The Glass House Mountains

On a still morning the Glass House Mountains float blue and serene on the inland horizon, and nothing about the scene suggests violence. They look about as old and settled as country can get, which, in a sense, they now are. But those calm blue peaks are the cooled stumps of volcanoes. Somewhere between twenty-six and twenty-seven million years ago this stretch of coast sat over a furnace and burned, and what you climb today is the wreckage, whittled into its skyline shapes across a span of time too long to picture. When you look at the peaks you are not looking at the volcanoes at all. You are looking at their frozen plumbing: the hard cores laid bare long after the volcanoes themselves rotted away.

The trick lies in the lava. Thick, silica-rich lava is too stiff to flow. It clogged the throats of the old volcanoes and set there as rhyolite and trachyte, some of the hardest, least-fractured rock in the region. Each of these sheer towers is the choked neck of its own volcano, which is why they stand scattered rather than lined up in a single ridge. And here is the part that sounds wrong until you sit with it: the volcanoes did not build the Glass House Mountains. Erosion did. The peaks were never heaved into the sky. The sky stayed roughly where it was, and everything around them was worn down instead, hundreds of metres of softer country stripped back and carried grain by grain out to sea, until the hard plugs were left marooned, standing sheer and improbable off the flat plain. Tibrogargan and Beerwah stand tall today because the country beside them, once as tall but softer, has gone. Geologists call the shape a volcanic neck — a stump too hard to rot.

Look closely at the north face of Beerwah and you can read the rock’s very first day. The tall trachyte columns there, aptly called the Organ Pipes, are cooling-contraction joints, cracks the lava made as it shrank and hardened, the way drying mud cracks, dividing itself into neat polygonal pillars. It is a small marvel of physics, molten rock spontaneously splitting into columns; it is also, over the following millions of years, a set of ready-made doorways for the weather. Even the way a rock will one day come apart was, in some measure, decided at the moment it was born.

That hard, unbroken rock makes hungry country. Climb one of these peaks and you pass, in an afternoon, from open eucalypt forest on the thin, dry soil of the slopes, up through a low montane heath of hardy plants gripping the bare stone near the top, while a pocket of rainforest waits in the damp of a gully below. Substrate and water, not rainfall, decide what grows where, the same rule that orders the whole coast, drawn steep and clear on one plug. And because so much of it is fire-shaped forest and heath, the same patchwork of burning applies here as everywhere else: intervals frequent on the heath, rare in the gully, keeping the fire-hungry and the fire-shy in one landscape.

The peaks throw in a bonus. They carry a date. They belong to a chain of old volcanoes strung down eastern Australia, printed one after another as the plate crept north over a stationary plume of mantle heat, the same slow machinery that raised dead volcanoes the length of the seaboard. The Glass House cluster is the boldest of the coast’s plugs, and the region’s signature landform.

A note on the name, kept to the public record: James Cook, sailing past on 17 May 1770, thought the abrupt peaks resembled the glass furnaces, the “glass houses” of his day, and set the name on his chart. It is a curiously apt piece of accidental geology, a man naming volcanic cores for furnaces with no idea that fire was exactly what had made them. The peaks had been named and known for many thousands of years before that, and carry Kabi Kabi names and meanings that are not this book’s to relate.

Read the shape first. A sheer, lonely, sharp-summited peak on this coast is a volcanic plug, and the rest of the hill reads itself. Give erosion a few more million years, though, and even these hard hearts will follow the volcanoes that made them.