The chemistry of decay
The striking images of Buddhas and Bodhisattvas at Tamgaly Tas, carved into dark sandstone cliffs along the Ili River, are steadily being erased by chemical reactions. The primary agent is water, which facilitates a suite of destructive processes. Rainwater absorbs carbon dioxide from the atmosphere to form a weak solution of carbonic acid. This acid attacks the mineral cement holding the sandstone's grains together, slowly dissolving the rock and blurring the petroglyphs' sharp lines.
This process is compounded by hydrolysis, where water molecules directly react with minerals within the sandstone, particularly feldspars. The reaction converts these relatively hard minerals into softer, weaker clay minerals. This fundamentally alters the rock's composition, causing it to lose integrity and crumble. Oxidation, another chemical process, affects iron-bearing minerals in the stone. Exposure to air and water causes these minerals to rust, expanding and creating internal stresses that fracture the rock from within. These combined chemical attacks relentlessly weaken the stone, making it susceptible to physical forces.
The physical assault
The climate of the Almaty region directly affects the physical destruction of the petroglyphs. The area experiences significant temperature fluctuations, both daily and seasonally. This leads to a powerful mechanical weathering process called freeze-thaw or frost shattering. Water from rain or melting snow seeps into microscopic cracks in the acid-weakened sandstone. During the cold winters, when temperatures drop well below freezing, this water freezes and expands by approximately 9%. The immense pressure exerted by the expanding ice acts like a wedge, widening existing cracks and creating new ones. Repeated cycles of freezing and thawing progressively break the rock apart.
The site also endures thermal stress, where the intense daytime sun heats the outer surface of the rock, causing it to expand, while the interior remains cooler. At night, the surface cools and contracts. This constant cycle of expansion and contraction fatigues the outer layers of the sandstone, causing them to flake off in a process called exfoliation.
Biological agents also contribute to the degradation. Lichens, which are common on rock surfaces, are particularly destructive. Their thread-like hyphae penetrate the rock's pores, physically prying grains apart. Lichens also produce various acids, including oxalic acid, as a byproduct of their metabolism. These organic acids are potent chemical weathering agents, dissolving minerals and accelerating the overall decay of the ancient artwork.