6.2 - Critical Zone Formation Flashcards
What forms the critical zone?
Weathering rock
What is the definition of soil?
Soil is the collection of natural bodies on earth’s surface containing living matter and supporting, or capable of supporting plants.
It’s upper limit is the atmosphere (air) or water, and its lateral margins it grades to deep water or barren areas of rock and ice.
It’s lower limit is normally considered to be the lower limit of the common rooting zone (root zone) of the native perennial plants, a boundary that is shallow in the deserts and tundra and deep in the humid tropics.
Pedogenesis = soil formation
What is pedogenesis
The formation of soil.
Late 19th century = Hillard (USA) and Dukuchaev (Russia) independently suggested that pedogensis is principally controlled by vegetation and climate.
What soils are most common throughout the UK?
- Inceptisols = barely developed soils
- Spodosols (aka podsols) = formed under coniferous forests
- Histosols = high in organic matter (eg. Peat bogs)
- Mollisols = Grasslands soils
What is the critical zone grouped into?
The CZ is grouped into horizons
O = Humus or organic (It is the topmost layer of decomposing leaves and lots of organic matter.)
A = topsoil (It is a mineral soil, which contains lots of roots, microbial activity, worms and minerals for growing plants
E = Eluviated layer or exit layer (Soils are well developed which show leached portion of ‘A’, the source region of clay Fe, Ai. Materials, minerals, organic matter, and clays exit the soil profile)
B = subsoil (It is highly weathered bedrock, with accumulation of clay/iron oxides from weathering. Minerals from upper horizons stop here.)
C = parent material (It is poorly weathered bedrock, with some accumulation of soluble products. It is the earth’s surface that soils develop from)
R = bedrock
What is CLORPT?
CLORPT is a mnemonic for Hans Jenny’s (1940’s) famous state equation for soil formation
Soil = f(cl,o,r,p,t)
- CLimate (primarily precipitation and temperature)
- Organisms (living biota, vegetation, micro- and macro organisms)
- Relief (topography: slope, aspect and landscape position)
- Parent material (geological or organic precursors to the soil)
- Time (duration of exposure of parent material
In general these processes DO NOT function independently
What is it called when one of the CLORPT influences is dominant?
In certain situations one of the soil-forming factors may exert a dominant influence of the formation of a set of soils.
Climosequence = when Climate influence is dominant
Biosequence = when organism influence is dominant
Toposequence = when relief influence is dominant
Lithosequence = when parent material influence is dominant
Chronosequence = when time influence is dominant
What are soil characteristic strongly influenced by?
Soil characteristics are strongly influenced by lithology and geological structure.
- Especially the chemistry and mineralogy of soils
How is parent material classified?
Soil “parent materials” are classified based on the initial location of soil precursors
Are they local (in situ) or transported from elsewhere?
1) In situ parent materials are classified as either residual or organic
Residual = rocks and minerals
Organic = plant debris
What is residual material?
It is inorganic parent material developed in place from weathering of underlying rock.
If the climate is warm and humid:
- Residual parent material is strongly leached and oxidised
- Bright red and yellow colours
Examples of this are Laterite and saprolite
Laterite = iron oxide/ clay layer rich in acid leached (but insoluble) elements like Fe, Mg, Si
Saprolite = it is isovolumetric, retains most original rock texture, but in warm/humid environments is oxidised, with the accumulation of soluble elements
What are laterites?
Laterite = iron oxide/ clay layer rich in acid leached (but insoluble) elements like Fe, Mg, Si
They are very common building material in sub-tropics/tropics. Hot and humid environments create oxidising conditions. This leads to red/orange rock from iron oxidisation
They are also a large source of aluminium, iron, and nickel ores
What is saprolite
Saprolite = it is isovolumetric, retains most original rock texture, but in warm/humid environments is oxidised, with the accumulation of soluble elements
Saprolite from a cool and dry environment = isovolumetric, it retains original rock texture and most of the chemistry. IT IS GREY
- Saprolite can be derived from granite in a dry, cold region
Saprolite from granite in a hot and humid environment = IT IS RED/ORANGE
Saprolite behaves differently in environmental conditions. Deep weathering causes many secondary/supergene ores of economic interest in subsoil horizons
What are examples of secondary/supergene ores?
- Bauxite And iron ores in laterites
- Saprolitic gold, supergene copper, uranium and many heavy metals in saprolites
Where is lots of peat formed across the globe?
Peat is most extensive in cool climates where precipitation is > evaporation and where drainage is poor.
About 75% of the worlds peat lands occur in northern Canada and Russia.
Organic parent material/deposits = peat
How is peat formed?
It is formed in wet places with poor drainage, and where decomposition of plant debris limited due to the lack of O2
(Organic matter accumulates in the soil without decaying due to lack of oxygen)
What are the 4 types of peat?
