Types of Rocks
Igneous rocks
They form under great heat and pressure, either intrusively or extrusively. Their formational environment does not contain water. Also they are crystalline, with complex mineral content.
Sedimentary rocks
These form from sediments near the earth’s surface, and in the presence of water. They occur as layers (strata) and may contain fossils. In addition, they have a relatively simple chemical and physical composition.
Metamorphic rocks
These maintain characteristics of the parent material
What is Weathering?
Weathering is the decomposition (via chemical weathering) and disintegration (via mechanical/physical weathering) of rocks in situ. Weathering is important for landscape evolution as it breaks down rock and facilitates erosion and transport.
Weathering produces a layer of rock fragments overlying the solid bedrock and soil known as REGOLITH.
The processes of mechanical, chemical and biotic weathering breaks down rocks into smaller pieces in a number of ways:
1. Shattering: this is when a rock breaks into irregular, angular fragments
(such as on a scree slope).It is mainly caused by mechanical weathering 2. Granular Disintegration: this is when a rock breaks down into individual
grain particles (example, sandstone sometimes disintegrates into the
individual quartz crystal from which it is formed) 3. Exfoliation: this is when surface layers of rocks become loose and ‘peel
off’. The result can be a rounded dome of rock that can look like a partly
peeled onionGranular disintegration and exfoliation can be caused by either mechanical or chemical weathering (often it is a combination of both).
Mechanical weathering increases the surface area on which chemical weathering takes place. Chemical weathering in turn promotes further mechanical disintegration.
The balance between both processes depends to a large extent on the nature of the rock and the climatic conditions.
Weathering vs Mass Movement vs Erosion
- Weathering refers to the decomposition of rocks ‘in situ’.
- Mass movement describes the process when gravity causes the downslope movement of the weathered material.
- Erosion is the removal and transport of weathered material by agents of erosion, example wind, rivers, waves
The overall process of wearing down the land (by all three processes) is known as DENUDATION.
Types of Mechanical Weathering
- Freeze-thaw (Ice Crystal Growth)
When water freezes, it expands by approximately 9% in volume. If water collects in cracks and pore spaces in rocks and then freezes and thaws over many times, the effect is to cause ‘fatigue failure’ (the rock eventually cracks and shatters).
Freeze thaw action is most effective when there are frequent changes of temperature above and below freezing. Where there is a diurnal (daily) cycle of freezing at night and melting in the day, the weathering process is more active
than if there are only seasonal temperature changes.
The composition, structure and strength of rock affect how it resists frost shattering. Rocks like granite have a higher tensile strength (resistance to
being pulled apart) than rocks like sandstone. Therefore rocks like sandstone are more prone to freeze thaw weathering than granite.
The permeability of the rock is also an important factor. Water must penetrate into the rock and be trapped by initial surface freezing for maximum pressure to be exerted. Therefore freeze thaw is more effective in well jointed, sedimentary rocks (e.g. Limestone)
- Salt Crystal Growth
This form of weathering occurs when salt accumulates (mostly occurs in arid and semi-arid regions). In hot, dry climatic conditions, evaporation and capillary
action can draw the water which contains mineral salts upwards and deposit a layer of salt on the rock surface. They can also be deposits salt spray inland from coastal areas. When salt crystals form in surface rock pores, they exert
pressure as they grow (sometimes up to three times their original size). The pressure exerted can weaken and eventually shatter rock. - Exfoliation
In hot desert regions, there can be a daily temperature range of more than 40°C, as skies are generally cloudless and daytime insolation is intense while at night the heat escapes into the atmosphere. This causes the surface layer of rocks to heat and expand in daytime and contract at night when it is cooler. As rock is a poor conductor of heat, stresses occur only in the outer layers and cause peeling or exfoliation to occur. Within the rock, there is also differential expansion and contraction because some crystals which have different colours and chemical compositions heat up and expand faster than others. - Pressure Release (Sheeting)
Rocks deep within the Earth’s crust are under massive pressure from the weight of overlying layers. If this overlying material is removed by erosion there is ‘pressure release’ and the buried rocks expand in volume. This can cause cracks to form parallel with the surface in a process called SHEETING. For example, granite is formed deep underground, under great pressure and is therefore particularly prone to sheeting. Once the layers of rocks and cracks are exposed to the atmosphere, other weathering processes (physical and chemical), start to work.
