Physical Geography - Rocks And Weathering

Divergent Plate Margins (Constructive)

Sea floor spreading

Oceanic crust pulling apart -> Magma rising from the asthenosphere, magma that is 'basaltic' (low silica content), selth's hardens, underwater chains of shield volcanoes

Convergent Margins (3 types)

Collison ->  continental + continental crust meeting. Eurasian + Indian has created the Himalayan mt belt

Subduction -> Oceanic + continental crust

Different densities, oceanic crust more dense and pulled into the Benioff zone

Oceanic + Oceanic subduction

same density oceanic crusts moving into eachother, both being pulled into the asthenosphere for melting. Plate that is melting become magma that will rise and form chains/volcanic island arcs

Pacific and Philippine plates forming Marianna's trench + Marianna's islands

Conservative Margins

Plates sliding past each other causing huge amounts of friction/pressure build up, shallow foci earthquakes, therefore no obvious tectonic landforms e.g. San Andreas fault

Intraplate Boundaries

Hotspots

Driven by the top of a large/hot convection cycle (current)

When the convection current meets the underside of the lithosphere, it will melt the plate  

e.g. Ring of fire, EARV

Fold Mountains

Fold mountains form when two tectonic plates collide, this can be where two continental plates move towards each other (collision margin) or a continental and an oceanic plate converge (destructive plate margin). The movement of the two plates forces layers of sedimentary rock upwards into a series of folds

Ocean Trenches

An ocean trench is an indentation in the ocean floor, the deepest area of the ocean. It is formed through subduction, which occurs when tectonic plates collide and push one plate beneath the other.

Volcanic Arc

Volcanic arcs form at convergent plate boundaries where an oceanic plate subducts beneath a continental plate or another oceanic plate. The subducting plate melts and generates magma, which rises to the surface, creating a series of volcanoes known as a volcanic arc.

Regolith

a region of loose unconsolidated rock and dust that sits atop a layer of bedrock

Biological weathering

Biological weathering is weathering caused by the movements of plants and animals (chelation, root action)

Physical Weathering

Physical weathering occurs when physical processes affect the rock

Chemical Weathering

when rocks decompose because of chemical reactions

Freeze-Thaw (Physical Process)

Peri-Glacial Environments

A rock that a weakness gets water inside, which then freezes and expands, causing the weakness to become more significant, eventually breaking the rock

Salt Crystallisation (Physical Process)

Occurs when salt solutions in the pores or joints of a rock crystallise due to evaporation, the crystals then expand and force the rock apart e.g. sodium or calcium chloride

 The process is particularly effective at temperatures of 27°C where temperature fluctuations produce expansion rates of 300%.

Block/Granular Disintegration (Physical Process)

Rocks usually are composed of different types of minerals. The dark coloured minerals e.g. mica in granite, absorb more heat and so are heated up faster. Lighter coloured minerals e.g. quartz, feldspar, can reflect light and are heated up slower due to their albedo

As a result of alternate expansion and contraction of minerals in rocks, the rock cracks and breaks down into small peices

(More effective in areas with a high diurnal range)

Exfoliation (Physical Process)

Rocks are generally poor conductors of heat. The effect of daily heating and cooling is confined to surface layers of the rock. 

The outer layer of rock is heated up and expands during the day while the inner layer of rock remains cool. 

The outer layers eventually peel off (like an onion skin), exposing the inner layers to the surface for further exfoliation.

Dilatation (Physical Process)

This removes weight from previously buried rocks e.g. a granite pluton which formed under immense pressure several kilometres under the Earth’s surface.

This removal of weight leads to expansion in the upper parts and allows cracks to occur parallel to the ground surface, known as pseudo-bedding planes

A similar process may occur where horizontal pressure is released by rock falls on a cliff face, allowing the growth of vertical cracks which, in turn leads to further rock fall.

Dissolution

Weathering by water

Hydrolysis (Chemical Process)

This is when a mineral is broken down by a reaction with hydrogen ions.

Feldspars are commonly found in igneous rocks. It is a process which leads to the breakdown of feldspars

Feldspar + Water + Carbon Dioxide à Kaolinite (clay)

Hydration + Dehydration (Chemical Process)

Wetting and drying can cause the addition or removal of water from the molecules of some minerals, causing expansion or contraction which assist in granular disintegration.

