The Magma That Forged a World of Green and Black: Tasmania's Serpentine Hills

In northwest Tasmania, near the town of Savage River, the hills are a toxic green. The soil there contains so much nickel and chromium that most plants cannot grow. What survives is stunted, sparse, and distinctly Australian — a flora adapted to ground that would poison almost anything else.

This landscape is made of serpentinite, a rock forged not on the surface but in the mantle of the Earth, 25 kilometres down. It was brought to light by the same tectonic forces that shattered Gondwana.

The Skin of an Ancient Ocean Floor

Serpentinite begins as peridotite, the dark, dense rock that forms the upper mantle. When that rock encounters seawater — usually where the ocean floor spreads apart at mid-ocean ridges — it undergoes a chemical transformation called serpentinisation. Olivine and pyroxene react with water to become serpentine minerals: antigorite, lizardite, chrysotile. The rock swells, cracks, and turns a waxy green-black, veined with white.

The serpentinite of northwest Tasmania is around 500 million years old. It was once part of the seafloor of the Cambrian ocean, a body of water that has since vanished completely. That ocean floor was pushed onto the edge of the ancient Australian continent during a period of subduction and collision, a process geologists call obduction. A slice of the mantle, hundreds of square kilometres, was ramped up and emplaced onto what is now Tasmania.

Today, these serpentinite bodies outcrop in a rough belt from the Arthur River area in the north down to the Pieman River in the south. The largest exposure covers about 50 square kilometres around the Savage River valley.

A Chemistry That Selects What Lives

Serpentinite soils are among the most hostile on Earth. They are rich in nickel, chromium, and cobalt, but poor in calcium and nitrogen. The magnesium-to-calcium ratio is wildly skewed — often 50 to 1 — which interferes with how plants take up nutrients. Most species simply cannot tolerate it.

Yet some plants have adapted. The Tasmanian serpentinite flora includes the endemic serpentine heath (Epacris barbata), the nickel-hyperaccumulating shrub Stackhousia tryonii, and a stunted form of the native pine Athrotaxis cupressoides. These species grow slowly, store heavy metals in their tissues, and reproduce sparingly. They are not abundant. They persist.

Where the serpentinite meets normal soils, the boundary is stark. On one side, tall eucalypt forest. On the other, low heathland, open and grey-green. The line can be walked in a few paces.

The Nickel That Stayed Behind

Serpentinite is not just a botanical curiosity. It is also a nickel ore. The same chemical process that makes the soil toxic also concentrated nickel sulphides into deposits worth mining.

At the Savage River mine, open since the 1960s, the serpentinite hosts a magnetite-nickel ore body that has produced millions of tonnes of iron and nickel concentrate. The ore formed when hot fluids, driven by the same tectonic collision that emplaced the serpentinite, remobilised nickel from the surrounding rock and deposited it in fractures and shear zones. The result is a rock that contains both valuable metals and a record of the violence that brought it here.

The mine is one of only two significant nickel operations in Tasmania. It operates year-round, processing ore that averages 0.8 per cent nickel — a low grade by global standards, but economic because the magnetite can be sold as an iron ore co-product.

The hills around Savage River are not just green from the serpentinite. They are green because of what the serpentinite forbids.

A Landscape That Resists Erasure

Serpentinite weathers slowly. Its surface tends to form rounded, hummocky hills with shallow soils and frequent rock outcrops. Drainage is poor; the valleys are often swampy. In Tasmania, the serpentinite belt supports a mosaic of buttongrass moorland, sedgeland, and patches of rainforest where the chemistry is less extreme.

The rock itself is slippery when wet — the name serpentine comes from the Latin serpentinus, meaning serpent-like, for its mottled, scaly appearance. It polishes to a dark green sheen. Cut slabs are used as decorative stone, though the asbestos content of some varieties (particularly chrysotile) has limited its use in recent decades.

There is a quiet strangeness to walking on serpentinite. The ground feels different underfoot: harder, more fractured, and oddly warm in the sun because the dark rock absorbs heat. The plants are unfamiliar. The air is still. It is a piece of the deep Earth, exposed not by erosion alone but by the slow machinery of plate tectonics that pushed the floor of an ancient ocean onto a continent that did not exist yet. The green hills of northwest Tasmania are a ghost of that ocean, still poisonous, still productive, still here.