Rocks & Weathering

3.1 Plate tectonics

Nature of tectonic plates and their global patterns

Earth, our planet, is made up of a number of different layers which have different properties. Note, you will see some different figures depending on your sources when reading. This is because these figures are averages.

The crust is the brittle outer layer of our planet. It is broken into a number of tectonic plates that have different characteristics. Oceanic plates are young (under 200 millions years), thin (around 10km thick) but dense (3.2 g/cm³). Oceanic plates are usually composed of basalt.

Continental plates, on the other hand, are old (over 1500 million years), thick (around 50km thick) but less dense (2.7 g/cm³). Continental plates commonly consist of granite.

The lithosphere consists of the crust and upper mantle, which is largely solid. The asthenosphere is much hotter and more fluid than the lithosphere. As depth increases, so does the temperature, which means the outer core is liquid due to melting. Towards the centre of Earth, density increases, which is why the inner core is solid - due to the high pressure.

Plates move due to convection currents at around 1 to 10cm per year. Plates move in three ways:

• Ridge push - magma is forced into intrusions causing the plates to move apart e.g. Mid-Atlantic Ridge.

• Slab pull - as an oceanic plate is subducted due to its high density, it drags the rest of the plate with it e.g. Philippine Trench.

• Convectional drag - convection currents cause the plates above to move.

Evidence for plate tectonics

Abraham Ortelius was likely to be the first person to consider that the tectonic plates we see today were once joined together and have drifted to their present day positions. Alfred Wegener put forward his theory of continental drift in 1912, proposing that around 260 million years ago the large super continent, Pangaea, existed and has broken up over time. This work was further developed by Harry Hess in the 1960s.

These scientists used the following evidence:

• Coastline fit

• Fossils, flora and fauna

• Fit of orogenic belts

• Fit of rock types

• Palaeoclimatic evidence

The Earth’s magnetic field has different polarity in different periods of time. The magnetism is weak but permanent, unless the rocks are re-heated. This is palaeomagnetism or fossil magnetism. Lava cooling on the sea floor acquires the polarity of the earth’s magnetic field at the time of cooling. In 1963 Vine and Matthews discovered that magnetic polarities were symmetrical on either side of a mid-ocean ridge.

Types of plate boundary, processes, and associated landforms

Convergent (or destructive) plate boundaries are those where the plates are moving together due to the convection currents. They can involve either oceanic-oceanic or oceanic-continental plates.

The Andes are an example of an oceanic-continental boundary. Where the two plates meet, the denser oceanic plate is forced to subduct beneath the less dense continental plate. An oceanic trench is formed where the oceanic plate is subducted. This is a long trench, around 10km deep, that runs parallel to the fold mountains formed at this boundary. The trench is asymmetric with the steepest side on the continental side and a more gentle slope where the plate gradually bends into the trench on the oceanic side. Deep in the mantle, the oceanic plate partially melts due to the high heat. This magma collect in a magma chamber and can rise through cracks and fissures in the continental plate, eventually reaching the surface and forming a volcano (usually composite/strato-volcanoes). The Benioff zone is an area of deep earthquake foci caused by the build up and release of pressure form the subducting plate. Fold mountains are formed due to the compression of the continental plate. This crustal thickening causes anticlines (downward-sloping strata) and synclines (upward-sloping strata) to form. Material that is scraped off the subducting oceanic plate is then deposited on the leading edge of the continental plate forming an accretionary wedge/prism. This material also contributes to orogenesis (mountain building). Earthquakes occur at this boundary type.

Where two oceanic plates meet, the landforms are similar expect there are no fold mountains. Volcanic island arcs are formed here where the subducted oceanic plate partially melts (this plate is the thicker and therefore more dense plate). The magma that collects in the magma chamber rises through cracks and fissures to form strato-volcanoes. An example of this landform is the Aleutian Islands. The Mariana trench is a good example of an oceanic trench that is formed at this plate boundary type. The curved shape of volcanic island arcs is due to the angle of the descending lithosphere.