Moss peat = derived mainly from sphagnum, but also from brown mosses
Herbaceous peat = from sedges, reeds, and cattails
Woody peat = woody plants, including trees and shrubs
Sedimentary peat = aquatic plants, including algae and fecal matter from aquatic animals
What do the types of peat depend on?
The type of peat depends on the type of original plant material.
Most organic deposits have > 1 type of peat because of ecosystem succession. If a peat drains out, decomposition happens quickly.
Is peat useful for agriculture?
Peats can be used for acrigulte and is very productive in the short term (especially for root veg) but in the long run it is unsustainable due to the lack of residual.
It is also likely to release a lot of co2 which is not good for climate change.
What are uncommon soil forming regions (unless uplifted)
Lake deposit = lacustrine
Ocean deposit = marine
- Many of the minerals that make up these sedimentary materials are chemically stable and resistant to weathering, which further limits the availability of nutrients for plant growth. The lack of plant growth and organic matter accumulation means that these sediments are not able to form the complex soil profiles that are found in upland environments.
- However, if these deposits are uplifted and exposed to the atmosphere, they can be subject to weathering and erosion, which can gradually transform them into soils over time. The uplift and exposure of these sediments can also create new habitats for plant growth, which can further enhance soil formation processes.
What are common soil forming regions?
Streams = alluvial (fluvial)
Ice = till (moraine)
Wind = eolian
Transport from gravity = colluvial
What characteristics do colluvial debris have?
By gravity.
- caused mainly by physical weathering (eg. Then falls off cliff)
- Coarse, rock and angular
- Particle sizes interspersed (not layered)
Eg. Scree at the bottom of a cliff
What is colluvium soils?
Colluvium soils are well-drained (rock piles leave voids)
Colluvium soils are prone to future landslides
They contain mix of rock sizes and angularity fragments
Where are alluvial deposits found?
At floodplains, alluvial fans and deltas.
Floodplains have nutrient rich deposits which are good for agriculture.
Alluvial fans = form where stream channels emerge from steep canyons/valleys and are common along the base of arid/semi arid mountains. (Can find alluvial fans on mars)
Deltas = they are like alluvial fans but are underwater. Both alluvial fans and deltas form as velocity of flow drops and sediment is deposited. (Eg. Nile river delta)
Glaciation on earth in the past
The last glaciation covered around 20% of earths land surface - Mostly N and C Europe. All of Canada and top bit of US (Ice age N America)
(St Andrews had 1km of ice over it only 15,000 years ago).
As the glacier retreats, they leave behind ‘glacier drift’ deposits. This provides new parent material for soil formation.
- Glacial soils depend on the parent material that is transported.
What does the term drift mean for glaciers?
General term for glacial parent material (deposited by ice or associated water)
What is till?
Till is more specific and is deposited by ice.
Till has many similarities to colluvium but till is more rounded and more compacted
- Till is heterogenous, containing a random assortment of particles, ranging in size from boulders to clay but generally rounded clasts.
What is outwash?
It is deposited by meltwater
- Stratified and particle sizes are sorted
Wind transported parent material (aeolian or eolian deposits)
US Midwest during the 1930’s’, prolonged drought and poor agricultural practices led to much of the topsoil being blown away.
Today, huge amounts of aeolian material comes from china and the Sahara desert.
What are the different types of aeolian transported parent material?
Dune sand = mostly quartz but impurities add nutrients
Loess = silt with very fine sand and clay
Dust - aerosilic dust
Volcanic ash
What are the global distributions of loess and dune sand?
Loess can be blows for 100’s of kilometres. The deposits farthest from the source are thinnest and consist of the finest particles. Much of the loess deposits source from barren expanses of till and outwash left when glaciers retreated at the end of the last ice age. Up to 8m think in North America.
Dune sand can be found in places such as North Africa and Australia.
What are characteristics of loess
- They are mostly silt and some are very fine sand and clay. The clay binds together but is vulnerable to rain.
- They form silty soils (silty loam)
- In china, there are loess deposits of 30-100m thick
The wind transported material from deserts of Asia.
What is aerosolic dust
Particles sizes are 1-1o microns and are transported 1000’s km
They generally do not blanket landscapes like loess
Their particle composition is variable (Eg. SiO2, CaCO3 etc.)
Dust from the Sahara desert is a hugely important source of nutrients to Caribbean islands and amazon basin.
(Sahara dust storms can cross the Atlantic Ocean)
Volcanic ash soils
Ash soils form within a few 100s km of a volcano
Ash soils (Andisols) are very light, porous and accumulate organic matter more rapidly than other soils nearby
Ash weathers quickly and provides lots of nutrients.
What are soils classified based on?
Soils are classified based on silt/sand/clay percentages
(Triangle to measure each percentage of silt vs sand vs clay)
