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Erosion and deposition 1+2 10.12.18 (Sedimentary rocks (Bioclastic…

- - - - Physical
        
        Breakage of the rock by the cyclic application and release of pressure. Often exploits lines of weakness in the rock.
        
        Agents of physical weathering- growth of crystals within the rock, salt, Ice (freeze thaw), and penetration of plant roots into rocks, exfoliation (decrease of overburden pressure).
        
        Physical weathering makes the clasts for clastic rocks.
        
        It also increases the surface area/ volume ratio of a rock, elevating its rate of chemical weathering.
      - Chemical
        
        Most rocks form under conditions different to those at the Earth's surface. They are out of equilibrium with surface conditions.
        Igneous rocks- form by cooling of molten rock.
        Metamorphic rocks- form in the Earth's interior.
        
        Agents of chemical weathering
        
        Carbonic acid. Solution of CO2 from the atmosphere and soils in rainwater.- H2O + CO2= H2CO3
        
        Sulphuric acid. Solution of atmospheric sulphur dioxide in rainwater (acid rain).
        
        e.g. Limestones are made of calcite (CaCO3). Calcite is very soluble and dissolves completely- chemical weathering of limestone is Carbon Neutral. Limestone is metamorphosed into marble.
        
        Chemical weathering of igneous rocks
        
        Most igneous rocks are made mainly of silicate minerals.
        
        Chemical weathering of silicate minerals (e.g. feldspar- anorthite) forms new stable minerals (e.g. clay mineral kaolinite) and liberate ions to solution.
        
        Chemical weathering rates of silicate minerals vary according to the temperature at which the mineral crystallized from molten rock.- highest temperatures weather most rapidly.
        
        Silicate mineral weathering rates are the reverse of Bowen's reaction series.
        
        Chemical weathering of silicate minerals that are rich in Ca and Mg (e.g. anorthite) removes CO2 from the Earth's atmosphere, which is eventually locked up as limestone.
        
        If the Earth's CO2 concentration rises- temperature increases, the weathering rate of silicate minerals increases, more Ca, Mg and bicarbonate are added into the Oceans, these solutes form limestones, this then results in a decrease in atmospheric CO2 and temperature.
        
        Determinants of chemical weathering rates
        
        Minerals within the rock.
        
        Acidity of the water.
        
        Availability of water.
        
        Temperature.- a given rock type will weather much faster in hotter (and wetter) climates.