Types of Chemical Weathering
During chemical weathering, rocks are decomposed. Their internal mineral structure is altered and new minerals are formed.
Water is important in the process, as it plays a direct part in some chemical reactions and in others it transports the elements that do the work. Therefore this process is least effective in deserts and polar regions where there is little
rainfall or where the water is frozen. In general, chemical reactions are faster in high temperatures than in low temperatures, and as such, chemical weathering is most active in equatorial zones that have hot, humid climates.
For most rocks, the potential for chemical weathering increases with acidity and acidity arises in three main ways:
1. Rainwater combines with carbon dioxide in the atmosphere to form dilute
carbonic acid
2. Where air pollutants such as sulphur dioxide or nitrogen oxides are present
the effect can be to create dilute sulphuric acid or nitric acid.
3. When rainwater washes into the soil, it combines with organic acids that are
formed by decomposing vegetation.The main types of chemical weathering are:
- Hydrolysis
This is a chemical reaction between water and a mineral. Wetting the mineral causes a chemical exchange to occur in which hydrogen ions from the water displace ions in the mineral. For example, granite consists of feldspar which when exposed to weakly acidic rainwater, decomposes. - Hydration
This process occurs when minerals absorb water. It causes a chemical change and also a physical change in that the mineral swells which weakens the rock.
For example: when water comes into contact with anhydrite to form gypsum. The gypsum is relatively soluble in water and is washed away. - Carbonation
This process is a chemical reaction that occurs when rainwater (dilute carbonic acid) reacts with calcium carbonate to form calcium bicarbonate. This is soluble
and is carried away by water. Carbonation (and solution) is most effective in areas consisting of limestone. - Oxidation
This process occurs when a mineral combines with oxygen which might come from the air or it might be dissolved in water. Iron is particularly prone to this type of weathering, as it readily oxidises and occurs in many rocks. Oxidation changes blue-grey ferrous iron compounds to ferric compounds that are rusty red. The process weakens the mineral structure and also makes the rock more vulnerable to other forms of weathering. - Chelation
This process occurs when rocks and soil weather through the action of organic acids. These acids, for example humic acid, are produced by bacterial action when vegetation decomposes. In the chelation process, rainwater mixes with organic acids and then combines with aluminium and iron in the rock . These metals are washed out of the soil as rainwater percolates through.
Biotic Weathering
This is caused by the action of plants and animals (biota).It can be a mechanical process (via plants roots) or a chemical process (via organic acids from decaying
vegetation).
- MICRO-ORGANISMS
Micro-organisms such as lichens, algae, fungi and bacteria grow directly on bare rock and can cause weathering to occur. They attach to the rock and extract nutrients in dissolved form. This produces holes in which the organisms can live,
gaining protection from the elements and from predators. This ‘pitting’ of the rock surface increase the surface area exposed to attack from other forms of weathering. - PLANTS
Some plants send roots deep below the ground surface that start as fine hairs penetrating pore spaces and joints and enlarge as the plant grows. This widens cracks and forces rock apart. Trees can exert massive force in this way. Sometimes when trees fall or blow down, huge amounts of rocks are loosened.
Weathering by root systems is relatively important in areas where most plant biomass is below the ground surface. - ANIMALS
Animals can weather rocks directly or indirectly. For example, burrowing animals open up cracks and joints in soil and in weathered rock fragments. This exposes the rock to weathering and allows increased percolation of rain water. Animals also mix rotting plant material into the soil. As this material decomposes, organic
acids are formed which can seep through the soil and chemically weather the bedrock.
Factors Influencing the Weathering Process
- Characteristics of the bed rocks (rock type and structure)- hard or soft, soluble or insoluble. Joints increase the surface area of rock exposed to physical and
chemical weathering - Climatic elements (precipitation, temperature)
- Dry, cold climate: physical weathering dominated
- Wet and warm climate: chemical weathering - Geographic orientation- slope face orientation (aspect) controls the slope’s exposure to Sun, wind, and precipitation
- Vegetation
- Ground water and water movement
Slopes
The term slope refers to an inclined surface or hillslope. Slopes can be seen as examples of open systems. Inputs include energy (insolation) and mass (water and sediments). Outputs include energy (re-radiated heat) and mass (water, regolith).
