Orange Lichen on Stone

This bright orange lichen grows pressed tightly against the stone, its edges spreading outwards in small, finger-like lobes. Like many crust-forming lichens, it grows from the edge, so the oldest part lies at the centre. As the centre ages, it can crack, fade or break away, leaving rings and islands of colour like those seen here, a record of the lichen's slow growth over many years.

Orange lichens of this kind are especially common on lime-rich surfaces such as limestone, mortar and concrete. As well as natural rock, they readily colonise walls, old masonry and other human-made surfaces, which behave like limestone to the lichens that settle on them.

Slowly, lichens also wear away the stone they live on. Their fungal threads work into tiny cracks and swell and shrink as they wet and dry, prising grains apart, while the acids they release dissolve the rock's surface. This weathering frees minerals such as calcium, potassium, magnesium and iron, which wash into the surrounding ground and help form the first thin soils. From there, those minerals pass into plants, the animals that eat them, and eventually into us, which has led some to suggest that some of the minerals in our own bodies may once have passed through a lichen.

The area shown here is only a few centimetres across, yet framed in a circle it could be a distant planet. Up close, a patch of lichen on an old stone becomes its own small world, one that may have taken decades to form.