Unit 2.2 Earth Flashcards
Describe how igneous rocks are formed.
Cooling of magma, underground (intrusive) or on the surface (extrusive).
List the principal elements that make up the Earth’s crust.
- oxygen (45%)
- silicon (27%)
- aluminium (8%)
- iron (6%)
- calcium (5%)
- sodium
- potassium
- magnesium
Describe how fragmentary (sedimentary) rocks are formed.
Accumulation of sediments, which undergo lithification through compression and cementing.
Describe how metamorphic rocks are formed.
Igneous or sedimentary rocks which are subjected to extreme heat and pressure, to the point at which their constituent minerals reorganise.
What are the distinguishing features of igneous rocks?
Interlocking crystals with random orientation.
What are the distinguishing features of sedimentary rocks?
Grains which may be cemented but not interlocking.
The presence of fossils would suggest fragmentary rock, but may not be present.
What are the distinguishing features of metamorphic rocks?
Interlocking crystals arranged into bands / oriented in the same direction.
Name the major rock forming silicate minerals, describe their structures at a molecular level, and account for the place of metal ions in the crystal structure.
Mafic minerals have simpler structures and contain Mg, Fe (and Ca), while felsic minerals are more complex and may contain Al, K and Na.
Quartz comprises silicon and oxygen molecules arrange in a 3D structure.
Feldspar also has a 3D structure. All feldspars contain Al, but orthoclase feldspar is rich in K. Plagioclase feldspar ranges from Na- to Ca-rich (mafic).
Mica has a 2D sheet structure. It ranges from Al-rich to Mg- or Fe-rich.
Amphibole contains Mg, Fe, Ca and Al, arranged in a 2D double chain.
Pyroxene contains Mg, Fe and Ca in a 1D singe chain structure.
Olivine contains Mg and Fe in isolated groups.
Place the major silicate-based rock-forming minerals in order of melting.
Felsic rocks have lower melting points, and so partial melting produces a magma which is more felsic than its source.
- quartz
- feldspar
- mica
- amphibole
- pyroxene
- olivine
Place the major silicate-based rock-forming minerals in order of formation from magma.
Mafic minerals crystallise at lower temperatures than felsic minerals, so forming magma earlier.
- olivine
- pyroxene
- amphibole
- feldspar
- mica
- quartz
Place the major silicate-based rock-forming minerals in order of susceptibility to weathering.
Mafic minerals, with their simpler structures, are the most susceptible to weathering.
- olivine
- pyroxene
- amphibole
- feldspar
- mica
- quartz
What is orthoclase feldspar?
Feldspar which is rich in potassium.
What is plagioclase feldspar?
Feldspar which ranges from Na-rich (felsic) to Ca-rich (mafic).
What rock is formed as a mafic magma cools within the Earth?
Gabbro.
What rock is formed as a mafic magma cools at the surface?
Basalt.
What rock is formed as an intermediate magma cools within the Earth?
Diorite.
What rock is formed as an intermediate magma cools at the surface?
Andesite.
What rock is formed as a felsic magma cools within the Earth?
Granite.
What rock is formed as a felsic magma cools at the surface?
Rhyolite.
Describe conglomerate in relation to fragment size and composition.
Rounded, pebble-sized fragments of rock, above 2mm in diameter.
If the rock is composed of angular fragments, it is termed breccia.
Describe sandstone in relation to fragment size and composition.
Feldspar, quartz grains and mica flakes, between 2mm and 0.63mm.
Describe mudstone (shale) in relation to fragment size and composition.
Grains below 0.063mm in diameter. Predominantly clay minerals.
Describe the different types of limestone and explain the origins of their constituents.
Oolitic limestone is formed of lithified ooids: tiny nuclei, potentially grains of sand or shell fragments, which attract a build-up of calcium carbonate.
Shelly limestone is formed from the calcium-rich shells of marine creatures.
Crinoidal limestone is formed from crinoids, a marine animal.
Outline the general structure of kaolinite, and (in general terms) its formation from other silicate materials.
