Deep time — the making of the rock

The peaks standing over the hinterland are the roots of dead volcanoes. The deep red soil beneath the range is their lava, rotted down over ages. The bright sand along the shore is younger again, and arrived a different way entirely. Three origins, three ages, and the gaps between them settled almost everything that would later grow. Deep time is the first force and the slowest, and it did its work long before there was any biology to feel the result.

Start with the number, because the number is the hard part. The volcanoes that built the bones of this region fell silent something like twenty-five million years ago, and twenty-five million years is not a figure the human brain can hold; beyond a few generations the zeros blur and “millions of years ago” lands in the mind as a vague synonym for “ages ago.” The only cure is to shrink it to something the body understands. So squash the whole span into a single twenty-four-hour day. On that scale the volcanoes erupt and die in the first moment after midnight. The entire human presence in Australia, tens of thousands of years, is the last few seconds before the following midnight. Every town, road and farm on the Sunshine Coast is the final thousandth of a second, the time it takes a camera to click. Everything you think of as the history of this place is that click. Everything here is the other twenty-four hours.

Now the fire. Twenty-six or twenty-seven million years ago this stretch of coast was volcanic country, and the reason ranks among the most elegant ideas in geology. The continents drift (slowly, about as fast as your fingernails grow, but relentless), and Australia has been sliding north for tens of millions of years and is sliding still. Fixed in the deep mantle beneath the wandering plate sits a plume of abnormally hot rock, a hotspot that stays put while the continent moves over the top of it. Where the crust passes over the flame it melts and erupts. Then the plate carries that patch onward, the volcanoes die, and fresh ones light up over the hotspot behind. The result is a trail of dead volcanoes laid down the eastern edge of the continent like pastry dragged across a blowtorch, oldest at one end, youngest at the other. The Glass House peaks stand on that trail, on its coastal line rather than on the famous two-thousand-kilometre inland track, the longest continental hotspot scar known on Earth.

The same fire cast two utterly different rocks, and that one split decides most of what follows. The thick, silica-heavy lava froze in place as rhyolite and trachyte, so hard it scarcely shifted while the surrounding country washed away, and that stubborn stuff is what stands on the skyline today as the signature peaks: the Glass House Mountains, Mount Coolum, Mount Tinbeerwah. Mount Ninderry, behind Yandina, is the exception: same hard rhyolite, but Late Triassic, some two hundred million years older than the rest. The thin, silica-poor lava was basalt, which spread out flat and, given rain and enough time, rots to the deep chocolate-red clay of the Blackall Range and Buderim, the richest soil the coast owns. One kind of lava raised the postcard mountains; the other laid down the farmland.

Beneath both fire-rocks lie two quieter, far older layers most people never notice. First, dinosaur-age sandstones: the Landsborough Sandstone that floors much of the low rolling country the towns actually sit on. Older again, the folded Palaeozoic basement, the worn roots of mountains welded together over three hundred million years ago. And then, youngest of all, the coast got a fourth material by water rather than fire: quartz sand, ground from distant coasts and swept north grain by grain, heaped into the dunes of Cooloola whose oldest ranks date to some seven or eight hundred thousand years. Four grounds, stacked in order of age, with an abyss of time between them: some of the most ancient rock in the country sitting within an hour of some of the youngest land in it.

Then the fires went out, and the only force more patient than deep time took over: the weather. Every day for twenty-five million years, rain and river have quietly taken the country to pieces, erasing whole volcanoes and lowering the land by hundreds of metres, until only the toughest cores were left standing as the peaks we climb. So the most permanent-looking things on the horizon are temporary, durable leftovers already halfway through vanishing. Deep time got here first. As that rock crumbles into soil, the living world at last gets its foothold.