Divergent (or constructive) plate boundaries involve two oceanic plates moving apart from each other. The Mid-Atlantic Ridge is a good example of this boundary type. Through the gap that is created, magma rises to cool and form new lithosphere. A ridge is formed because of the expansion of the rock here as it is hot. Shield volcanoes are common at this boundary type, although Iceland has a number of composite volcanoes such as Eyjafjallajökull, which erupted in 2010. Earthquakes also occur here. Rift valleys, offset by fracture zones form between the double central ridge when a central block falls.

Conservative (or transform) boundaries involve no subduction, so there is no volcanism at this type of boundary. Here the plates can be moving past each other in different directions, or in the same direction at different speeds. A good example of this boundary type is the San Andreas Fault, that extends roughly 1 200 km through California, USA. Shallow earthquake foci, which are particularly devastating, can be found here.

Collision boundaries such as the Himalayas, Nepal, are formed where two continental plates collide. In this case, the Indian and Eurasian plate. Here there is no volcanism but often shallow earthquake foci.

Most volcanic activity takes plate at plate boundaries but hot spots, like Hawaii, are situated above a mantle plume and form volcanoes like Mauna Loa.

3.2 Weathering

Physical (mechanical) weathering processes: freeze-thaw, heating/cooling, salt crystal growth, pressure release (dilation), vegetation root action and chemical weathering processes: hydrolysis, hydration, and carbonation

Weathering is the wearing away of material in situ, that is, where it is. Erosion is the breaking down and removal of material. Weathering is an important process in creating the landscapes we see around us. You need to understand the physical and chemical weathering processes at work.

Physical weathering processes

Freeze-thaw

Also known as frost shattering, this involved water flowing into cracks in a rock and then freezing (at night, for example, when it is colder). When water freezes, it expands by 9%. This expansion exerts pressure on the rock and can cause it to fracture. It is a very effective weathering process in alpine and periglacial environments. What is most important for this weathering process is the number of cycles i.e. constant freezing and thawing of the water in the cracks.

Thermal fracture

When rocks are subjected to rapid changes of temperature, they crack. This results in stress fractures. The large diurnal (daily) range in temperatures in deserts results in thermal fracturing of rocks. Another process worth mentioning here is exfoliation. This involves heating and cooling of material, but is different to thermal fracture as it involves the addition of water, which may also lead to salt crystal growth in cracks in the affected rock.

Salt crystal growth

When salts in rock pores or joints crystallise, they expand and this exerts pressure on the rock which may cause it to disintegrate. In areas with temperatures around 27 °C sodium sulphate and sodium carbonate expand as much as 300%. Similarly, when salt crystals left behind after evaporation are heated, they begin to expand and this can also weather rocks. Rocks in hot deserts are prone to this type of weathering.

Dilation

Also known as pressure release. This involves the removal of overlying material by erosion which causes the underlying material to crack as it expands from the release of the pressure. A good example of this is the melting of an ice sheet resulting in dilation joints (expansion joints) in the bare rock that was beneath it. Dilation is largely responsible for the creation of pseudo-bedding planes (cracks parallel to the ground surface) in inselbergs (a steep-sided hill of solid rock) and tors (a well-jointed rock that appears from a smooth slope).

Vegetation root action

Roots can penetrate soil and help break-up underlying bedrock by the pressure they exert in cracks and fissures.

Chemical weathering processes

Water plays a key role in chemical weathering. You need to remember the chemical formulas for each process, too.

Hydrolysis

Hydrolysis is when water reacts with minerals in rocks to break them down. When this happens kaolin (china clay), a soft white clay, is produced. It affects feldspar minerals in granite, in particular orthoclase feldspar. Slicic acid and potasssium hydroxyl are also produced.

2KAlSi₃O₈ + 2H₂O --> Al₂Si₂O₅(OH)₄ + K₂O + 4SiO₂

Hydration

When some minerals absorb water they expand, such as anhydrite and gypsum. Contraction also helps assist disintegration. Shales and mudstones can expand by more than 1000%.

CaSO₄ + 2H₂O <-- --> CaSO₄.2H₂O

Carbonation

When rainfall combines with dissolved carbon dioxide, a weak carbonic acid is formed. This is known as carbonation-solution.

CO₂ + H₂O <-- --> H₂CO₃

Limestone, which contains calcium carbonate, reacts with this carbonic acid to calcium bicarbonate which is removed by the water.

