In the last few chapters, we've looked at the Earth's crust—its mineral composition, its lithospheric plates, and the landforms created by volcanic and tectonic activity. Now let's examine the shallow surface layer in which life exists. We'll look first at how rocks are softened and how they break up. Later, we'll see how the resulting rock materials move downhill under the force of gravity.
Weathering describes the combined action of all processes that cause rock to disintegrate physically and decompose chemically because of exposure near the Earth's surface. There are two types of weathering.
In physical weathering, rocks are fractured and broken apart. In chemical weathering, rock minerals are transformed from types that were stable when the rocks were formed to types that are now stable at the temperatures and pressures of the Earth's surface. Weathering produces regolith—a surface layer of weathered rock particles that lies above solid, unaltered rock—and also creates a number of distinctive landforms.
One of the most important physical weathering processes in cold climates is frost action. Unlike most liquids, water expands when it freezes. If you've ever left a bottle of water chilling in the freezer overnight only to find a mass of ice surrounded by broken glass the next morning, you've seen this phenomenon first-hand. As water in the pore spaces of rocks freezes and thaws repeatedly, expansion can break even extremely hard rocks into smaller fragments.
Water penetrates fractures in bedrock. These fractures, called joints, are created when rocks are exposed to heat and pressure, then cool and contract. Joints typically occur in parallel and intersecting planes, creating natural surfaces of weakness in the rock. Frost action then causes joint-block separation. Water invades sedimentary rocks along their stratification planes, or bedding planes. Joints often cut bedding planes at right angles, and relatively weak stresses will separate the joint blocks. Water can also freeze between mineral grains in igneous rocks, separating the grains to create a fine gravel or coarse sand of single mineral particles. This process is called granular disintegration.
All climates that have a winter season with cycles of freezing and thawing show the effects of frost action. On high mountain summits and in the arctic tundra, large angular rock fragments can accumulate in a layer that completely blankets the hard rock underneath. The result is a rock sea, or
rock glacier that is constantly churned by frost action. On cliffs of bare rock, frost action can detach angular blocks that fall to the base of the cliff. These loose fragments are called talus, and if block production is rapid, a talus slope of coarse rubble forms.
A similar physical weathering process occurs in dry climates. Salt-crystal growth in rock pores can disintegrate rock, and this process carves out many of the niches, shallow caves, rock arches, and pits seen in sandstones of arid regions. During long drought periods, ground water moves to the rock surface by capillary action—a process in which the water's surface tension causes it to be drawn through fine openings and passages in the rock. The same surface tension gives water droplets their round shape. The water evaporates from the sandstone pores, leaving behind tiny crystals of minerals like halite (sodium chloride), calcite (calcium carbonate), or gypsum (calcium sulfate). Over time, the force of these growing crystals breaks the sandstone apart, grain by grain.
Rock at the base of cliffs is especially susceptible to saltcrystal growth. Salt crystallization also damages masonry buildings, concrete sidewalks, and streets. Salt-crystal growth occurs naturally in arid and semiarid regions, but in humid climates, rainfall dissolves salts and carries them downward to ground water.
OTHER PHYSICAL WEATHERING PROCESSES
Unloading, or exfoliation, is another widespread process that weathers rocks. Rock that forms deep beneath the Earth's surface is compressed by the rock above. As the upper rock is slowly worn away, the pressure is reduced, so the rock below expands slightly. This expansion makes the rock crack in layers parallel to the surface, creating a sheeting structure. In massive rocks like granite or marble, thick, curved layers or shells of rock peel free from the parent mass below, producing an exfoliation dome. Thermal expansion from hot fires can also generate or enhance exfoliation.
Although first-hand evidence is lacking, it seems likely that daily temperature changes also break up surface layers of rock that have already been weakened by other weathering agents. Most rock-forming minerals expand when heated and contract when cooled, so intense heating by the Sun during the day alternating with nightly cooling exerts disruptive forces on the rock.
Plant roots can also break up rock as they wedge joint blocks apart. You've probably seen concrete sidewalk blocks that have been fractured and uplifted by the growth of tree roots. This process also happens when roots grow between rock layers or joint blocks.
Chemical reactions can turn rock minerals into new minerals that are softer and bulkier and therefore easier to erode. And some acids can dissolve minerals, washing them away in runoff. These processes are examples of chemical weathering.
Chemical reactions proceed more rapidly at warmer temperatures, so chemical weathering is most effective in the warm, moist climates of the equatorial, tropical, and subtropical zones. There, hydrolysis and oxidation, working over thousands of years, have decayed igneous and metamorphic rocks down to depths as great as 100 m (about 300 ft). The decayed rock material is soft, clay-rich, and easily eroded. In dry climates, oxidation and hydrolysis weather exposed granite to produce many interesting boulder and pinnacle forms.
Acid action is another form of chemical weathering. Carbonic acid is a weak acid formed when carbon dioxide dissolves in water. It is found in rainwater, soil water, and stream water, and it slowly dissolves some types of minerals. Carbonate sedimentary rocks, such as limestone and marble, are particularly susceptible to carbonic acid action, producing many interesting surface forms. Carbonic acid in ground water dissolves limestone, creating underground caverns and distinctive landscapes that form when these caverns collapse.
In urban areas, sulfur and nitrogen oxides pollute the air. When these gases dissolve in rainwater, we get acid precipitation, which rapidly dissolves limestone and chemically weathers other types of building stones, stone sculptures, building decorations, and tombstones. Soil acids that form as microorganisms digest organic matter also rapidly dissolve basaltic lava in the wet low-latitude climates.