For example anhydrite converts to gypsum with the addition of water and reverses when the water is removed

Anhydrite + Water --> Gypsum

Carbonation (Chemical Process)

 

Is the process of mixing water with carbon dioxide to make carbonic acid and is important in making caves known as karst scenery.

 Rainwater combined with carbon dioxide forms a weak acid known as carbonic acid. This acid reacts with calcite, an abundant mineral in sedimentary rock to form calcium hydrogen carbonate (sometimes known as hard water). e.g. Shillin, China

Water+Carbon Dioxide --> Carbonic Acid (+Calcite) --> Calcium Hydrogen Carbonate

Solution (Chemical Process)

 

Solution – water acting as a solvent (dissolving) most effective when acidulated such as carbonation/limestone

Oxidation (Chemical Process)

Oxidation is a form of chemical weathering where the oxygen in water reacts with iron in the rock/soil, often causing a reddening of the material. Eg, from ferrous to ferric iron oxide. 

Root Action (Biological Physical Process)

Occurs when soil is shallow, the seeds and roots of trees find their way in joints and cracks in the bedrock.

As seeds germinate and the roots get bigger; they make cracks wider and deeper, eventually breaking up the bedrock.

Chelation (Biological Chemical Process)

As organic matter decomposes it releases humic acids into the rocks

Factors Affecting Weathering

Climate

Structure (rock)

Texture (rock)

Composition (rock)

Type (rock)

Vegetation

Relief

Human Activity

Van Hoffs Law

The rate of chemical weathering increases 2-3 times for every increase in temperature of 10 degrees. (up to a maximum temperature of 60 degrees)

What weathering happens most in polar/sub-polar regions

Mainly affected by freeze-thaw action (peri-glacial)

What type of weathering occurs in humid temperate areas?

Physical weathering is limited, making chemical more common

What type of weathering occurs in arid/semi-arid areas?

Chemical rates are reduced → More Physical (Exfoliation, Thermal stress)

What type of weathering occurs in humid tropical areas?

• Most rapid and intense rates of weathering, Accelerated by chemical reactions from high temps and lots of moisture (Hydrolysis, oxidation, solution, carbonation)

What are the 7 different types of rock?

• Sandstone

• Shale

• Slate

• Basalt

• Dolerite

• Limestone

• Granite

What does Peltier’s Diagram show?

It shows how weathering is influenced by the temperature and levels of rainfall

Mass Movements

Is the term for the downslope movement of rock and weathered material by gravity alone

Factors affecting slope processes

Aspect

Soil Composition

Geological Structure

Rock Type

Vegetation

Regolith

Climate

What Human factors can cause slope processes

Excavations, Waste Heaps, Slope Loading, Removal of Vegetation, Blasting, Ploughing, Destroying soil structure

Falls (Type of slope process)

Movements of dry material that occur very quickly. They can create cones of material and the base of slopes, known as talus cones. the space that has been excavated is known as a scar. e.g. rock falls

Slides (Type of slope process)

Slides are movements of wet material, which tends to happen relatively quickly. Water in the earth or rocks tend to create a slip plane on which the material slides. e.g. rotational slumps

Creeps (Type of Slope Process)

Movements of less than 1 cm per year. Occurs mainly in humid tropical areas where there is lots of vegetation and a thick regolith (soil layer).

Flows (type of slope process)

Often referred to as mudflows, these involve the rapid movement of rock and weathered debris mixed with water down valleys. They are a turbulent and structureless mixture of sediment and water.

Occurs when rainfall adds weight to the top soil and lubricates it so that it flows down the slope due to gravity.

Strategies to increase slope stability

Pinning

Netting

Gabions

Afforestation

Shotcrete

Grouting

Drainage Channels

Slope regrading

Case Study: Pines Hotel, Swanage - Hard Engineering

• Located on the South Coast, Swanage, Dorset

• 10m from the edge of 30m high cliffs

• In 2012, landslides almost caused the loss of the hotel

• Funding was done by the council and pines hotel, costing £2.5 million for various hard techniques such as Groynes, Nailing, Pinning, Netting, Shotcrete

Case Study: California, USA (Soft): Causes

1. Intensive rainfall

2. Steep relief and Unconsolidated rocks

3. Wild fires

4. Road and Housing construction

5. Oil and water extraction

6. Earthquakes

Case Study: California, USA (Soft): Management

1. Monitoring

2. Hazard Zoning maps

3. Management techniques (Rock bolts, netting, shotcrete)