Weathering Flashcards
Defining characteristics of sedimentary rocks.
- Are made up of pre-existing rocks or by living organisms.
- Particles which comprise them are transported through different mediums - water, wind, ice - by traction, suspension, solution.
- Often have distinct layering or bedding.
Characteristics of clastic sedimentary rocks.
- Made up of clasts of pre-existing rocks.
- Weathering loosens rock fragments which are then transported elsewhere. (via water, wind, or gravity)
- Sediment becomes lithified (compacted and cemented) with burial.
The order of clasts based on grain sizes.
- Clay
- Silt
- Sand
- Pebbles
What do clay minerals describe?
Clay minerals are both a type and size of grain.
1. Size - clay-size particles are the smallest clast size. Associated with low energy environments. Forms shales.
2. Type - clay minerals (mica’s, kaolinite, etc.) tend to have basal cleavage and are very layered.
Characteristics of biochemical (organic) sedimentary rocks.
Form when large numbers of living things die and are cemented to form a rock.
1. Chert - radiolarians, diatoms.
2. Limestone - calcareous organisms or biologically-mediated precipitation in water.
3. Coal - water lain plant detritus.
How is coal formed?
- Plant material dies and collects in water (often swampy areas)
- Degradation of plant material initially dead (pulls oxygen from water.)
- Further collection of dead plant material which does not degrade due to low oxygen levels in the water; high acidity levels limit decomposing organisms.
- compaction and cementation -
Characteristic features of chemical (inorganic) sedimentary rocks.
- Form when mineral constituents become supersaturated and inorganically precipitate.
- Common rocks include evaporite minerals (gypsum, sylvite, barite and halite)
Components of physical/mechanical weathering.
Produces smaller particles without altering chemistry/mineralogy. Exceeds tensile strength of the rock.
Types of physical weathering.
- Freeze-thaw
- Insolation
- Salt
- Wetting/drying
Describe process of unloading/exfoliation.
- Formation of rocks in high-pressure environments.
- Exhumation of rock/ removal of overlying material results in unloading.
- Pressure is relieved when rock is exposed, allowing the rock to expand upward.
- Layer separation and breakage as top layers expand faster than lower layers.
- Tension received by cracking of rock layers. Large, flat slabs on surface.
How does surface area affect the weathering process?
As physical weathering proceeds, surface area increases (breakdown of large chunk to more and more smaller pieces). Surface area : volume increases.
Increased surface area increases likelihood of chemical weathering on the rock.
Components of chemical weathering.
- Chemical weathering results in changes in the chemical and mineral composition of a rock. Often with a net loss of material and/or a change in particle size.
- Reduces integrity of matrix and minerals, making it more susceptible to physical weathering.
Types of chemical weathering.
- Dissolution.
(ex. calcite/limestone: Rain combined with CO2 or an organic acid -> carbonic acid. Reaction with calcite and forms calcium bicarbonate)
(ex. evaporites: ) - Hydration/dehydration. Often happens in highly saline areas.
(ex. anhydrite and gypsum - same formula, gypsum has water. Reversible process, depends on environment) - Hydrolysis - Principle means of weathering Si+ minerals and produces dissolved silica. Not really reversible. Commonly occurs under acidic conditions.
(ex. orthoclase feldspar + carbonic acid + H2O => Kaolinite + silicic acid) - Oxidation - Mineral alters under the influence of water and oxygen (loss of e- or increase in ox. state of atoms).
(ex. Fe2+ + H2O + O => Fe3+ hydroxides/oxides - goethite/limonite/hematite)
How does grain size affect rate of weathering?
Smaller particles have a buffer of air when they collide, reduces the breakage. Size-buffer effect
Which weathering process results in particulate residues?
Physical weathering (Sandstones, conglomerates, mudrocks)
