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.
How to Make a Granite Sphere
Spheroidal weathering is the geomorphic process responsible for the round boulders found throughout the Rokko Mountains. It begins underground, where water seeps into the jointed, rectangular blocks of granite. Weathering attacks all exposed surfaces, but it works fastest on the corners, which have the most surface area. Edges weather more slowly than corners, and the flat faces of the block weather the slowest of all.
Over immense periods, this differential rate of decay rounds the angular blocks, much like a tumbling machine polishes stones. The result is a relatively untouched spherical core of solid granite, known as a corestone, surrounded by the soft, crumbly masado saprolite. These corestones, which can range from 0.4 to 1.5 meters in diameter or larger, are often only exposed after the surrounding decayed material erodes away, leaving them scattered across the landscape scattered across the landscape. The entire process happens below the surface, meaning the boulders are "born rounded" before they are ever exposed to the sky.