Rock and weathering booklet

Rock & weathering

As Geo Revision

Plate boundaries & landforms

Mid-Atlantic Ridge

Sea Floor Spreading

Earth’s ocean floors move like conveyor belts, carrying the continents along with them. This movement begins at the mid~ocean ridge. The ridge forms along a crack in the oceanic crust. At the mid-ocean ridge, molten material rises from the mantle and erupts. The molten material then spreads out, pushing older rock to both sides of the ridge.

As the molten material cools, it forms a strip of solid rock in the center of the ridge. Then more molten material splits apart the strip of solid rock that formed before, pushing it aside. This process, called sea-floor spreading, continually adds new material to the ocean floor. Convection currents in the asthenosphere along with the process of slab pull at a subduction boundary act to pull the two sides of the ocean floor apart.

Convergent plate boundaries

Nazca plateS. American plate

Peru-Chile Trench

Philippines plate

Pacific plate

MarianasTrench

Convergent plate-C & C

Eurasianplate

Indianplate

Himalayas Mts

Continent & Continent plates

Peltier Diagram

Factors affecting rate & types ofWeathering:1. Climate2. Rock Types 3. Vegetations

ClimateFrom Peltier diagram, it describe the relationship between climate (temperature and rainfall) and weathering processes. Chemical weathering tends to be most intense in wet and warm climates where high temperatures promote chemical reactions, and heavy rainfall provides the moisture necessary for the processes to operate. Deep weathering profiles are common in area such as tropical climates and the characteristic red soils reflect active oxidation. Physical weathering active in cold climates where frost shattering dominates. In desert environments physical weathering might be expected to dominate due to absence of water, chemical weathering is important, particularly oxidation.

Rock characteristicsRate of chemical weathering is affected by chemical composition of a rock. Some minerals are more prone to chemical change than others. For example, in granite, both minerals feldspar and mica are chemically vulnerable, whereas quartz weathers extremely slowly. The presence of iron minerals and salts will affect oxidation and hydration respectively, and rocks rich in calcium carbonate (i.e. limestone) will be affected by carbonation. Joints and bedding planes promote weathering as they enable water to penetrate deep into rocks.

Vegetation

(C ) Vegetation:Presence of vegetation also promotes weathering, in that organic acids speed up hydrolysis, and plant roots may prise, apart jointed rocks.Some form of vegetation, such as moss, cling to rock surfaces, holding water against them like a wet sponge thereby encouraging chemical weathering.However this same vegetation might, at the same time, protect rock surface from temperature extremes, reducing effect of physical weathering.

Slope system-factors affecting slope processes

Slope as a system

It is an example of an open system because there are inputs from outside such as heat and precipitation and outputs e.g. water and weathered rock into other systems.

(a) Rock type- the tougher the rock the more able it is to support a steep slope. Igneous and metamorphic rocks are extremely strong and are capable of supporting near-vertical slopes, whereas sands and gravel can only support very gentle slopes. (b) Geological structure-Rock slabs may become detached along bedding planes or joints, promoting rockfalls and landslides. (c ) Permeability and porosity- An impermeable rock will be liable to surface water flow, and deep gullies may forms. (d) Climate-The climate of an area will affect the type of weathering that operates on a slope. It will also depends on presence or absent of water and vegetation. Most of mass movement such as mudflows and soil creep are dependent on aspects of climate, particularly precipitations. (e) Vegetation-If a slope is forested or covered with bushes and grass it is less likely to be active. This is because it will protect a slope from direct rainfalls and help bind together particles of rock and soil. (f) Weathering-Weathering affects the upper slopes, particularly bare rock outcrops. In general mechanical weathering, particularly frost shattering will lead to a more jagged, angular, bare rock surface whereas chemical weathering with its tendency to dissolve and produce fine clays, will produce more rounded slopes. (g) Basal excavation- It can takes place in the form of river undercutting a slope or the sea cutting a notch in a cliffline. Human activities such as road construction can have same effect. Basal excavation can lead to a steepening of a slope, so making it unstable. (h) Human activity- People are capable of altering slopes directly by mining and quarrying, construction roads and housing estates, and terracing land for farming. Slope can also be altered indirectly when, for example, forests are cut down for firewood or to make way for agriculture. This deforestation will encourage more surface runoff and soil erosion may occur.(i) Time- Time a slope exposed to weathering is also important controlling factor. Newly formed landscapes that are steep and unvegetated are actively weathered and eroded until they assume a shape that is balance with their environmental conditions. But if environment changes (global warming, for example) the balance may be upset and the slope profile will be forced to adapt.

