Sedimentary Rocks, Weathering and Erosion

Sedimentary rocks are types of rock that are formed by the deposition of material at the Earth’s surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle and accumulate or minerals that precipitate from a solution. Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment was formed by weathering and erosion in a source area and then transported to the place of deposition by water, wind, ice, mass movement, or glaciers.

Weathering is the breaking down of rocks, soil, and minerals as well as artificial materials through contact with the Earth’s atmosphere, biota, and waters. Weathering occurs in situ, or “with no movement”, and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, snow, wind, waves, and gravity.

Two important classifications of weathering processes exist – physical and chemical weathering; each sometimes involves a biological component. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice, and pressure. The second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals, also known as biological weathering, in the breakdown of rocks, soils, and minerals.

The materials left over after the rock breaks down, combined with organic material, create soil. The parent material determines the mineral content of the soil thus a soil derived from a single rock type can often be deficient in one or more minerals for good fertility, while a soil weathered from a mix of rock types (as in glacial, aeolian, or alluvial sediments) often makes more fertile soil.

• Erosion: The process of picking up material.
• Transportation: The movement of material by wind, water, and gravity.
• Deposition: The laying down of material in different environments. Control on erosion, transportation, and deposition is the amount of Energy. High energy = carry bigger sizes.

Physical Weathering

Physical weathering, also known as mechanical weathering, is the class of processes that causes the disintegration of rocks without chemical change. The primary process in physical weathering is abrasion (the process by which clasts and other particles are reduced in size). Physical weathering can occur due to temperature, pressure, frost, etc.

Figure Above: Different processes of physical weathering are shown. These include wedging by frost and minerals, fracturing, and wedging by roots and other biological activity.

Thermal stress

Thermal stress weathering results from the expansion and contraction of rock caused by temperature changes. For example, the heating of rocks by sunlight or fires can cause the expansion of their constituent minerals. As some minerals expand more than others, temperature changes set up differential stresses that eventually cause the rock to crack apart. Because the outer surface of a rock is often warmer or colder than the more protected inner portions, some rocks may weather by exfoliation – the peeling away of outer layers. This process may be sharply accelerated if ice forms in the surface cracks. When water freezes, it expands with a force of about 1465 Mg/m^2, disintegrating huge rock masses and dislodging mineral grains from smaller fragments.

Frost weathering

Frost weathering, frost wedging, ice wedging, or cryofracturing is the collective name for several processes where ice is present. Certain frost-susceptible soils expand or heave upon freezing due to water migrating via capillary action to grow ice lenses near the freezing front. This same phenomenon occurs within pore spaces of rocks. The ice accumulations grow larger as they attract liquid water from the surrounding pores. The ice crystal growth weakens the rocks, which, in time, break up. It is caused by the approximately 10% expansion of ice when water freezes, which can place considerable stress on anything containing the water as it freezes.

Pressure release

In pressure release, also known as unloading, overlying materials (not necessarily rocks) are removed (by erosion or other processes), which causes underlying rocks to expand and fracture parallel to the surface. Intrusive igneous rocks (e.g., granite) are formed deep beneath the Earth’s surface. They are under tremendous pressure because of the overlying rock material. When erosion removes the overlying rock material, these intrusive rocks are exposed, and the pressure on them is released. The outer parts of the rocks then tend to expand. The expansion sets up stresses which cause fractures parallel to the rock surface to form. Over time, sheets of rock break away from the exposed rocks along the fractures, a process known as exfoliation. Exfoliation due to pressure release is also known as “sheeting.”

Salt-crystal growth 

Salt crystallization causes the disintegration of rocks when saline solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. These salt crystals expand as they are heated up, exerting pressure on the confining rock. The salts which have proved most effective in disintegrating rocks are sodium sulfate, magnesium sulfate, and calcium chloride. Some of these salts can expand up to three times or even more. It is normally associated with arid climates where strong heating causes strong evaporation and, therefore salt crystallization.

Chemical Weathering

Chemical weathering changes the composition of rocks, often transforming them when water interacts with minerals to create various chemical reactions. Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near-surface environment. New or secondary minerals develop from the original minerals of the rock. More chemical weathering occurs in environments that are wetter and warmer.

Figure Above: The different processes of chemical weathering are shown. These include oxidation, biological reactions, hydrolysis, and dissolution.

Dissolution/Carbonation

Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0.

One of the most well-known solution weathering processes is carbonation, the process in which atmospheric carbon dioxide leads to solution weathering. Carbonation occurs on rocks that contain calcium carbonates, such as limestone and chalk. This occurs when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid that reacts with calcium carbonate (the limestone) and forms calcium bicarbonate.

CO₂ + H₂O → H₂CO₃

carbon dioxide + water → carbonic acid

H₂CO₃ + CaCO₃ → Ca(HCO₃)₂

carbonic acid + calcium carbonate → calcium bicarbonate

Carbonation on the surface of well-jointed limestone produces a dissected limestone pavement. This process is most effective along the joints, widening and deepening them.

Hydrolysis (Hydration)

Mineral hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral. When rock minerals take up water, the increased volume creates physical stresses within the rock. For example, iron oxides are converted to iron hydroxides, and the hydration of anhydrite forms gypsum.

Oxidation

Within the weathering environment, the chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe2+ (iron) and combination with oxygen and water to form Fe3+ hydroxides and oxides such as goethite, limonite, and hematite. This gives the affected rocks a reddish-brown coloration on the surface, which crumbles easily and weakens the rock. This process is better known as ‘rusting’, though it is distinct from the rusting of metallic iron. This process can occur through the atmosphere or under water.

Biological weathering

Plants and animals may create chemical weathering by releasing acidic compounds, i.e., the effect of moss growing on roofs is classed as weathering. Mineral weathering can also be initiated and/or accelerated by soil microorganisms. Lichens on rocks are thought to increase chemical weathering rates.