Kaolinite is a 1:1 clay. Every silicon-oxygen tetrahedral sheet (SiO4) is stacked on an aluminium-oxygen octahedral sheet. The interlayer between each pairing is negligible.
Kaolinite is the most widespread clay, and is the residual end product of weathering: when feldspars are dissolved by aqueous carbon dioxide, they release metal ions and soluble silica, leaving behind kaolinite.
Outline the general structure of illite, and (in general terms) its formation from other silicate materials.
Illite is a 2:1 clay. Each aluminium-oxygen octahedral sheet is sandwiched by two silicon/aluminium-oxygen sheets. The interlayer space is larger than kaolinite, and contains K+ ions, which serve to hold the layers together.
Illite is produced from the weathering of mica. Illite may also be produced when kaolinite is subjected to heavy concentrations of metal ions, as in seawater. As such, illite is the major clay mineral of mudstones.
Outline the general structure of montmorillonite, and (in general terms) its formation from other silicate materials.
Like illite, montmorillonite is a 2:1 clay. Each aluminium/magnesium/iron-oxygen octahedral sheet is sandwiched by a silicon-oxygen tetrahedral sheet.
It is the most complex of the clays. The interlayer is larger than illite, and contains various cations: Na+, Mg2+ and Ca2+. The larger interlayer allows more free exchange of cations with outside solution, and also allows water to enter, causing swelling of the clay.
Montmorillonite can be created by the chemical weathering of mafic rocks.
Describe how ores are formed by separation from magma.
When minerals like olivine and pyroxene crystallise early from hot basaltic magma they are sometimes accompanied by dense non-silicate minerals containing iron, chromium and titanium.
These minerals also sink to the bottom of the magma chamber. When the magma solidifies, layers of ore are found at the base of the magma chamber.
Another way in which minerals can separate from sulfur-rich basaltic magma is as liquid sulfides. Molten iron sulfide forms blogs of liquid that also settle to the bottom of the magma chamber, like blobs of water in oil. This liquid, which is even denser than the first crystallising silicates, also dissolves some metals more effectively than does magma, in particular copper and nickel. The liquid cools and solidifies to form layers of copper, iron and nickel sulfides.
While elements like chromium, nickel and titanium are segregating out, magma is consequently being relatively enriched in other minerals, including tin and tungsten.
Describe how ores are formed by hydrothermal fluids.
At high pressures associated with depth, magma contains a considerable amount of dissolved water, but as the anhydrous minerals, like olivine and pyroxene, crystallise, water separates and moves to the top of the magma chamber. Water at high temperature and pressure is an effective scavenger of metal ions from the magma. It also extracts metals from the surrounding rocks as it comes into contact with them, bringing about the process of alteration. As this hot, aqueous hydrothermal fluid approaches the Earth’s surface it cools, gases may be given off, and it infiltrates cracks in the colder surrounding rocks.
Explain the processes, agencies and results of physical weathering.
- as rocks approach the surface through uplift, pressure reduces and the rocks expand, causing splitting known as jointing.
- freeze-thaw weathering occurs as water infiltrates gaps, and then freezes; as ice has a greater volume than water, the gap is pushed apart
- attrition is the physical breakdown of rock fragments in transport
Explain the processes, agencies and results of bioweathering.
Plants (including algae and lichens) growing, and decaying, on rocks.
- roots will release organic acids
- respiration produces carbon dioxide, which dissolves in water to produce an acid (carbolic acid)
Explain how water can become acid thus enhancing chemical weathering.
Carbon dioxide dissolves in water to produce ions of hydrogen and hydrogen carbonate.
H2O(l) + CO2(g) = CO2(aq) = H+(aq) + HCO3-(aq)
Describe the dissolution and repricipitation of calcium carbonate during weathering.
Carbon dioxide dissolves in water to produce an acid. This acid dissolves carbonate rocks, producing the highly soluble calcium hydrogen carbonate: similar to limescale. The moving water carries the calcium until it evaporates, causing the calcium to remprecipite. The result is (e.g.) stalactites and stalagmites.
