Types of weathering

Weathering

 The texture of granite changes as it weathers.

Weathering refers to the combination of processes that break up and corrode solid rock, eventually transforming it into sediment. Geologists refer to rock that has not undergone weathering as unweathered or “fresh” (figure above). Rock exposed at the Earth’s surface sooner or later crumbles away because of weathering. Just as a plumber can unclog a drain by using physical force (with a plumber’s snake) or by causing a chemical reaction (with a dose of liquid drain opener), nature can attack rocks via two types of weathering: physical and chemical. 

Types of weathering

Physical Weathering 

Clasts are classified by grain diameter.

Physical weathering, sometimes referred to as mechanical weathering, breaks intact rock into unconnected clasts (grains or chunks), collectively called debris or detritus. Each size range of clasts has a name (table above). Many different phenomena contribute to physical weathering, as described below. 

Jointing 

Rocks buried deep in the Earth’s crust endure enormous pressure due to the weight of overlying rock or overburden. Rocks at depth are also warmer than rocks nearer the surface because of the Earth’s geothermal gradient. Over long periods of time, moving water, air, and ice at the Earth’s surface grind away and remove overburden, a process called erosion, so rock formerly at depth rises closer to the Earth’s surface. As a result, the pressure squeezing this rock decreases, and the rock  also becomes cooler. A change in pressure and temperature causes rock to change shape slightly. Such changes cause hard rock to break. Natural cracks that form in rocks due to removal of overburden or due to cooling are known as joints.

Joints (natural cracks) break bedrock into blocks and sheets, which can tumble down a slope.

Almost all rock outcrops contain joints. Some joints are fairly planar, some curving, and some irregular. For example, large granite plutons may split into onion-like sheets along joints that lie parallel to the mountain face; this process is called exfoliation. Sedimentary rock beds, however, may break into rectangular blocks bounded by joints on the sides and bed (layer) surfaces above and below (a in figure above). Regardless of their orientation, the formation of joints turns formerly intact bedrock into separate blocks. Eventually, these blocks topple from the outcrop at which they formed. After a while, they may collect in an apron of talus, the rock rubble at the base of a slope (b in figure above).

Wedging is one type of physical (mechanical) weathering.

Frost wedging

Freezing water bursts pipes and shatters bottles because water expands when it freezes and pushes the walls of the container apart. The same phenomenon happens in rock. When the water trapped in a joint freezes, it forces the joint open and may cause the joint to grow. Such frost wedging helps break blocks free from intact bedrock (a in figure above). 

Salt wedging

In arid climates, dissolved salt in groundwater precipitates and grows as crystals in open pore spaces in rocks. This process, called salt wedging, pushes apart the surrounding grains and weakens the rock so that when exposed to wind and rain, the rock disintegrates into separate grains. The same phenomenon happens along the coast, where salt spray percolates into rock and then dries (b in figure above). 

Root wedging

Have you ever noticed how the roots of an old tree can break up a side walk? As roots grow, they apply pressure to their surroundings, and can push joints open in a process known as root wedging (c in figure above). 

Thermal expansion

When the heat of an intense forest fire bakes a rock, the outer layer of the rock expands. On cooling, the layer contracts. This change creates forces in the rock sufficient to make the outer part of the rock break off in sheet-like pieces. Recent research suggests that the intense heat of the Sun’s rays sweeping across dark rocks in a desert may cause the rocks to fracture into thin slices.

Animal attack

Animal life also contributes to physical weathering: burrowing creatures, from earthworms to gophers, push open cracks and move rock fragments. And in the past century, humans have become perhaps the most energetic agent of physical weathering on the planet. When we excavate quarries, foundations, mines, or roadbeds by digging and blasting, we shatter and displace rock that might otherwise have remained intact for millions of years more.

Chemical Weathering 

Up to now, we've taken the plumber’s-snake approach to breaking up rock. Now let’s look at the liquid-drain-opener approach. Chemical weathering refers to the many chemical reactions that alter or destroy minerals when rock comes in contact with water solutions and/or air. Common reactions involved in chemical weathering include the following:

Dissolution is one type of chemical weathering.

• Dissolution: Chemical weathering during which minerals dissolve into water is called dissolution. Dissolution primarily affects salts and carbonate minerals (a and b in figure above), but even quartz dissolves slightly.
• Hydrolysis: During hydrolysis, water chemically reacts with minerals and breaks them down (lysis means loosen in Greek) to form other minerals. For example, hydrolysis reactions in feldspar produce clay. 
• Oxidation: Oxidation reactions in rocks transform iron bearing minerals (such as biotite and pyrite) into a rusty brown mixture of various iron-oxide and iron-hydroxide minerals. In effect, iron-bearing rocks can “rust.” 
• Hydration: Hydration, the absorption of water into the crystal structure of minerals, causes some minerals, such as certain types of clay, to expand. Such expansion weakens rock.

Not all minerals undergo chemical weathering at the same rates. Some weather in a matter of months or years, whereas others remain unweathered for millions of years. For example, when a granite (which contains quartz, mica, and feldspar) undergoes chemical weathering, most of its minerals except  quartz transform to clay. Until fairly recently, geoscientists tended to think of chemical weathering as a strictly inorganic chemical reaction, occurring entirely independently of life forms. But researchers now realize that organisms play a major role in the chemical-weathering process. For example, the roots of plants, fungi, and lichens secrete organic acids that help dissolve minerals in rocks; these organisms extract nutrients from the minerals. Microbes, such as bacteria, are amazing in that they literally eat minerals for lunch. Bacteria pluck off molecules from minerals and use the energy from the molecules’ chemical bonds to supply their own life force.

Physical and Chemical Weathering  Working Together 

So far we've looked at the processes of chemical and physical weathering separately, but in the real world they happen together, aiding one another in disintegrating rock to form sediment.

Physical and chemical processes work together during the weathering process.

Physical weathering speeds up chemical weathering. To understand why, keep in mind that chemical-weathering reactions take place at the surface of a material. Thus, the overall rate at which chemical weathering occurs depends on the ratio of surface area to volume the greater the surface area, the faster the volume as a whole can chemically weather. When jointing (physical weathering) breaks a large block of rock into smaller pieces, the surface area increases, so chemical weathering happens faster (a in figure above).

Similarly, chemical weathering speeds up physical weathering by dissolving away grains or cements that hold a rock together, transforming hard minerals (like feldspar) into soft minerals (like clay) and causing minerals to absorb water and expand. These phenomena make rock weaker, so it can disintegrate more easily (b in figure above).

Differential weathering.

Weathering happens faster at edges, and even faster at the corners of broken blocks. This is because weathering attacks a flat face from only one direction, an edge from two directions, and a corner from three directions. Thus, with time, edges of blocks become blunt and corners become rounded 9a in figure above). 

In rocks such as granite, which do not contain layering that can affect weathering rates, rectangular blocks transform into a spheroidal shape (b in figure above).
When different rocks in an outcrop undergo weathering at different rates, we say that the outcrop has undergone “differential weathering.” As a result of this process, cliffs composed of a variety of rock layers take on a stair-step or sawtooth shape (c in figure above). You can easily see the consequences of differential weathering if you walk through a graveyard. The inscriptions on some headstones are sharp and clear, whereas those on other stones have become blunted or have even disappeared (d in figure above). That’s because the minerals in these different stones have different resistances to weathering. Granite, an igneous rock with a high quartz content, retains inscriptions the longest. But marble, a metamorphic rock composed of calcite, dissolves away relatively rapidly in acidic rain.

Credits: Stephen Marshak (Essentials of Geology)

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