Sedimentary Rocks

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Sedimentary Rocks surface to peel off like the skin of an
Types of Weathering: onion. Onion-skin weathering.
Sedimentary rocks form around 5% of the There are three important types of weathering ­ - Frost shattering is caused by the
volume of the lithosphere but their importance lies chemical, physical and biological. expansion and contraction of the minerals
in the fact that they are 75% of the exposed rocks and the pore waters in the rocks. The
of the continents. Chemical Weathering- is only significant when water may be held in joints, cracks,
They are of crucial economic importance since water is present in its liquid form, therefore this bedding planes and pore spaces and
they form reservoir rocks for water and gas as type of weathering isn't of great significance in when the water freezes it expends.
well as containing coal, mineral deposits and either very dry areas such as deserts or very cold Tremendous pressured can result which
building materials. areas such as mountainous or permafrost will literally burst the rock apart. Evidence
regions. Ordinary rainwater contains small of frost shattering often occurs on steep
Detrital Rocks: amounts of acid, and when it comes into contact mountain slopes where angular
These are formed when the parent rock, which with rock the acid attacks it and causes the rock fragments of rock fall under gravity to
can be I/S/M, is subjected to a degree of to rot and crumble away. An example of this can collect as scree below.
weathering and/or erosion which will break the be seen in gravestones in churchyards, buildings
parent rock up. The fragments are than and statues where the stone has been worn away
transported to a greater or lesser degree before and pitted. Water and heat made the chemical
being deposited. Examples include weathering occur quicker, so it is greatest in
conglomerates, brecha and sandstone. places that are both warm and wet.
Chemical weathering greatly modifies the
Carbonate Rocks: mineralogy of the particulate material which is
These are chemical precipitates which are passed on down the chain to form a new
usually formed in shallow marine basins where sedimentary rock.
there is a high rate of evaporation. The sea water Solution- rock is dissolved entirely by water and
becomes saturated with salts, especially CaCO3 washed away. Example, limestone
which precipitates out onto the sea floor to form Carbonation- carbonic acid dissolved in
rich lime muds. When these muds become rainwater attacks limestone, converting the
lithified or hardened, they form an important calcium carbonate into calcium bicarbonate and
group of rocks best known as limestones or washing it out. A residue is clay.
carbonates. Hydrolysis- acids in rainwater attack feldspar,
an important mineral found in many rocks. The
Organic Sediments: feldspar is converted into clay and washed away,
These are the deposits that are the remains of leaving other minerals behind so that the rock
life. For this reason they are often referred to as structure crumbles. Granite contains a lot of
fossil fuels and include coal, oil and gas. feldspar is weathered away in this fashion.
Physical Weathering- this type of weathering is
Weathering: more likely to occur in very cold or very hot
Weathering is the term given to the break down locations than in temperate regions, but it can
of rocks that take place in situ through physical, occur anywhere. The break up of rocks in situ can
biological and chemical means. Weathering will occur by;-
fractionate the source of the material because - Expansion or contraction of the
each mineral in a rock has a different resistance minerals in the source rocks due to the
to the various agents of weathering. extremes of day and night temperatures,
For example, when granite is subjected to which s more common in deserts. As it is
weathering it will be broken up with quartz being heated the outer layer of the rock
the most dominant mineral in the sediment that expands slightly and as it cools it
forms as a result since quartz is a very resistant contracts. The continual expansion and
mineral. contraction causes small pieces of the


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