The profile of the slope creates a store of potential energy, due to the difference in height between the crest and the base of the slope. This potential energy is converted into kinetic energy through mass movement and erosion.
Slope Development
There are some models to help explain slope development. One such model is one by A. Woods, which divided the slope into straight segments and curved segments. Woods suggested that slopes were made up of 3-4 units which contained a scree slope developing beneath a free face (cliff).In humid areas, an upper convexity and a lower concavity developed, caused by weathering at the crest and transport of fine material to the base of the slope.
Factors controlling Slope Development
- Rock Type
- Primary control of slope steepness because the angle that a particular rock
can support depend on its shear strength
- Primary control of slope steepness because the angle that a particular rock
- Structure
- This is dependent on whether the rock strata dips inwards (results in a stable
slope) or if the strata dips downslope (results in slope instability as masses of
rock can readily slide over the bedding planes)
- This is dependent on whether the rock strata dips inwards (results in a stable
- Earth Movements
- This can influence the formation of fault scraps due to uplifting - Climate
- Influences the rate and type of weathering, which in turn, influences the
thickness of the slope regolith. It will also influence the rate of slope
transportation processes like rainwash and mass movement - Vegetation
- This can influence weathering and slope transport.
More vegetation would stabilise regolith and maintain
slope steepness; sparse vegetation will result in the
dislodgement and transport of soil particles by rainfall
and runoff
What is Mass Movement
Mass movement is the downhill movement of regolith (weathered material), caused by gravity. The movement might be a slow creep or a fast landslide.
The material which moves might be rock, soil or mud (or a combination of all three). New material is added to a slope by the weathering of bedrock and by the downhill movement of material from further upslope. The material eventually accumulates at the foot of the slope or, more often, it is removed by the agents of erosion (waves, rivers or glaciers)
Downslope movement depends upon the balance between two
forces:
1. Shear stress: the downhill pull exerted by gravity and the weight of material. (slide)
- Shear strength: counterbalancing this is shear strength, which is the
resistance to downhill movement. (stick). It depends upon friction and cohesion.
Most rock and soil particles have rough surfaces that increase the friction between them. In addition, their shapes might help them interlock like jig-
saw pieces. Also, particles of clay within slope material carry an electric
charge that increase cohesion (the clay ‘glues’ material together and therefore increase shear strength)
If shear strength > shear stress, mass movement does not occur. If shear stress > shear strength, slope failure occurs and material starts to move downhill.
Factors affecting Slope Failure
- Angle of slope (Steep vs. Gentle slope)
- The amount of water present (Dry vs Saturated areas)
- The vegetation cover (Presence of trees)
- Earthquakes
Types of Mass Movement
- SLIDE
A slide is a movement along a ‘slip plane’ (a line of weakness in the rock or
soil structure).The moving material slides downslope along the slip plane, more or less intact, until it reaches the bottom of the slope where impact usually breaks it up. A slide of mixed rock and soil is called a debris slide while when a slab of solid rock break loose along a bedding plane or line of weakness it is known as a rock slide.
A stump (also known as a rotational slip or slide) is a type of slide that involves a rotational movement along a curved plane.
- FLOW
A flow consists of mixtures of rock fragments, mud and water that move down slope as a viscous (thick) fluid. Speed of flow can range from very slow (1 cm per day) to very fast (100 metres per second).The greater the water content, the greater the velocity, as water decreases the friction between particles and
increases the weight of the material. They cause a ‘scar’ where the flow starts and a ‘lobe’ of material that spreads out at the base of the slope. Mudflows consist of high percentage of fine silt and clay sized particles. They tend to occur when the ground is saturated after sudden thaws or heavy rain.
The high water content (up to 30%) causes clay and other soil materials to reach their ‘liquid limit’ (the point at which they act as a liquid).Mudflows are most common in areas with sparse vegetation cover and where torrential rainstorms occur. Lahars are a type of mudflow that occur when water washes away unconsolidated ash and dust on volcanic slopes. They create torrents of mud capable of washing bridges, vehicles and buildings away.