Physical and chemical weathering

• Weathering-decay or decomposition of rocks in situ resulting from physical or chemical actions.

• Erosion-the picking up and transportation of rock material by agents such as rivers, glaciers and sea.

• Physical weathering-involves the disintegration rocks into smaller fragments without any chemical change taking place.

• Chemical weathering-involves chemical change, causing rocks to decompose.

How slopes can be stabilised

(i) Plant vegetation to bind soil together and intercept rainfall(ii) Improve drainage to prevent the slope becoming saturated and to stop lines of weakness, for example bedding planes, becoming lubricated(iii) use wire nets and metal stakes to hold a slope together(iv) reduce the gradient by adding material to the base of a slope.

Physical weathering Chemical weathering

Insolation weathering-expand & contract outer layers of rock due to diurnal change temperature (rock is poor conductor of heat-only outer layer expand & contract)Outer layers peels off result granular disintegration.

Solution-dissolving minerals in water. E.g. rock salt (halite)Calcium carbonate (limestone) easily dissolve in rainwater that has absorbed carbon dioxide from atmosphere to form carbonic acid. This form of solution is called carbonation.

Salt weathering-High temperature, high evaporation rate & low rainfall, result concentration salt lying on, below ground surface, growth crystal acts frost shattering causes stresses within rocks and result break down rock.

Hydrolysis-Associated with process of Hydration. Chemical change often occurs when mineral absorbs water. Mineral feldspar in granite vulnerable to hydrolysis, weakly acidic water causes feldspar to change into white clay called kaolin.

Pressure release-released of overlying rock by erosion, cause rocks to expand and lead formation of cracks which later exploited by weathering.

Oxidation- oxygen dissolves in water react with minerals, such as iron converted to iron oxide. (weaken the bond and more vulnerable to other weathering processes.

Hydration-expansion of mineral or salts resulting in absorption of water in rock i.e. clay, causes stress within rocks and break.

Chelation-Effects of organic acids in rock, These acids derived either from decomposition of humus (rotted vegetation) in soil, or direct secretion from organism such as lichen. Chelation important in promoting the effects of hydrolysis and carbonation, because weathering of rock under soil seems to be more active than where bare rock is exposed to the elements.

Frost Shattering-Water enter joints, on freezing expand volume, as temperature rises above freezing, thawing takes place, stresses released. Cycles of melting & thawing of ice joint enlarged, result shatter, angular rocks collect at foot called scree.

Mass movement & effects of rocks

Rockfall Landslide

Rotational slip/slide Mudflow

Solifluction

Soil Creep

Granite

• Granite forms deep below the ground and is only exposed on the surface after many millions of years of erosion.

• A physically tough rock, which is resistant to erosion, and commonly forms uplands, for example Dartmoor and Bodmin in S.W England.

• One of the most common features associated with granite is a barely rocky outcrop found on hilltops called a tor.

Granite’s Landform (Tors)

Formed by weathering deep underground before the granite became exposed on the surface.

A barely rocky outcrop found on hilltops called a tor.

Formation of Tors• Over a long period of time, the weaker parts of the rock may have been

weathered to greater depths than the more resistant parts. If subsequent erosion, most likely by river action in the context of Puebla, stripped away all the weathered rock, it would leave the more resistant rock as upstanding craggy outcrops called tors. (Theory by Linton, 1971)

• Despite being physically strong granite is very vulnerable to chemical weathering. The feldspar readily reacts with acidic water to form a clay, and this chemical processes is hydrolysis that weakens the granite causing it to crumble apart.

• Granite is heavily jointed and the density of jointing is believed to have been a critical factor in the formation of tors.

Limestone

• Carboniferous limestone is a common form of limestone that was formed some 300 million years ago during the Carboniferous geological period. It outcrops throughout the UK, from the Gower in S.Wales to Pennine Hills in Yorkshire.

• The limestone in these areas has resulted in a characteristic landscape known as karst. (both overground & underground features)

Karst

cavern

Karst Scenery

Limestone Pavement: Bare rocky surface criss-crossed by enlarged joints, separating blocks of limestone. Enlarged joints are grykes & blocks of limestone in between them -clints Swallow hole

Water flow impermeablerocks

Sink Holes

Surface depressionsSurface weathering

Rainwater passes Underground joints & Along bedding planes,Weathering & erosion To form caverns.

Drips –long tapered featureDown cavern roof-stalactite

Shorter, stubbier feature on floor of cavern-stalagmite

Both join to formColumn or pillar