The Basalt Stairs: The Monaro Volcanics

High on the western shoulder of the Great Dividing Range, the earth is paved with the black, hexagonal bones of a forgotten fire. These are the Monaro Plains of New South Wales, where a vast sea of basaltic lava once flooded the landscape, cooling into a high-altitude plateau that today remains one of the coldest and most austere environments in Australia.

The Flood of the Monaro

Between 34 and 55 million years ago, during the Eocene and Oligocene epochs, the ground beneath southeast Australia began to pull and fracture. This was not the explosive volcanism of a single peak, but a series of "fissure eruptions" where the crust split open to weep molten rock. The resulting Monaro Volcanics covered an area of roughly 4,200 square kilometers, burying the older Paleozoic basement rocks under layers of fluid basalt.

Unlike the steep cones of the Glass House Mountains to the north, the Monaro flow created a trap—a Swedish word for "stairs"—where successive pulses of lava stacked atop one another. Each layer represents a distinct event, a moment when the earth’s interior spilled out to level the topography. Today, these layers are visible in the stepped hillsides near Cooma and Nimmitabel, where the grass-covered rises reveal the rhythmic pulse of ancient eruptions.

The Architecture of Cooling

When basaltic lava flows are thick and cool slowly, they shrink and crack with mathematical precision. This process, known as columnar jointing, creates vertical prisms that look less like geology and more like masonry. In the Monaro, these columns are often hidden just beneath the soil, but where creeks have cut deep or roadworks have bitten into the hills, the geometry is revealed.

The basalt does not merely sit upon the land; it dictates the life that can exist there, holding the frost in its dense mass and yielding a soil that is as rich as it is stubborn.

The mineralogy of this rock is dominated by olivine and pyroxene, crystals that formed under intense heat and pressure. As these minerals break down, they release calcium, magnesium, and iron into the soil. This creates the "Black Soils" of the Monaro—fertile, heavy clays that support the iconic treeless grasslands. While the surrounding granite country grows thin forests of eucalypt, the basalt plains remain open, a high-veld of tussock grass and wind.

A Landscape of Resistance

The Monaro represents a transition in the tectonic story of Australia. As the continent drifted north away from Antarctica, the stretching of the crust allowed these deep-seated magmas to reach the surface. This volcanism was a herald of the continent's isolation, a thermal signature of a landmass finding its own path across the mantle.

Today, the basalt acts as a protective cap. It is harder than the surrounding sedimentary rocks, resisting the wind and the biting sleet of the Southern Highlands. The plateau sits at an average elevation of 800 to 1,000 meters, a high island of volcanic stone that catches the first snows of winter. The rivers here, like the Snowy and the Murrumbidgee, have had to work for millions of years to carve through this dark crust, leaving behind the deep, rocky gorges that define the region's rugged periphery.

The Monaro is a place of heavy stillness. To stand on its plains is to stand on the surface of a solidified lake of fire, now cold, silent, and draped in the pale gold of winter grass.