CaCO₃ + H₂CO₃ --> Ca(HCO₃)₂

General factors affecting the type and rate of weathering: climate, rock type, rock structure, vegetation, relief and specific factors affecting the type and rate of weathering: temperature and rainfall (Peltier diagram)

Climate

Climate affects the rate of weathering but also determines which processes will actually occur. Van't Hoff's law states that the rate of chemical weathering will increase 2-3 times for every 10 °C increase in temperature, up to a maximum of 60 °C. Strakhov's 1967 model shows the variation in depth of weathered material whereas Peltier's 1950 diagram illustrates the relationship between temperature and precipitation.

In general, humid tropical areas have the fastest weathering rates. Polar and sub-polar areas experience freeze-thaw weathering . Rates are determined by the number of cycles. Carbonation may take place as carbon dioxide is more soluble at lower temperatures. Humid temperate areas have low physical weathering rates but chemical weathering rates are high due to the wetter climate. Arid and semi-arid areas have the slowest rates of weathering due to the lack of moisture.

Rock type

The chemical make-up of the rock, as well as the presence of any joints or bedding planes both affect the rate of weathering.

Some examples:

• Granite - quartz is very resistant so is not easily weathered but feldspar will break down due to hydrolysis.

• Limestone - carbonation weathers carbonate minerals and insoluble clay minerals are left behind as a residue. Deep weathering can take place due to the presence of joints and bedding planes.

• Sandstone - clastic rocks are attacked at different rates depending on mineralogy e.g. quartz is very resistant but calcium carbonate is susceptible to carbonation. Joints and bedding planes may also increase the rate of weathering.

Rock structure

Coarse-grained rocks will whether fastest owing to the pore space inside them. Fine-grained rocks may have a greater surface area and also have fast weathering rates.

Joints and bedding planes can lead to differential weathering (a greater rate of disintegration in some areas of a rock compared to others).

Porosity and permeability also affect the rate of weathering.

Porosity is a measure of the number of void spaces (holes) in a material.

Permeability is a measure of how easily fluids can pass through a material.

Vegetation

Vegetation can increase the rate of chemical weathering through the release of organic acids, important in chelation (the breakdown of material due to organic material). Rates of carbonation also increases with the presence of vegetation as the increase in carbon dioxide from plant respiration.

Vegetation provides thermal insulation which means that rates of physical weathering will be lower, although roots can add stress to cracks and joints that result in the rock fracturing (biological weathering).

Relief

Steeper slopes allow the removal of talus and therefore further weathering to continue. This cannot happen on shallower slopes. However, if the slope is too steep, water will runoff and therefore medium slope angles are the most susceptible to weathering. Aspect (the direction the slope faces) is also important. South-facing slopes are usually warmer than north-facing slopes and this results in more freeze-thaw weathering on the north-facing slopes and greater rates of physical weathering.

3.3 Slope Processes

Slope processes, conditions under which each occurs and effects on slopes

A slope is defined as an inclined surface. Slopes can be thought of as an open system with inputs and outputs.

Inputs to the system include energy from the sun as well as mass (water and sediment).

Outputs from the system include energy (re-radiated heat), and mass (water and regolith (unconsolidated rock debris))

Some factors occur on the outside of the slope (exogenic).

Some factors occur on the inside of the slope (endogenic).

Mass movements are large-scale movements of the Earth's surface that are not accompanied by a moving agent (e.g. river, glacier).

Mass movements can be classified by their speed and water content, as shown in the classification by Carson and Kirkby (1972).

The likelihood of a slope failing is determined by the relative strength/resistance of the slope compared with the forces acting on it.

For slope failure to occur there needs to be a one of the following situations, or a combination of the two:

• A reduction in shear strength/resistance (the internal resistance of the slope itself)

• An increase in shear stress (the forces acting on a slope)

Mass movement: heaves, flows, slides and falls

There are actually five types of mass movement that you need to learn:

• Heave/creep (technically different but you can use them interchangeably)

• Flow

• Slide

• Fall

• Slump

You should be able to produce annotated diagrams to show the key features of each.

