Weathering — how rock becomes soil

Somewhere on a dry ridge you come across a heap of great rounded boulders, as though a giant piled them there and wandered off. Nobody did. They are corestones, the last unrotted hearts of what was once a single mass of solid rock, and they hold the whole story of weathering, caught in the act. Water crept into the rock along its cracks and rotted each block inward from every side until only a sound, rounded kernel remained, sitting in a matrix of its own decay; wash the soft matrix away, as rain and slope eventually do, and the cores tumble together at the surface, stone spheres that were never round when they formed. Every such pile is a working diagram of how rock becomes soil.

Because that is what weathering does. It takes rock to pieces and rebuilds the pieces, chemically, into something new: the slow everyday work of rain and air on every exposed surface. It comes in two kinds, working together.

The first is physical, the breaking-apart of rock along its weaknesses. Rain does not attack a rock evenly, all over at once. It gets in wherever there is a way in, along the joints and fractures that split almost every body of rock, some of them cracks that formed the moment the lava cooled and shrank. A rock laced with joints is riddled with doorways and crumbles from the inside out. A rock with hardly any, dense and unfractured, cooled slow and whole deep in a volcanic throat, offers the weather almost no opening, and simply endures. This is why the plugs stand. The sheer peaks of the Glass House Mountains, Mount Coolum, Mount Ninderry were never pushed up; they are rock that refused to rot while everything softer around them was carried off to sea. The corestone on the ridge and the mountain on the horizon are the same principle at two sizes: the weather goes at the cracks first, and the sound rock last.

The second kind is chemical, and it does the real rebuilding. Where physical breakage opens the rock, chemistry rots the exposed minerals into new ones, mostly clays, dissolving what will not stay put and letting the water carry it away. In the warm, wet subtropics both processes run hard and deep, which is why weathering here reaches down metres, sometimes tens of metres, into the ground.

The parent rock sets the product. Feed the same rain to two different rocks and you get two utterly different grounds, because each rock has different stuff to weather into. Basalt, rich and mineral-crowded, rots down under high rainfall into deep, water-holding, chocolate-red clay — the most fertile soil on the coast, and the reason rainforest and orchards sit on the ranges. Quartz sand is the opposite: almost pure silica, chemically dull, with next to nothing in it to begin with and no way to hold onto what little arrives. There is nothing to weather it into. So rain simply rinses it, year after year, stripping the iron coatings from the grains until they bleach bone-white over a buried, coffee-coloured hardpan. That is a podzol.

And one loss, above all, is permanent. Nitrogen a soil can rebuild from the air; phosphorus comes only from rock and is never resupplied, so as weathering grinds on it is leached below the roots and locked away for good. Measured along the Cooloola dunes, the soil’s total phosphorus falls by something like ninety per cent from the youngest sand to the oldest. Weathering sets that decline running.

This is what happens on the day geology hands over to biology. Basalt’s weathering builds wealth. Sand’s builds poverty. That one divergence, turning on nothing grander than what each rock was made of, is the raw material the whole reef-to-range gradient gets sorted out of. Every soil on the Sunshine Coast, and very nearly everything that grows on it, is a record of which rock met the weather, and how it came apart.