Outline the weathering process for silicate-based rocks and sulphite ores.
The free H+ ions (produced when CO2 dissolves in water) react with silicate minerals to produce silicic acid: Si(OH)4
Mg2SiO4 + 4H = 2Mg + Si(OH)4
Describe the changes experienced by metallic elements during the weathering of silicate-based rocks.
They are released as ions.
Iron may become oxidised.
Describe the changes experienced by silica during the weathering of silicate-based rocks.
Silica is released as silicic acid (soluble silica).
Outline the formation of hydrated oxide clays, and the conditions necessary for their formation.
Hydrated oxide clays are oxides of iron, aluminium and manganese which that are formed by the reaction of these elements, in ionic form, with water and oxygen.
Hydrated oxide clays are found in highly weathered soils, especially in the tropics, under climatic conditions of high temperatures and heavy rainfalls. The heavy rainfall leaches out the more soluble cations, leaving behind resistant Fe, Al and Mn compounds.
Explain why the course of weathering is dependant on climate and latitude, and outline the differences observed.
Intensity of weathering is related to temperature and precipitation.
As chemical reactions generally occur more rapidly at higher temperatures, more intense weathering is observed in tropical regions, especially as levels of precipitation are also high in these climates.
Describe the three components that comprise a soil texture.
Proportion of: sand, silt and clay. Each defined by particle size.
Sand particles are between 2.0 to 0.02mm large
Silt particles are between 0.02 to 0.002mm large
Clay particles are less than 0.002mm in size
Outline the overall physical and chemical properties that sand, silt and clay contribute to soil.
- water-holding capacity and drainage rate
- susceptibility to wind and water erosion
- cohesion
- inherent fertility
- ease of pollution leaching, and ease of compaction
Sand has a low water retention, due its fast drainage rate. It is moderately vulnerable to wind erosion, but not susceptible to water erosion. Sand has little inherent fertility.
Silt has moderate water retention and drainage. It is highly vulnerable to both wind and water erosion. It has a moderate inherent fertility.
Clay has a high water-holding capacity, and low drainage rate. It is moderately susceptible to water erosion, but resistant to wind erosion. Although it holds pollution, it has high inherent fertility.
Explain the physical properties that soil organic matter imparts to a soil.
Fungal strands and fine roots help to bind the soil together, reducing compaction and allowing water to pass to the lower layers.
Humus acts as a cement between particles of clay, silt and sand, binding them together. Organic matter-rich soils are generally less vulnerable to erosion, and allow for easy root penetration and seedling emergence.
Organic matter also enhances water holding efficiency indirectly, through its effect on soil structure.
Worms and other detrivotes help to mix soils.
Explain the chemical properties that soil organic matter imparts to a soil.
The decomposition of organic matter releases nutrients into the soil, along with carbon dioxide, which increases acidity and enhances weathering.
Soil organic matter is the primary source of nitrogen, and a major source of phosphorus and sulfur to plants. In addition, organic matter provides energy and carbon for soil organisms, and its decomposition releases beneficial vitamins and amino acids.
Describe the effects of cultivation on the organic matter content of a soil.
Cultivation greatly reduces the amount of organic matter. This reduction is due to removal of plant material by harvesting or weeding, and to increased microbial decomposition caused by greater soil aeration.
When a virgin soil is first cultivated, there is a rapid decline in organic matter content, especially in tropical ecosystems. However, the organic matter content eventually stabilises at a new, lower level.
Describe the effects of climate on the organic matter content of a soil.
Organic matter is greatest in cool, moist regions and lowest in warm, dry areas such ass deserts.
Describe the effects of vegetation on the organic matter content of a soil.
Vegetation with extensive rooting systems, such as grasses, provide more soil through organic matter than do forests, which contribute organic matter mainly through above-ground leaf litter.
Explain how isomorphous substitution occurs in clay minerals and the type of charge it generates.
Isomorphous substitution occurs when ions of a similar shape and size are swapped between the soil solution and clay sheets. It occurs most readily in 2:1 clays, as they have a more open structure, and it imparts upon the clay a negative charge which is not affected by pH.
