AN ORANGE LICHEN CLINGS TO A CRACK near the base of a granite boulder. It holds a little rain, catches dust that has blown in from the Great Basin, and begins to secrete acids, which work into the grain of the rock. Water makes a wedge in the crack, and following it, a birch seedling sends out its root along the edge of the fracture, living on the products of its leaves’ decay and the few minerals that the water and the lichen have liberated from the rock.
Water tears rock, and so there begins to be soil. Sir Isaac Newton, in a book on alchemy, De Natura Acidorum, suggested that all substances could be reduced to water. Since his time, scientists have found the basis of matter in far smaller units than water, yet it might still be said that all substances are reduced to their own fundamental parts by water.
Water gets into things. It soaks them, drenches them, permeates them. No watch or coat is truly waterproof. There is no legal definition of the word “waterproof,” because there is no such thing. Nothing resists water indefinitely. Even my Stetson hat, which features a picture of a cowboy using the hat to water his horse, soaked through after four hours in a driving Pacific storm.
The same thing happens to rocks. Some are porous to start with; some contain elements, like calcium carbonate, that dissolve in water, creating channels. Sometimes a mechanical shock starts the process of disintegration: a moving fault or a crashing wave produces a microscopic fracture in the crystalline structure. A thin film of water insinuates itself into the crevice. From that moment, soil becomes possible and, with it, life.
Water behaves strangely when it gets into a crack. In that environment, it is called interstitial water, vicinal water, orthowater, or ordered water. The key thing is that it does not freeze.
The thin film of water in the crack starts to freeze, but it can’t. When the temperature drops, the water molecules try to migrate, reordering themselves into ice crystals, but they are already in an ordered relationship with respect to the crystals of the rock walls. A tug of war ensues. The tension is enough to pull rock grains apart. Equivalent force, on a larger scale, would be a wind so strong that it ripped the facades off skyscrapers or a pull sufficient to part a bridge cable.
It takes 210,000 pounds of pressure to break the surface tension of a one-inch column of pure water. So as invisible as the process is to our eyes, it is nevertheless explosively powerful. The results can be seen deep in the profiles of older soils, where the digger comes upon huge boulders of granite or gneiss that crumble at a touch. They perfectly resemble the parent rock, but water has eaten out their structure from the inside.
In nature, however, water is seldom pure. When salt-impregnated water enters a crack, the salt may gather more water around its strongly attractive ions, a process called “hydration,” forcing the crack apart. If, on the other hand, the water evaporates, the salts may crystallize into a solid state again, gradually splitting the rock as the crystals grow. Furthermore, since salts expand more when heated than do most rocks, hot salt will open a breach further.
What does it mean, then, to be as solid as a rock? Better to consider the fragility of rock, and its transformation into soil. One third of the sedimentary rock in the world is derived from clay—all of which is derived from weathering of other rocks. At first, the stone breaks down into smaller and smaller parts, but at a certain point the process of destruction concludes and the fragments begin to build a new structure: one of the earth’s indispensable clays. If you spread all this clay evenly, it would make a layer one mile thick over the whole surface of the Earth.
Even human monuments show the delicacy of stone. The 4,500-year-old step pyramids of Egypt are among the oldest human structures known. Each terrace of the pyramids is covered with a thick talus of cracked-off, crumbled stone, much like the talus found at the base of mountains. Each building stone has turned from a rectangular piece to a rounded boulder. The Great Pyramids, too, are beginning to turn to boulder piles. Up until a thousand years ago, when their polished facing stones were carried off to Cairo to use in building mosques, the core of the pyramids was protected. Since that event, the more vulnerable limestones among the blocks have been losing matter at a rate of fifty cubic centimeters per year.
The castles and churches of Europe promise to be even more ephemeral. A nine-hundred-year-old castle in Austria, made of sandstone blocks, is a ruin that sprouts grass from the clay soil that has formed atop what once were its walls. A 770-year-old church of the same sandstone is deeply weathered but still intact. A five-hundred-year-old church is just starting to show wear. And a one-century-old church is as fresh as when it was built.
Limestone and even marble tombstones weather so fast that the memorial to grandfather may scarcely outlive those who knew him, disappearing like the writing on a magic writing tablet after you have lifted the top page. When polished marble weathers, it first gets reddish iron oxide streaks, then the stone roughens as the polished surface grains are split from the underlying stone; finally, the grains form a sugary coating that can be be scraped off by hand.
The landscape of the whole Earth is itself little more than a monument to the different weathering rates of its constituent minerals. Peaks and valleys, beds and swales all result from the variable weathering of stone into soil.
Generally speaking, sandstone is quickest to go, then marble and schist, then granite and gneiss, so the resistant granites occupy the mountain heights, the middling-resistant marbles the uplands and slopes, and the quick-melting sandstone the valleys. In an Edinburgh cemetery, the marble tombstones lost three and a half inches of their surface in a millennium; a granite stone lost only one tenth that amount.
The sum of the processes of destruction can work so fast that they become perceptible within the scale of a human lifetime. In 1881, Captain Henry H. Gorringe brought an elaborately carved Egyptian memorial obelisk from Alexandria to New York’s Central Park, where it was set up with fanfare and dubbed Cleopatra’s Needle. Within a year, the beautiful hieroglyphs of the west and north faces were already fading; after three years, piles of flakes could be collected at the needle’s base. Eighty years later, the west face had lost one and a half inches of its surface, which effectively erased its entire inscription.
The monument was made of a hard but coarse-grained syenite granite, quarried in Aswan, where only one to three inches of rain fell per year. There, the stone weathered at a rate of only one-half inch in two thousand years. In New York, where forty-three inches of rain fall each year, you might expect faster weathering. But salt and temperature are the more telling agents. The obelisk had lain for several hundred years on its side in the Nile silt, where Persian invaders had toppled it. During that time, the muddy faces had imbibed the natural salts of the Nile.
When Captain Gorringe set up the salt-impregnated stone in rainy New York, the increased moisture rose through the base, forcing the hidden salts to the surface, where together with the immense pressures of freezing and thawing, they made short work of the inscribed names and the mighty deeds of Queen Hatshepsut, Thutmose III, and Ramses II.