- CREEP
This is a very slow, almost imperceptible downslope movement of soil and rock particles.
Signs include:
- Build up of soil behind walls
- Telegraph poles and fence posts on a slope start to tilt downwards as soil builds up behind them and is removed from in front
- Formation of step like features called terracettes
Soil creep is caused by expansion and contraction in the surface layer of regolith, mainly due to two processes: wetting and drying and freezing and
thawing. When soil particles become wet, they expand in volume. They push up from the slope at a perpendicular angle. Then, when they dry, they contract in volume and sink down vertically. The result is a zig-zag downhill motion for the soil particles. The same process occurs when soil moisture freezes and expands. Material is lifted and then moved downslope when the ice thaws.
- FALL
It occurs after the rock has been loosened up then it acted on by gravity which carries the rock downslope which produces scree or talus (unconsolidated rock)
Freeze thaw is the main agent that is responsible, so falls are more common in areas which experience cold conditions at least some of the time.
What is hydrology?
It is the study of the Earth’s water molecules and their movement through the
hydrological cycle.
The hydrosphere
It is the “realm of water in all its forms and the flows of water among oceans, land and the atmosphere”. Water does not come into or leave planet earth as it is continuously transferred between the atmosphere and the oceans. This is known as the global hydrological cycle. This system is a closed system, as there are no inputs or outputs.
The hydrological cycle
The hydrological cycle, also known as the water cycle describes the continuous
movement or circulation of water on, above and below the surface of the Earth.
Water can change states among liquid, vapour, and ice at various places in the water cycle.
This open system has a range of inputs, outputs, stores, transfers and flows.
INPUTS – WATER COMING INTO THE SYSTEM
Inputs include precipitation which are all forms of moisture (including rain and snow) that reach the Earth’s surface and solar energy for evaporation.
STORAGE – WATER STORED IN THE SYSTEM
Interception: this is when precipitation lands on buildings, vegetation and concrete before it reaches the soil. Interception storage is only temporary as it is often quickly evaporated.
Vegetation storage: this is water taken up by vegetation. It is all the moisture in vegetation at any one time.
Surface storage: the total volume of water held on the Earth’s surface in lakes, ponds and puddles.
Groundwater storage: the storage of water underground in permeable rock strata.
Channel storage: the water held in a river or stream channel.
FLOWS AND PROCESSES – WATER MOVING FROM ONE PLACE TO ANOTHER
Interflow: water flowing downhill through permeable rock above the water table (within the zone of aeration).
Percolation: the flow of water within soil due to gravity
Stemflow: water running down a plant stem or tree trunk.
Surface Runoff: the movement of water over the surface of the land, usually when the ground is saturated or frozen or when precipitation is too intense
from infiltration to occur.
Throughflow: the movement of water downslope within the soil layer. Water also flows through the soil in percolines, which are lines of concentrated water flow between soil horizons.
Baseflow: water that reaches the channel largely through slow throughflow and from permeable rock below the water table (movement of water within the zone of saturation). This delayed flow will take a longer time to reach the stream, sometimes arriving days after a rainfall event and results in a flatter hydrograph
Channel flow: the movement of water within the river channel. This is also called a river’s discharge.
Groundwater flow: the deeper movement of water through underlying permeable rock strata below the water table. Limestone is highly permeable with lots of joints and can lead to faster groundwater flow.
Infiltration – the downward movement of water into the soil surface.
OUTPUTS – WATER LEAVING THE SYSTEM
Evaporation: the transformation of water droplets into water vapour by heating.
Evapotranspiration: the loss of water from a drainage basin into the atmosphere from the leaves of plants + loss from evaporation.
Transpiration: evaporation from plant leaves.
River discharge: the amount of water that passes a given point of the river, in a given amount of time.
Infiltration
The maximum rate at which rain can be absorbed by a soil in a given condition is known as the INFILTRATION CAPACITY.