The most common exchange is between Al3+ (solution) and Si4+ (clay), or between Mg2+/Fe3+ (solution) and Al3+ (clay).
Explain how pH-dependant charge is generated in clay minerals, humus and hydrous oxide clays.
The broken edges or surfaces of clay and humus contain hydroxy (-OH), phenolic (-OH) and carboxylic acid (-COOH) groups. In these groups, the hydrogen atom is only weakly bonded and, as electrons are distributed towards the O, they are partially positively charged.
If the soil solution has a low H+ content (i.e. it has a high pH), then the hydrogen from these groups will be attracted into solution, leaving behind a negative charge.
pH dependant charge occurs more in 2:1 clays, which have more broken surfaces than kaolinite.
Define cation exchange capacity.
The ability of a soil to adsorb and exchange cations with the surrounding soil solution.
Clay and humus particles hold a negative charge, and so attract cations from the soil solution. These cations surround the particle; some in the spaces between the layers of clays, others in a shell or halo around the clay or clay-humus particle. Together, these achieve electrical neutrality.
Describe the factors that contribute to cation exchange capacity in a soil.
- charge density: amount of negative charge per area, both permanent and pH dependant, of the clay or humus particle; this is greater in 2:1 clays
- amount of surface area exposed to soil solution; i.e. loosely layered montmorillite has a greater surface area than tightly-layered illite
- humus particles are highly disorganised, with a large concentration of freely exposed phenolic and carboxylic acid groups, and have the highest CEC of any soil constituent
Name the factors that affect the strength of adsorption of different cations to soil exchange sites.
Charge: Al3+ > Ca2+ > Mg2+ > NH4+ > K+ > Na+
Relative abundance of cations. If one cation is present in abundance, then its adsorption would increase. For example, liming a soil will displace Al3+ and H+ with Ca2+ even though it has a weaker charge.
Describe the differences between exchangeable and non-exchangeable cations in a soil.
An exchangeable cation is held by the soil, but remains mobile and can be absorbed by plant roots.
Non-exchangeable cations are not mobile, and so unavailable to plants. For example, heavy metals such as copper, zinc, lead, iron and nickel, can form strong bonds to the oxygen and hydroxl ions at the surfaces of clays and organic matter.
Calculate cation exchange capacity given the molar composition of a CEC percolate solution.
Units: moles of charge (molc) per kilogram: molc/kg
Total CEC is the sum of the moles of each ion multiplied by its charge.
E.g. 2 mol of Na+ in 1kg is 2 molc/kg
E.g. 2 mol of Mg2+ in 1kg is 4 molc/kg
E.g. together, total CEC is 6 molc/kg
Given full soil chemical analysis including LOI, pH, vegetation type, and exchangeable ion concentrations, calculate CEC.
…
Given full soil chemical analysis including LOI, pH, vegetation type, and exchangeable ion concentrations, calculate base saturation.
Base saturation = ( molar charge of base cations / total exchangeable molar charge ) x 100
The base cations are Ca2+, Mg2+, K+ and Na+. Though the ions are not base, the oxides of these elements form soluble hydroxides that dissociate in solution to generate hydroxide anions (OH-).
Define base saturation of soils.
The ratio of the basic cations Ca2+, Mg2+, K+ and Na+ to the total CEC.
How do CEC and base saturation relate to soil fertility and resistance to acidification?
… P264
Define anion exchange capacity.
The capacity of a soil to retain anions.
Describe the types of soil which have the highest anion exchange capacity.
Soils with a low pH and a high proportion of hydrous oxide clays, such as highly weathered tropical or semi-tropical soils, can have substantial anion exchange capacity.
Describe the relationship between soil water content at field capacity and soil texture, and explain the motives for this relationship.
… P270
Describe the variables in Darcy’s law as they pertain to calculating the speed of water as it moves down a slope through unsaturated soil.
Speed = gradient multiplied by hydraulic conductivity.
Gradient is height divided by distance.
