Lecture 8 - From Rocks to Soils and then Plants Flashcards
What is weathering?
The chemical alteration and mechanical breakdown of rock.
What does weathering do?
Converts rock to saprolite (in situ weathered rock) and then to soil.
What is physical weathering linked to?
The exploitation and utilisation of joints.
What is haloclasty?
Saline water seeps into rocks and then precipitates a salt.
The salt crystals grow and apply a force on the rock.
What is granular disintigration?
Grain-by-grain breakdown of rock masses.
What is frost wedging?
Water seeps into cracks in the rock and then freezes.
This increases the volume by 9% and so a force is exerted onto the rock.
How does physical weathering occur via biology?
Plant roots can infiltrate into rocks and grow.
What are the methods of weathering by plants?
Root exploration of cracks.
Retention of water.
Organic acid excretion from roots.
Uptake of selected ions causing increased solubility.
Decomposition of root and leaf remains releasing organic acids as decay products.
Encouraging biota to graze and disturb rock materials.
How does moss colonising limestone cause it to weather?
Moss attracts and retains water which changes the chemical and physical environment of the rock.
This accelerates the process of chemical weathering.
What are the four types of chemical weathering?
Hydrolysis
Acid dissolution.
REDOX reactions.
Chelation of polyvalent metals.
What is hydrolysis?
Minerals of the rock dissolving and leaching out of the rock.
What is acid dissolution?
Acidic water dissolves/reacts with rock minerals.
What is an example of a weathering redox reaction?
Fe2+ oxidising to Fe3+.
How does chelation of polyvalent metals cause weathering?
Citric acid from plant roots and microbes dissolving iron and aluminium.
Which minerals are the most susceptible to weathering?
Olivine
Ca-rich plagioclase
Pyroxene
Amphiboles.
Which minerals are the least susceptible to weathering?
Biotite
Na-rich plagioclase
K-feldspar
Muscovite
Quartz
How do temperature and rainfall affect the type of weathering that occurs?
High rainfall and high temperatures lead to high rates of chemical weathering.
Lower rainfall and low temperatures lead to high levels of mechanical weathering.
What is the rate of weathering proportional to?
The specific surface area (m² g⁻¹).
What is a positive feedback mechanism that occurs during weathering?
As the mineral weathers, the specific surface area increases which causes the weathering rate to increase.
What soil conditions do ultrabasic rocks cause?
High levels of magnesium, iron, nickel and chromium.
Low levels of calcium, potassium and phosphorus.
What plants can live in soils created by ultrabasic intrusive rocks?
Highly selective, unique, endemic floras.
What are zonal soils?
Well developed soils that reflect the influence of climate as the major soil-forming factor.
What are intrazonal soils?
Well-developed soils formed where some local factor is dominant.
What are azonal soils?
Soils that are immature and poorly developed.
