Weathering is the process that takes place when rocks are exposed to the elements at the Earth’s surface. While most rocks originate deep within the crust, sedimentary rocks are exceptions. In the crust, rocks are subjected to constant temperatures, high pressure, and minimal exposure to the atmosphere or moving water. However, when rocks are brought to the surface due to erosion of the overlying layers, they encounter a drastically different environment. They experience wide temperature fluctuations, reduced pressure, abundant oxygen and other gases, and, in most climates, plentiful water.

Weathering encompasses two distinct processes. The first is mechanical weathering, which involves the physical fragmentation of the rock, and the second is chemical weathering, which alters the rock’s minerals into forms that are stable at the surface. Mechanical weathering exposes new surfaces for chemical processes to act upon, while chemical weathering weakens the rock, making it more prone to mechanical weathering. These combined processes yield two important outcomes: sedimentary clasts and ions in solution that can eventually form sedimentary rock, and the soil that is vital for life on Earth.

Mechanical Weathering

Intrusive igneous rocks form deep underground, where the temperature and pressure are high. Sedimentary rocks form when layers of sediments are buried and compacted over time. Most metamorphic rocks form when rocks are subjected to high temperature and pressure in the crust. These rocks can only be exposed at the surface after they are uplifted by tectonic forces and eroded by wind, water, and ice.

Mechanical weathering breaks down rocks into smaller pieces by physical forces. Some of the main factors that cause mechanical weathering are:

• The release of pressure when overlying rock is removed
• The expansion and contraction of water in rock cracks when it freezes and thaws
• The growth of salt crystals in rock pores and cracks
• The penetration of plant roots and animal burrows into rock fractures

When a rock is unloaded by erosion, it expands and cracks along curved surfaces. This is called exfoliation. Granite is a common rock that exfoliates because it is uniform and has no preferred direction of cracking. Other types of rocks, such as sedimentary and metamorphic rocks, tend to crack along existing planes of weakness.

Frost wedging is a type of mechanical weathering that occurs when water seeps into rock cracks, freezes, and pushes the cracks apart. Frost wedging is more effective in cold climates where freezing and thawing cycles are frequent. In warm climates where freezing is rare, cold climates where thawing is rare, or dry climates where water is scarce, frost wedging has less impact.

In many mountainous regions, the transition between freezing nighttime temperatures and thawing daytime temperatures occurs frequently, from tens to hundreds of times a year. Even in warm coastal areas, freeze-thaw transitions are common at higher elevations. A typical feature in areas with active frost wedging is a talus slope, a fan-shaped deposit of fragments dislodged by frost wedging from the steep rocky slopes above.

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Frost heaving is a process that affects loose materials on gentle slopes. It happens when the water in the soil freezes and expands, lifting the material above it. The expansion of water is due to the fact that ice has a lower density than liquid water and, therefore, occupies more space. When the ice melts, the material settles back, creating cracks and gaps. This process causes damage to roads in many parts of North America during winter.

Salt weathering is another process that can weaken and break rocks. It occurs when saltwater infiltrates rocks and evaporates on a hot sunny day, leaving behind salt crystals that grow inside cracks and pores. These crystals exert pressure on the rock and separate the grains. Salt weathering can happen not only near the coast but also in other environments that have some salt content. Some examples of salt weathering are honeycombed stones in sea walls, tafoni, cavernous weathering in sandstone, and pedestal rocks in desert areas.

One factor contributing to mechanical weathering is the presence of plants and animals. As plants grow, their roots can penetrate small cracks in the rocks and exert a strong force that widens and splits them. Animals do not usually dig through solid rock, but they can remove large amounts of soil and expose the underlying rock to other weathering processes.

Another factor that enhances mechanical weathering is erosion, which carries away the weathered material and exposes more rock surfaces for weathering. At the top of the cliff, where the rock face is steep, pieces of rock have been detached by ice wedging and then fallen down by gravity. This is an example of mass wasting, which will be discussed later in this chapter. Other important agents of erosion that help to remove the weathered material are water in streams, ice in glaciers, and waves on the coasts.

Chemical Weathering

Chemical weathering occurs when minerals undergo chemical transformations due to surface conditions. Different minerals and environmental factors affect the type and rate of these changes. Quartz is very resistant to chemical weathering, while feldspar is easily modified. Warm and wet climates tend to enhance chemical weathering, while cold and dry climates inhibit it. The main factors that contribute to chemical weathering are water (in liquid or vapor form), oxygen, and carbon dioxide, which forms a weak acid when dissolved in water.

Effects of Weathering and Erosion

Weathering and erosion are two different processes that produce unconsolidated materials on the Earth’s surface. Weathering is the breakdown of rocks into smaller particles by physical, chemical, or biological agents. Erosion is the removal and transport of weathered materials by water, wind, ice, or gravity. The characteristics of the unconsolidated materials — such as their composition, size, degree of sorting, and degree of rounding — depend on the type of rock that is weathered, the weathering and erosion processes involved, and the climate conditions.

The table below summarizes the weathering products of some common minerals found in rocks.

The rock material created from weathering ranges widely in size and shape, depending on the processes involved. If and when deposits like these are turned into sedimentary rocks, the textures of those rocks will vary significantly. Importantly, when we describe sedimentary rocks that formed millions of years ago, we can use those properties to make inferences about the conditions that existed during their formation.