From Sediments to Rocks
Sediments are broken bits of rock. They can be many different shapes, sizes, compositions and can be found in many different types of environments. In fact, all of these variations are unique to specific depositional environments.
There are three steps necessary to the formation of sedimentary rocks: Diagenesis, Recrystallization, and Lithification.
Step 1: Diagenesis
- Weathering: The generation of detritus via rock disintegration.
- Erosion & Transportation: The formation and dispersal of sediments by wind, water, and ice.
- Deposition: The accumulation after transport stops
These three processes form and move sediments.
Soils and sediments are the end products of weathering. There are three types: Mechanical, Chemical, and Biological.
Mechanical Weathering involves the physical breaking apart of Earth materials.
- Frost Wedging - Water fills cracks in a rock, then freezes. The volume of water increases as it solidifies in the frozen state, widening the crack. This allows more water to fill the crack, which widens more when freezing. Eventually the crack widens so much that part of the rock breaks off.
- Root Wedging - Cracks in rocks fill up with water and soil, and plant roots will eventually find their way into those cracks. The cracks widen as the roots grow. Eventually the crack widens so much that part of the rock breaks off.
- Jointing - Joints are fractures in a rock along which there is no appreciable movement (that would make them a fault). Joints can form due to a variety of methods - unloading of the overlying rock can cause expansion joints to form in igneous rocks, while contraction during the formation of sedimentary rocks can also form joints.
- Unloading - Intrusive igneous rocks solidify inside the Earth, surrounded by the host or country rock. The pressure of the surrounding rock on the pluton is constant. Erosion of the overlying rock not only removes the overlying rock, but lessens the pressure on the pluton as it is exposed. The pluton then expands as the pressure surrounding it is released. Sometimes, huge slabs of rock can 'peel off' the rock like the skin of an onion. This process is called exfoliation.
Chemical Weathering alters the internal structure of minerals, usually via water. Minerals differ in their tendency to weather chemically. This is dependant upon the type of bonding within the mineral, the type of cleavage present (or not present), in addition to the temperature and pressure at which the mineral forms. Minerals that form first in Bowen's Reaction Series are the least stable, while those that form last are the most stable.
- Dissolution - Some minerals dissolve (halite, gypsum, calcite). Acidity (i.e. acid rain) enhances this effect.
- Hydrolysis - Water breaks cation bonds in silicate minerals, which yields dissolved cations, and alteration residues (Clay minerals, Iron oxides)
- Oxidation - A reaction whereby a metal loses electrons. EX: Rust (FeO2)
- Hydration - Absorption of water into a mineral structure, which results in a volume increase (expansion). minerals.
Biological - Organisms often important chemical weathering agents
- Plant roots.
Physical weathering speeds chemical weathering. An increase in surface area accelerates chemical attack, while the chemical weakening of the substance increases surface area via breakage. The shape of the object is also important. Block geometry influences weathering. The corners weather fastest; as there are 3 sides of attack. Edges weather at a moderate rate (2 sides). Flat faces weather slowest (1 side). Weathering rates vary due to changes in mineral stability, the degree of compaction or cementation and subtle differences in texture, etc. These in turn control the surficial expression of a rock or landscape.