The factors that affect the rate of infiltration include:
1. Duration of rainfall
2. Type of soil (Soil porosity)
3. Type of surface (pervious vs. impervious)
4. Amount of vegetation cover
5. Raindrop size
6. Angle of slope
7. Antecedent soil moisture (the amount of water
already existing in the soil)
INFILTRATION VS SURFACE RUNOFF
- As infiltration decreases, surface runoff increases
- As the duration of rainfall increases, the rate of infiltration decreases while surface runoff increases
- The greater the porosity of the soil, the greater the amount of infiltration and less surface runoff
- More vegetation means that there is more interception thus more infiltration, therefore less surface runoff
Evaporation
The factors that affect the rate of evaporation include:
- Temperature
- Humidity
- Amount of water available
- Vegetation cover
EVAPOTRANSPIRATION
The evaporation plus transpiration from a vegetated surface with unlimited water supply is known as potential evaporation or potential evapotranspiration
(PE) and it constitutes the maximum possible rate due to the prevailing meteorological conditions. Thus PE is the maximum value of the actual
evaporation (Et)
PE = Et when water supply is unlimited
Actual evaporation is the amount of water which is evaporated a normal day. Example, if for instance, the soil runs out of water, the actual evaporation is
the amount of water which has been evaporated and not the amount of water which could have been evaporated if the soil had had an infinite amount of
water to evaporate.
Factors that Influence Storage and Transfers
- PRECIPITATION
The main characteristics of precipitation that affect the hydrology of a local area include:1. The total amount of precipitation 2. Seasonality 3. Intensity 4. Type of precipitation (snow, rain, etc.) 5. Geographic distribution 6. Variability - TEMPERATURE
In cold temperatures, the ground is frozen as such water remains on the surface(no infiltration). Also, water turns to ice so water ‘flows’ become water
‘stores’. Plants are dormant therefore less interception and transpiration There is also a lack of heat thus slowing/preventing evaporation.
In warm temperature, there are more plants so there is more interception and higher rates of evaporation and transpiration. As such there is less lower infiltration and percolation rates
- LAND USE
Amount of Natural Vegetation- More forest cover = Higher interception rates
- More deforestation = Increased surface runoff
Agricultural Land Use
- Permanent cultivation = Less water flows than annual crops (as land is left
bare for a period in the year)
Urban vs. Rural
- Urbans areas = more impermeable surfaces = less infiltration
- SOIL AND ROCK TYPES
- Impermeable rocks & soils = no/less infiltration (clay,granite)
- PHYSICAL CHARACTERISTICS OF THE DRAINAGE BASIN
- Gradient of area (steep areas = faster overland flow than low-lying areas)
- High drainage density = faster water movement
Groundwater
Groundwater refers to subsurface water.
The permanently saturated zone within solid rocks and sediments is known as the PHREATIC ZONE (ZONE OF SATURATION). It is here that nearly all the pore
spaces are filled with water.
The zone that is seasonally wetted and seasonally dries out is known as the AERATION ZONE or the VADOSE ZONE. This zone is above the zone of saturation.
The WATER TABLE divides one zone from the other. The water table varies seasonally.
An artesian well is an aquifer where the ground water is confined under high pressures thus the water level rise above the top of the confined aquifer. This well therefore does not need any pumping for the water to reach the surface. Such a well occurs in a depressed area of land known as a artesian basin
An aquifer is a water-bearing stratum underground that can store and transmit large quantities of groundwater.
Perched groundwater is an isolated body of groundwater that is perched above and separated from the main groundwater and water table by an impermeable layer of rock such as clay.
An aquiclude or aquifuge is a rock which will NOT hold water. These are impermeable rocks which prevent large-scale storage and transmission of water.
GROUNDWATER BALANCE
∆S=Qr – Qd
where:
∆S is the change in storage (+ or -)
Qr is the recharge to groundwater
Qd is discharge from groundwater
CAUSES OF GROUNDWATER RECHARGE
- Infiltration of part of the total precipitation at the ground surface
- Seepage through the banks and bed of surface water bodies such as ditches, rivers, lakes and oceans
- Groundwater leakage and inflow through adjacent aquicludes and from aquifers.
- Artificial recharge from irrigation, reservoirs, etc.
