava. These columns exhibit a high degree of regularity in their forms; in cross-section, they take the shape of polygons, typically hexagons or pentagons. The thermal joint forms in accordance with the principle that the longer axes of the developing columns are perpendicular to the cooling surface of the lava. The presence of vertical columns in the discussed quarry indicates that the observed basalt is a solidified lava flow that cooled from the surface. Further evidence of this is the extensive, flat shape of the hill on which the quarry is located. The regular structure of the columns, in turn, reveals that we are dealing with the central part of the lava flow, not the peripheral part in contact with older rocks.
The basalts are subject to weathering processes, and the effects of these processes are clearly visible on the walls. Erosive destruction primarily proceeds along the columnar joints, gradually ‘attacking’ the core of the columns as well. The degree of weathering of basaltic rocks is significantly greater in near-surface areas and decreases towards the depth.
In addition, near the surface in several places within the quarry, glacial sands and gravels can be observed—rocks formed as a result of glacial activity in this area during the Pleistocene period (2.5 million – 10,000 years ago). At that time, the ice sheet directly covered the basalt rocks, and the waters flowing from it deposited sands and gravels on them.