Creep/Heave

Creep is the slow, continuous movement of regolith and soil (unconsolidated thin layer of naturally occurring material) down a slope under the influence of gravity. It is hard to observe as it is a type of plastic deformation (change in a solid without failure or rupturing). It is common in winter and has very slow rates of moment, typically 1-10mm per year (higher rates in tropical areas and lower rates in temperate regions).

There are two types of heave:

• Soil creep (finer material)

• Talus (unconsolidated material deposited on slopes) creep (coarser material)

Particles are moved to the surface through expansion and contraction (heave). When particles expand, they move towards the surface, and when they contract they move back downslope. This occurs through wetting and drying, freezing and thawing and also heating and cooling. Water is essential for lubrication and can lead to solifluction (soil flow), for example in permafrost areas, and surface wash (transport of soil and regolith downslope by running water through rainsplash erosion and surface runoff). The influence of water can lead to high levels of denudation (the washing away of surface materials).

Factors that influence the rate of creep include the slope angle (a steeper slope leads to faster creep) and the soil type. Clays, like smectite, absorb water and expand leading to faster rates. The rate of soil creep decreases with depth as the climate has less of an impact here.

Soil creep creates terracettes, which were once thought to be caused by grazing animals on hillslopes, but this is now believed to accentuate these features.

Flow

A flow is the movement of a substance either as a fluid or by plastic deformation. There is no well-defined slip plane with this type of mass movement. They are fast (cm/day to m/sec), due to the water content that lubricates the material in the flow, and the material in a flow is either fine or small. Clay is a good example of the type of material susceptible to flow as it can absorb a lot of water.

Factors that influence a flow are the angle of the slope (steep slopes leading to higher flow velocities). Vegetation is also important as it's roots can help hold the slope together. Removing vegetation in some areas increases the likelihood of a flow. Similarly, heavy rainfall can lead to ground saturation. Traffic vibrations, earthquakes, and even slides can all lead to flows.

Slide

A slide is the downslope movement of rock, soil or superficial material along a slip plane. It is a widespread type of mass movement. Indeed, the term 'landslide' is commonly used in the news to cover a variety of mass movements. There are two types of slide:

• Transitional slides, which have a planar shear surface

• Rotational slides (covered later in slumps), which have a curved slip plane

Slides are common in areas of weaker rock with steep slopes. Undercutting is a common cause of a slide along a slip plane. Slip planes are likely to occur at a fault line, along a bedding plane and where the shear stress is greater than the shear strength (usually just below the surface on a slope). Clay is a good example of material that is prone to slides.

Factors that influence slides include slope angle, rock type and water content of a slope (pore water pressure decreases shear strength).

Fall

A fall involves the vertical movement of rock, or more unusually, soil under the influence of gravity. A steep slope is needed for a fall to occur (over 40°), and the material has to be weathered first, in order for it to be eroded and consequently fall down the slope. Freeze-thaw weathering is a good example of such a process.

Contributing factors to falls include undercutting, which is particularly active at coastal cliff faces. Any removal of material further increases the likelihood of a future fall as vertical cracks are opened up. A good example of this is the exposure of valley sides when glaciers melted in the UK around 15, 000 years ago, further eroding these slopes.

At the base of the cliff scree will collect. A straight scree slope is formed if the fall is short, and if the fall is long, this produces a concave scree slope. The size and shape of the fallen material is also a contributing factor to the shape of the scree slope.

Slump

The final type of mass movement you need to learn is a slump. A slump is a mass of surface rocks or superficial material that becomes detached from a hillside and moves along a slip plane (a type of slide). This slip plane is usually rotational (curved).

Slopes made of clay are liable to slump (on the coast, for example,) as they absorb water easily, and undercutting can be a contributing factor leading to this type of mass movement.

Water and sediment movement on slopes: rainsplash and surface runoff (sheetwash and rills)

Rainsplash

When rainfall is intense, it can cause soil particles to detach (rainsplash erosion). On steep, bare slopes, this can cause the material to move downslope.

Surface runoff

In areas of intense precipitation, where the infiltration rate exceeds the infiltration capacity, surface runoff (water moving over the ground without infiltrating) will occur. Sheetwash (the removal of surface debris by overland flow) can occur where the slope angle is gentle. Steeper slopes can engender gullies (deep, steep-sided channels). Rills (narrow, shallow, steep-sides channels) are found on intermediate slopes.