CAUSES OF GROUNDWATER LOSS
- Evapotranspiration particularly in low-lying areas where the water table is close to the ground surface
- Natural discharge by means of spring flow and seepage into surface water bodies
- Groundwater leakage and outflow through aquicludes and into adjacent aquifers
- Artificial abstraction (extracting water for consumption)
The Water Balance
It examines the balance between inputs and outputs, from a global level
(hydrological cycle) or at a local level (drainage basin cycle)
At a global level, oceans tend to experience greater outputs (evaporation) than inputs (precipitation). This is because oceans are large unshaded bodies of water that have regular winds blowing saturated air on land, allowing greater evaporation. In addition, oceans do not tend to suffer from the same amount of relief and convectional rainfall as land does.
On land, inputs (precipitation) tend to be greater than outputs (evaporation).
This is because lands suffers from larger amounts of frontal, relief and convectional rainfall, as well as much of the lands water being protected underground or in shaded areas reducing evaporation.
At a global level there is an equilibrium between inputs and outputs. The excess precipitation on land is returned to the oceans by channel flow, surface run-off and to a lesser extent groundwater flow. The excess of evaporation is returned to the land from the sea by winds blowing saturated air on land.
WATER BALANCE FORMULA
At a more local level, the following formula is usually used to calculate the water balance:
P = E + R ± S
P = Precipitation E = Evapotranspiration R = Surface runoff S = Changes (gains/loss) in groundwater storage
(Over a period of many years, S may tend to be constant and for that reason, is
sometimes omitted from the equation)
Because drainage basins are open systems, there can be an imbalance in inputs and outputs.
This equation gives the volume of water that is in the system.
In wet seasons, precipitation is greater than evapotranspiration which creates a water surplus. Ground stores fill with water which results in increased surface runoff, higher discharge and higher river levels. This means there is a positive water balance.
In drier seasons, evapotranspiration exceeds precipitation. As plants absorb water, ground stores are depleted. There is a water deficit at the end of a dry
season (negative water balance).
Storm Hydrographs
RIVER DISCHARGE
The rate and volume of flow of water in a river is known as RIVER DISCHARGE.
This volume is the total volume of water flowing through a channel at any given point and is measured in cubic metres per second (cumecs).The discharge from a drainage basin depends on precipitation, evapotranspiration and storage factors.
Drainage basin discharge = precipitation – evapotranspiration +/- changes in
storage
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COMPONENTS OF STORM HYDROGRAPHS
- The starting and finishing level of the graph show the base flow of a river.
The base flow is the water that reaches the channel through slow throughflow and permeable rock below the water table. (the normal level of the river, which is fed by groundwater) - As storm water enters the drainage basin the discharge rates increase. This is shown in the rising limb.
- The highest flow in the channel is known as the peak discharge.
- The fall in discharge back to base level is shown in the receding limb.
- The lag time is the delay between the maximum rainfall amount and the peak discharge.
The shape of a hydrograph varies in each river basin and each individual storm event.
Rural areas with predominantly permeable rock increases infiltration and decreases surface runoff. This increases lag time. The peak discharge is also lower as it takes water longer to reach the river channel.
Analyzing a Storm Hydrograph
It is generally drawn with two vertical axes. One is used to plot a line graph showing the discharge of a river in cumecs (cubic metres per second) at a given
point over a period of time. The second is used to plot a bar graph of the rainfall
event which precedes the changes in discharge.
The shape of the hydrograph varies according to a number of controlling factors in the drainage basin such as:
- The baseflow of the river represents the normal day to day discharge of the river and is the consequence of groundwater seeping into the river channel.
- The rising limb of the hydrograph represents the rapid increase in resulting from rainfall causing surface runoff and then later throughflow.
- Peak discharge occurs when the river reaches its highest level.
- The falling limb (or recession limb as it is sometimes known) is when discharge decreases and the river’s level falls. It has a gentler gradient than the rising limb as most overland flow has now been discharged and it is mainly
throughflow which is making up the river water. - Lag time
Urban vs Rural Storm Hydrographs
Rural area storm hydrographs tend to have longer lag times and short peak discharge as there is greater vegetation thus more infiltration and interception. This reduces the speed at which water enters the river channel and increases the amount rainfall that is stored in the soil.
Urban area storm hydrographs tend to have short lag times and higher peak discharges as there is an increase in surface runoff due to urban development which reduces the amount of vegetation and increases the amount of impermeable surface (roads, pavements). Laying drains leads to the rapid transportation of water to river channels which reduces the lag time.
