Gabbro

The same rock, cooled slowly

Gabbro and basalt share a bulk composition: calcium-rich plagioclase, clinopyroxene, often some olivine, and no more than a trace of quartz. Erupt that magma and you get basalt. Let the same magma sit in a chamber a few kilometres down and it grows crystals of 2–10 mm, and the rock becomes gabbro. Geologists call this pair a compositional series with a grain-size split. It is also why gabbro is such a large share of the lower ocean crust: at mid-ocean ridges, the melt that does not erupt ponds and crystallises into a gabbro layer a few kilometres thick, sitting between the mantle below and the sheeted dykes above.

Identifying it in the hand

Gabbro's texture is unfriendly to quick identification because everything is the same dark grey. Work through it in a fixed order.

• Find a fresh broken face and wet it. Weathered gabbro is uniformly dull; fresh gabbro sparkles.
• Look at the plagioclase. It is grey and glassy with visible lath shapes — not pink, not chalky white.
• Tilt the specimen in the sun. Pyroxene cleavage faces flash as the angle changes; that glitter is diagnostic.
• Confirm no quartz. If there is translucent grey quartz, you are in granodiorite territory.

Layered intrusions: the real prize

The Bowen reaction series explains most of gabbro's behaviour, but layered complexes are what make it interesting to collect from. Where a large body of mafic magma sits and cools slowly, dense minerals — olivine, then pyroxene, then plagioclase — settle out in sequence, building a stack of layers that can be hundreds of metres thick and rhythmically banded. The Bushveld Complex in South Africa is the largest example, and its chromitite and merensky reef layers carry most of the world's platinum, palladium and chromium. In Skye's Cuillin centre, the same process operated at a small scale and the rhythmic banding is visible in a single cliff face, which is a rare convenience.

Where you meet it on the surface

Gabbro reaches the surface in three settings. It forms the guts of large ophiolites, the slivers of ocean crust that get obducted onto continents — the Oman ophiolite exposes a full section from mantle peridotite through gabbro to pillow basalt. It forms the lower parts of layered complexes, often exposed after kilometres of uplift. And it forms the cores of some large mafic intrusions, including the Skaergaard in Greenland, where decades of study produced much of what is known about how mafic magma differentiates. All three settings are worth a field visit precisely because the rock cannot form at the surface.

Field notes and safety

Gabbro weathers to a plain dark grey and looks unremarkable from a distance, so record the texture evidence in the field rather than trusting your memory later. It is also a durable, hard rock that resists the hammer — a 1 kg sledge or a chisel and a 2 kg lump hammer are more useful than a geological pick on a big exposure. Because many layered complexes host ore minerals, some are protected or actively mined; check whether the face you are approaching is a working quarry before you swing.

Sources

Hardness, streak, cleavage and specific-gravity values quoted on this sheet are checked against the references below.

• Geology.com Rocks — rock-type reference — descriptions and photographs of igneous, sedimentary and metamorphic rocks
• British Geological Survey — UK national geological survey — the three rock types, the rock cycle and mineral definitions