3.4 The human impact

Increasing and decreasing stability

Human activity can both increase and decrease the stability of slopes. Activities that reduce slope stability include removal of vegetation. This involves deforestation for urbanisation and other construction projects.

Surface loading is the addition of weight onto a slope. This can be either by building, or adding water. Water also has a lubricating effect. Waste heaps can collapse such as in the case of the Abderfan disaster of 1966, when, on the 21st October, 116 children and 28 adults were killed when a spoil heap from a nearby coal mine collapsed in Wales due to a build-up of water which saturated the material in the waste heap. Traffic vibrations, especially from heavy vehicles, can trigger mass movements. Excavations involve the removal of material from a slope, and if the slope is left at too steep an angle, can result in mass movement. Ploughing in the direction of slope gradient can develop rills. Similarly, soil structure can be destroyed through poor agricultural practices, which loosens slopes.

Strategies that can increase slope stability are pinning, where a metal rod is inserted into a slope, and a plate added to anchor loose material. Netting can be secured to slopes to stop material falling. Drainage channels remove excess water from a slope and mean the slope material cannot become lubricated. Grading reduces the slope angle and therefore the likelihood of mass movement occurring. Gabions (wire cages filled with rocks) can be used to stabilise slopes. Grouting involves injecting permeable rocks with cement to increase shear strength. Shotcrete can be sprayed onto loose surfaces to bind them. Lastly, hazard mapping can be used by governments to decide which areas are most likely to experience factors that could lead to mass movements e.g. areas of heavy rainfall.

Case study: Hong Kong

The entrepôt of Hong Kong has long been associated with landslides due to rapid urbanisation, steep topography and a sub-tropical climate bringing warm, wet conditions in summer (Chen and Lee, 2004; Malone, 1997; So, 1970).

Causes

Hong Kong has steep topography, and, as of 2019, had a population of 7.5 million. This means that development has had to take place on steep slopes, often removing vegetation which has reduced slope stability and shear strength. Slope loading from buildings has only increased shear stress.

The slopes are made of a mix of granitic, volcanic and sedimentary rocks. The granitic rocks are prone to deep weathering.

The climate is conducive to mass movements - hot and wet in summer. In the June 1966 heavy rainfalls were responsible for landslides - over 400mm of rainfall fell in one hour (Nagle and Guinness, 2016).

Impacts

Landslips were responsible for the death of more than 470 people in Hong Kong between 1948 and 1997 (Malone 1997). Rainstorms in June 1966 caused huge landslides that made over 2500 people homeless and 8000 people needed to be evacuated. 64 people died (Nagle and Guinness, 2016).

Responses

The Geotechnical Engineering Office (GEO) is a government-run organisation responsible for slope safety in Hong Kong. Private owners of slopes are fined up to HK $50 000, and up to one year in prison, if they do not take responsibility for slope safety. Since 1977, landslide risk has decreased by 50 % (Nagle and Guinness, 2016).

Greening techniques such as mulching, using long-rooting grass and fibre-reinforced soil systems have all increased shear strength of the slopes.

Bibliography

The following books and journal articles are useful for further reading around the topic:

Chen, H. and Lee, C. F. (2004) Geohazards of slope mass movement and its prevention in Hong Kong. Engineering Geology, 76, 3-25.

Fretwell, M., Kelly, D. and Nanson, J. (2015) Geography. Oxford: OUP.

Malone, A. M. (1997) Managing slope safety in a high density city prone to landslips. Proceedings of The Hong Kong Engineers Engineering for Public Safety Conference, HKIE, April 1997.

Nagle, G. and Guinness, P. (2016) Geography. 2nd ed. London: Hodder.

So, C. (1971) Mass Movements Associated with the Rainstorm of June 1966 in Hong Kong. Transactions of the Institute of British Geographers, 53, 55-65.

Summerfield, M. A. (1991) Global Geomorphology. Essex: Pearson.

Vine, F. J. and Matthews, D. H. (1963) Magnetic anomalies over oceanic ridges. Nature, 199, 947–949.

Whittow, J. B. (2000) Dictionary of Physical Geography. 2nd ed. London: Penguin.