Understanding Weathering, Diagenesis, and Sedimentary Rocks
Weathering: Alteration of Surface Rocks
Weathering is the alteration of surface rocks in contact with air and water.
Physics of Weathering
The physical aspect of weathering involves the disintegration or division of rocks into fragments without modifying their chemical and mineralogical composition. Living organisms also play a role.
Gelifraction (Freeze-Thaw)
Gelifraction occurs when water penetrates cracks in rocks, freezes, and expands, increasing its volume and causing fractures.
Haloclastia (Salt Weathering)
Haloclastia is the breakage of rocks produced by the growth of salt crystals. If water permeates a rock with dissolved salts, crystals form upon evaporation of these salts in the pores, favoring the disintegration of the rock.
Thermal Expansion
Temperature changes cause rocks to expand and contract, potentially leading to fracturing.
Decompression
Decompression occurs when slabs separate in rocks due to the removal of overlying pressure.
Chemistry of Weathering
The chemical aspect of weathering involves the alteration of the chemical and mineralogical composition of rocks caused by air and water.
Hydrolysis
Hydrolysis results from the dissociation of water into hydrogen (H+) and hydroxide (OH-) ions. These ions are very reactive and decompose many minerals.
Oxidation
In the presence of oxygen, elements lose electrons and are oxidized. Oxidation is more effective in the presence of water.
Carbonation
Carbonation is produced by carbonic acid in water, which dissolves joints in rocks.
Dissolution
Diagenesis: From Sediment to Sedimentary Rock
Diagenesis is the set of processes by which sediment becomes sedimentary rock.
Compaction
Compaction is the volume reduction of a rock, occurring as a result of pressure from newly deposited material on underlying sediment. This pressure reduces the rock’s volume and decreases pore water.
Cementation
Cementation consists of the precipitation of minerals dissolved in water circulating between sediment grains. The precipitated minerals act as cement, uniting the grains and decreasing the rock’s porosity.
Sedimentary Rocks: Classification
Sedimentary rocks are formed from fragments of other rocks that have been transported in a solid state. They are classified according to grain size and origin.
Detrital Rocks
Detrital sedimentary rocks are formed from fragments of other rocks.
Rudite
Rudites are coarse-grained rocks with rounded clasts (gravel) or angular clasts (conglomerate).
Arenitas
Arenites are medium-sized rocks formed from sandstone.
Shales
Shales are fine-grained rocks, including siltstones (formed from the diagenesis of silt) and argillites (formed from the diagenesis of clay).
Non-Detrital Rocks
Non-detrital rocks are formed from substances dissolved in water that have precipitated.
Carbonates
Carbonates are formed from CaCO3 and Mg.
- Chemical Origins: CaCO3 dissolved in water precipitates without biological involvement.
- Biochemical Origins: Precipitation is mediated by biological organisms.
- The rock is formed by the accumulation of organic debris.
- Dolomite is composed of CaCO3 and Mg.
- Marls are rocks composed of shale and limestone.
Evaporites
Evaporites are rocks formed by the precipitation of mineral salts dissolved in water that has undergone intense evaporation. They form in arid areas, lakes, and inland seas (e.g., halite, sylvite, gypsum).
Siliceous-Aluminous-Ferruginous Rocks
These rocks originate from altered soils in tropical climates.
Organogenic Rocks
These rocks originate from the remains of organisms after they have been altered.
- Natural Coals: Formed from plant debris in areas of abundant vegetation. These environments, poor in O2, undergo bacterial action, resulting in a partial decomposition and enrichment of C.
- Oil: Formed from the remains of plankton accumulated on the seabed, covered by clay or mud to prevent oxidation. Microbial fermentation removes O2 and N2, and the remaining material is enriched by N and C, forming a mixture of hydrocarbons.