Thursday 11 August 2016

Bubble-like caves in the Laoshan Granite of Shandong Province.

Large bodies of ice are known to have covered wide ares of Europe and North America during the Pleistocene galciations, however evidence for similar glaciation in China has been harder to find, which is puzzling as it is difficult to explain how China wouldhave excaped such a glaciation. One piece of evidence that has been used to support the idea of glaciation in northern China is the presence of a number of small cave systems in the Laoshan Granite, which outcrops around Qingdao in Shandong Province in northeast China. Granite is a volcaninc rock formed by the slow cooling of magmatic material deep beneath the ground, which enables large, visible, mineral grains to form. These larger grains are more prone to weathering by freeze-thaw conditions than the smaller mineral grains that make up extruded lavas and basalts, thus enhanced erosion of granites can be evidence of freezing conditions in the past.

In a paper published in the journal Acta Geologica Sinica in February 2016, Song Zaojun of the Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals at the Shandong University of Science and Technology, Lui Xiqing of the Qindao Institute of Marine Geology, Tang Wenjia, Gao Luo and Li Jianping, also of the Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals at the Shandong University of Science and Technology, and Luan Shaogang, of the Qingdao Laoshan Scenic Area Adminstrative Bureau, re-examine the caves of the Laoshan Granite, and propose an alternative hypothesis for their formation.

Song et al. note that the Laoshan Granite contains a number of small caves, with bubble-like shapes, averaging 1-2 m across, with the largest measuring 3.1 m in diameter. However they also note that the Laoshan Granite is mariolitic in nature; that is to say it contains, as well as the larger caves, smaller bubble-structures (vesicles) believed to have formed by pockets of gas evolved by the cooling magmatic rock which were unable to excape the burried granite. Such vesicles are common in granites, often cotaining larger crystals formed later from minerals disolved in water that percolated through the rocks (geodes). 

Bubble cave north of Laoshan Reservoir. Song et al. (2016).

Mairiolitic caves as large as the Laoshan Granite structures have never previously been recorded. Nevertheless Song et al. conclude that, based upon the morphology of the caves, a mariolitic origin is more likely that a glacial erosion based one.

Large cave near Tiaqinggong. Song et al. (2016).

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