High Island Hexagonal Rock Columns

Although it may not be obvious from its terrain, Hong Kong actually sits on the site of an ancient supervolcano from 140 million years ago. Although remnants of that volcano are found almost all around in the form of igneous rocks, the most visible reminders are the hexagonal rock columns found on High Island and the neighboring Sai Kung Peninsula. These are at the eastern edge of the territory in the UNESCO Global Geopark. Reminiscent of the Giant's Causeway in Northern Ireland, these volcanic tuffs are in a way more impressive in their size and scale, ranging up to 3 meters in diameter and 30 meters in length. At 100 square kilometers in area, some sources call them the largest area of hexagonal rock formations in the world.

Such bizarre, seemingly artificial structures formed from the rhyolitic ash tuff left after the supereruption. (This is in contrast to the lava that formed the Giant's Causeway.) The ash tended to cool at a series of evenly spaced centers. That in turn contracted the nearby tuff, forming a series of hexagonal cracks that then propagated downwards.