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Abstract:
Maar-diatreme volcanoes are small volcanic structures with a conical crater surrounded by a tephra-ring. They are formed by the explosive phreatomagmatic eruptions in a short time when groundwater comes into the contact with magma. We focus on the Bažina maar in the geodynamically active western Eger Rift (Czech Republic) and discuss results from multidisciplinary geophysical investigations calibrated by drilling in the frame of the ICDP Eger project (International Continental Drilling Program). Apart from the internal structure, we show how detailed pre-drill geophysical research contributed to a proper selection of the drill site which concluded in revealing unique structure of a maar volcano with several generations of eruptions. Two boreholes located in the maar crater evinced lithologies of the maar-diatreme filling and its contact with country rocks. They penetrated volcanic rocks and volcaniclastic deposits of the maar but also revealed pyroclastic cone with the olivine nephelinite feeding conduit. Such results are unique and disclose that the maar formation was followed by several subsequent eruptions within the maar. Further investigationsconfirmed long-lasting volcanic activity with very primitive magma and extremely low magma productivity, which distinguishes the western Eger Rift from other Cenozoic volcanic provinces in Europe. These results improve our understanding of volcanic processes, their significance is also general because they shed light on unique evolution of two volcanoes, one inside the other.