The hydraulic conductivity is the ability of the soil to conduct water, and is measured as a speed (m s-1). This is greatest in sandy soils.
Outline the major chemical components one mght expect to find in soil air.
- oxygen
- carbon dioxide
- methane
- nitrous oxide
Describe the major chemical components one may expect to find in the solution of a soil.
- ions derived from atmospheric deposition
- carbon dioxide from root respiration
- dissolved organic nutrient ions
- dissolved inorganic ions from weathering
Explain the difference between particle density and mass density.
…
Given soil porosity and particle density, calculate mass density.
… P275
Mass per volume of the different particles in the soil.
Given soil porosity and mass density, calculate particle density.
…
Describe the consequences of waterlogging soil in terms of soil gas and soil solution chemistry.
In a waterlogged soil, all pore space is filled with water. As a consequence, there is no oxygen within the soil for plant roots or respiring bacteria.
In the absence of oxygen, bacteria may begin to reduce nitrates into nitrogen, or Fe3 into the more toxic Fe2.
Describe and explain how the O, A, E and B horizons form
(O) Organic layer:
(A) ..
(E) ..
(B) ..
Briefly outline the chemical and physical properties used to classify soils.
… P283
Give examples of soil orders in either the USDA or British systems.
A brown soil is a well-mixed soil associated with broadleaf woodland. There is little apparent layering, as the soil supports a healthy population of worms, moles etc.
A podzol is a poorly mixed soil associated with coniferous forest: below a thin dark layer of organic matter lies a sandy layer, leached of colour by acid, sitting above a ‘pan’.
Briefly outline five buffer ranges that may be found in soils.
A solution is said to be buffered if its pH remains constant even though small amounts of acid or base are added to the solution.
Carbonate:
Silicate:
Exchangeable cation:
Aluminium:
Iron:
< … >
Describe how growing vegetation can acidify soil.
Organic acids are leached from roots and hydrogen ions are actively pumped into the soil solution by roots.
Decomposition also releases high concentrations of carbon dioxide, which forms an acid in solution.
P290
Describe the types of nutrient pools in soils.
… P295
Describe factors that influence the availability of nutrients in a soil.
… P296
Describe the concept of limiting factors.
Plants require a range of nutrients and micronutrients to function correctly, and a shortfall of any single one of these will limit growth, no matter how plentiful the others.
In this case, only an increase in the limiting factor will improve growth.
Outline some of the major consequences of soil loss through erosion.
Loss of productive land:
- removal of organic material and fine mineral particles
- reduced cation exchange and water holding capacity
- reduced biological activity
Damage to water courses:
- disruption of aquatic ecosystems (eutrophication)
- sedimentation of lakes or reservoirs
- contamination of drinking water
Wind-borne particles:
- may be hazardous to human health
- may create hazardous driving conditions
Outline the factors that make a soil more susceptible to erosion.
Disruption of vegetation, e.g. by cutting forests on hillsides, or through overgrazing or excessive tillage in arid or semi arid regions, can leave souls vulnerable to erosion by wind or water.
- intensity of rainfall or wind
- vegetation cover
- organic matter in soil
- slope
Describe the process by which soils become saline and how they may be remediated.
Soils containing an excess of soluble salts are said to be saline.
Saline soils occur when salt-rich groundwater flows above an impermeable layer. The water is either evaporated or transpired through pants, leaving the salts behind. As salt accumulation increases, the surface may become covered with a white salt crust.
Saline soils can be remediated through:
- improving drainage so that water can escape through pathways other than evaporation and transpiration
- excess sodium can be remediated through application of calcium
- salts must be leached from the soil
Describe gneiss, relate its nature, appearance and grain size to the conditions of metamorphism.
Large crystals suggest that it was formed in conditions of intense heat and pressure.
Describe schist, relate its nature, appearance and grain size to the conditions of metamorphism.
Crystals of an intermediate size suggest that it was formed in conditions of intermediate heat and pressure.
Describe slate, relate its nature, appearance and grain size to the conditions of metamorphism.
Small crystals suggest that it was formed in conditions of less intense heat and pressure.
