The Bishop Tablelands are one eruption. About six days of it. Roughly 765,000 years ago the magma chamber under what is now Long Valley emptied out and the roof fell in. Pyroclastic flows — ash and pumice moving as a hot ground-hugging current — swept over more than 2,200 square kilometres.…

There is a place so flat that satellites use it to check themselves. Salar de Uyuni sits at about 3,700 m in the Bolivian Altiplano: the largest salt flat on Earth, 9,000 km² with less than one metre of vertical relief across the whole of it.…

In the Grand Canyon you can put a hand flat on a billion years of nothing. Layers stack in order, oldest at the bottom. But the record only exists where sediment was piling up.…

Those sweeping diagonal lines in a sandstone wall are dunes. Actual dunes. Wind pushes sand up the gentle windward side of a dune and drops it over the crest. On the steep downwind face the sand piles up until it fails and slides — a small avalanche — and each slide leaves its own inclined layer.…

Some rock looks wet when it's bone dry. That's serpentinite. It starts as mantle rock — peridotite, made mostly of olivine and pyroxene. Where hot water gets into it, those minerals are converted into serpentine minerals: lizardite, antigorite, chrysotile.…

The colour of the wall usually isn't the rock. Those black streaks, brown patches and orange tapestries on a desert cliff are coatings — thin skins that sit on the surface and are compositionally different from the bedrock underneath.…

The coldest nights aren't the ones that break your crag. Everyone knows the line: water freezes, expands by about 9 %, and levers the rock apart. It is real physics — but the conditions it needs are strict.…

That wet streak on the wall isn't random. It's a horizon. Rain soaks into porous rock and keeps sinking, as long as there is pore space to sink through. Where it reaches a layer water can barely pass — mudstone, marl, shale — it stops going down and starts going sideways, piling up above that…

There is a rock that records the moment the planet started breathing. For most of Earth's early history there was no free oxygen. Iron from volcanoes and weathering stayed dissolved in seawater, because dissolved iron only rusts out when oxygen is around.…

Somewhere in the rock there is a clay layer a few centimetres thick that ends an era. About 66 million years ago an asteroid hit what is now the Yucatán and left the Chicxulub crater.…

Whether your local crag gives you ledges or big blank faces was decided before anyone climbed it. Sedimentary rock comes in beds, and each bed acts as its own mechanical layer.…

Much of the Frankenjura isn't limestone. It's dolomite — and that is why the towers are standing. About 150 to 160 million years ago, sponges and microbes built reef mounds in the shallow sea that covered what is now Franconia. That reef rock started out as calcite, like any limestone.…

Granite doesn't wear down. It falls apart. Sandstone is grains held together by cement, so weathering can attack the glue. Granite has no glue: quartz, feldspar and mica grew into one interlocking frame, and nothing holds it together but that fit. Weather only has to destroy two of the three.…

Some boulders simply do not belong where they are standing. A glacial erratic is a block that a glacier picked up, carried, and dropped somewhere else. Blocks fall onto the ice from the walls above it, or get quarried off the bed underneath, and then travel with the flow — routinely tens of…

There is no new rock. Everything you have ever pulled on has been something else before. Magma cools and crystals grow into each other: igneous rock. Weather takes it apart into grains, rivers stack the grains in layers, and burial cements them: sedimentary rock.…

An ice core is the only climate archive that hands you the actual air. Snow falls and buries the snow before it. As the pile compacts, the pores between the grains stay connected to the atmosphere for a while — only about 50 to 100 metres down do they pinch shut into sealed bubbles.…

Almost the whole Mid-Atlantic Ridge is two kilometres under water. In Iceland you can walk into it. A mid-ocean ridge is a spreading centre: two plates pulling apart, magma rising into the gap and freezing there as new crust.…

The rock on top of the Matterhorn did not grow in Europe. The Matterhorn is a stack of thrust sheets — nappes. Its summit belongs to the Dent Blanche nappe: gneisses that came from the Adriatic plate, a block that had rifted away from Africa.…

Lichen on a hold isn't just dirt. It's a slow attack on the rock — and sometimes its protection. A lichen is a fungus farming algae. The fungal threads work their way down between mineral grains and prise them apart, and the whole body swells and shrinks with every wetting and drying, loosening…

Those glassy glacier-polished slabs weren't just worn smooth. Something was added. Glacial polish used to be explained as pure abrasion — grit at the base of a glacier sanding bedrock down. That part is real; the striations are scratch marks.…

Slate doesn't split along its layers. It splits along its memory of being squeezed. Slate starts as mud, settling in flat layers on a sea floor. Then a mountain-building event squeezes it from the sides.…

That pale stripe across the wall is a healed crack — and often the best hold on the pitch. Quartz veins start as fractures. Hot water moving through the crust carries dissolved silica; when it cools or the pressure drops, the silica comes out of solution and crystallises on the fracture walls,…

There was a time when every crag on Earth was in the same place. From roughly 335 to 200 million years ago almost every continent was assembled into a single landmass — Pangaea — ringed by one vast ocean, Panthalassa.…

There is a whole food web down there that has never seen the sun. Along mid-ocean ridges, cold seawater seeps into cracks in young, hot ocean crust. Close to the magma it is heated to roughly 400 °C and strips metals and sulphide out of the rock on the way.…

Desert rock doesn't get carved into pockets. It gets hollowed out from the inside. Honeycombs — tafoni, alveoli, the huecos you jam a fist into — are largely salt at work.…

Devils Tower is a crack pattern you can climb. Those columns are contraction cracks. About 50 million years ago a body of phonolite magma came to rest near the top of the crust and cooled slowly.…

Conglomerate is a climbing gym that built itself. It is old river gravel, glued back together. Rivers and debris flows round the stones and dump them in fans and deltas; later, groundwater precipitates calcite, iron oxide or silica in the gaps and turns the loose pile into rock.…

The boulders at Chironico are the wreckage of one very bad day. Chironico, in Ticino's Valle Leventina, sits on a landslide deposit. About 13,400 years ago — roughly three thousand years after the glaciers had pulled out of the valley — around 530 million cubic metres of granite gneiss let go of…

Those stripes in the rock are the reason your feet stay on. Gneiss is a metamorphic rock whose minerals have been sorted into layers: pale bands of quartz and feldspar, darker bands rich in mica.…

Ice ages keep a beat — and it's written in Earth's orbit. Three slow wobbles change how sunlight reaches the planet: the shape of Earth's orbit (eccentricity, ~100,000 years), the tilt of its axis (obliquity, ~41,000 years) and its slow spin-wobble (precession, ~23,000 years).…