A Pliocene river of stone
On the eastern bank of the Irrawaddy River, near the town of Singu, a wall of dark, geometric pillars rises from the landscape. These are the Singu Basalt Columns, the eroded remains of a volcanic eruption from the Pliocene Epoch. The columns are the product of a geological process called columnar jointing. When a thick flow of basaltic lava cools, it contracts horizontally. This contraction creates tensile stress, which is relieved by the formation of cracks. As the lava cools uniformly, these cracks propagate downwards from the surface, forming a remarkably regular network of polygonal, often hexagonal, columns.
The lava that formed these columns originated from the Singu Plateau, also known as Letha Taung, a volcanic field covering approximately 100 square kilometers. This eruption occurred during the Pliocene, between 5.3 and 2.6 million years ago, pouring a sheet of fluid basalt over the existing Mio-Pliocene sandstones of the Irrawaddy Formation. The Singu lavas are classified as basaltic trachyandesite and alkali basalt. Their chemical composition shows they are intraplate basalts, meaning they formed far from the tectonic collision zones that typically produce volcanic activity. Instead, the eruption likely occurred along fissures associated with the active Sagaing Fault, a major transform boundary that runs under the region.
A thermal history in stone
The size and shape of basalt columns are a physical record of the lava flow's cooling history. The diameter of a column is inversely proportional to the cooling rate—the faster the lava cools, the smaller the diameter of the columns. The relatively large columns at Singu suggest a slow, steady cooling process. Analysis of the column diameters here allows geologists to calculate the original thickness of the lava flow, which exceeded 4 meters.
The lava that formed the Singu columns has a porphyritic texture, meaning it contains larger crystals (phenocrysts) embedded in a finer-grained groundmass. The primary mineral constituents are plagioclase and pyroxene, with smaller amounts of olivine and iron-titanium oxides. The presence of vesicles, or gas bubbles, indicates the lava was gaseous when it erupted. Some of these vesicles are now filled with secondary minerals like calcite. In some locations, the rock is amygdaloidal, where these vesicles are completely filled with these deposited minerals. The remarkable freshness of the rock makes it look like the lava flow could have cooled recently, rather than millions of years ago.