FORM FIVE SELECTIONS 2021** FORM 5 JOINING INSTRUCTIONS
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Formation of sedimentary rocks
Sedimentary rocks are products of weathering,
erosion, and deposition of materials from other rocks. Deposition is done in the sedimentary basin
at normal temperature and pressure.
The general processes involved in the formation of
sedimentary rocks are as follows:
1)
Weathering of the parent rock
2)
Transportation of the weathered products by running water, wind,
gravity, ice or in solution as dissolved material
3)
Deposition /precipitation of the material in a sedimentary basin
4)
Compaction and cementation of the sediments into a solid rock
Composition of sedimentary rocks
Most sedimentary rocks are composed of materials
that are abundant in other rocks and are stable at surface temperature and
pressure. The great bulk of most sedimentary rocks are composed of only four
constituents: Quartz, calcite, clay, and rock fragments.
Sedimentary textures
Textures in sedimentary rocks provide clues
concerning distance of transportation and environments of deposition. There are two basic types of sedimentary
textures.
i)
Clastic textures
ii)
Crystalline textures.
Clastic textures
These are textures that are characterized by
fragments of rock debris. They are
classified on the basis of grain size followed by the degree of sorting,
cementation and roundness of particles. The rock fragments may be rounded,
sub-angular or angular depending on the degree of abrasion they have suffered.
The sediments moved by ice or gravity tends to be very angular while those
carried by wind or water are more rounded depending on the degree of
abrasion. The grain size and roundness
give a rough measure of the distance the particles have been transported. The
sorting provides history of transportation and depositional environments.
Well-sorted particles are results of prolonged washing, transportation and
winnowing by current action. The glaciers do not do sorting. Particles of different
sizes are deposited together. The dominant cementing materials for the clastic
sedimentary rocks are calcite, silica,
iron oxides and clay.
Crystalline texture
Form as a result of deposition by chemical
reactions. The crystalline texture is characterized by interlocking aggregates
of crystals. The crystals may be small,
medium or large or may exhibit a mixture of sizes. The crystals are formed by precipitations
from saturated solution.
Major classes of sedimentary rocks
1)
Residual sedimentary rocks
2)
Transported sedimentary rocks
Residual sedimentary rocks:
These are rocks formed by
accumulation and consolidation of sediments left as residue during the
operation of weathering and transportation. They are insoluble products of rock
weathering which still mantle the rocks from which they have been derived.
They include the following
rock types:
a)
Terra-rosa: These are insoluble residue of clay and
other material left behind after solution of limestone.
b)
Laterite: a mixture of hydrated ferric-oxide with
hydroxide of aluminium in varying proportions
c)
Bauxite: a rock composed of composed of diaspore,
gibbsite and boehmite, which are aluminium hydroxide minerals. It results from prolonged weathering and
leaching of silica from aluminum bearing rocks under subtropical to tropical
climatic conditions. Bauxite is
generally whitish in colour. But sometimes found with patches of reddish colour
due to presence of iron oxides.
Transported sedimentary rocks
Result from materials
transported mechanically by traction and suspension and chemically in solution.
These categories of sedimentary rocks are further subdivided into: clastic and
non-clastic rocks
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