The Geology of Rot
The Rokko mountain range is built from a body of granite that formed during the Late Cretaceous period, approximately 70 to 78 million years ago. This rock, a massive pluton, cooled from magma deep beneath the Earth's surface. Tectonic activity associated with fault lines, such as the Rokko-Awaji fault zone, has been uplifting this granite block for millions of years. This uplift, which continues at a rate of about 0.5 millimeters per year, fractured the solid granite, creating a network of intersecting joints and cracks.
These fractures are the starting point for the granite's decay. The rock itself is primarily composed of minerals like quartz, feldspar, and biotite mica. When groundwater penetrates the network of joints, a slow chemical reaction begins. This process, a form of chemical weathering called hydrolysis, targets the feldspar and biotite, converting them into softer clay minerals. The quartz is more resistant and remains as sand grains. This slow, subsurface decay transforms solid granite into a loose, sandy, decomposed material known locally as masado.