Everything about Sedimentary Rock totally explained
Sedimentary rock is one of the three main
rock groups (the others being
igneous and
metamorphic rock). Rock formed from
sediments covers 75-80% of the Earth's land area, and includes common types such as
chalk,
limestone,
dolomite,
sandstone,
conglomerate and
shale.
Sedimentary rocks are classified by the source of their sediments, and are produced by one or more of:
The sediments are then
compacted and converted to rock by the process of
lithification.
Formation
Sedimentary rocks are formed because of the
overburden pressure as particles of
sediment are
deposited out of air, ice, wind, gravity, or water flows carrying the particles in
suspension. As sediment deposition builds up, the overburden (or 'lithostatic') pressure squeezes the sediment into layered solids in a process known as
lithification ('rock formation') and the original
connate fluids are expelled. The term
diagenesis is used to describe all the chemical, physical, and biological changes, including
cementation, undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface weathering.
Sedimentary rocks are laid down in layers called beds or strata. That new rock layers are above older rock layers is stated in the
principle of superposition.There are usually some gaps in the sequence called
unconformities. These represent periods in which no new sediments were being laid down, or when earlier sedimentary layers were raised above sea level and eroded away.
Sedimentary rocks contain important information about the
history of Earth. They contain
fossils, the preserved remains of ancient
plants and
animals. Coal is considered a type of sedimentary rock. The composition of sediments provides us with clues as to the original rock. Differences between successive layers indicate changes to the environment which have occurred over time. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that don't destroy fossil remains.
The sedimentary rock cover of the continents of the Earth's
crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 5% of the total. As such, the sedimentary sequences we see represent only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks.
Classification
Sedimentary rocks are classified into three groups. These groups are clastic, chemical precipitate and biochemical or biogenic.
Clastic
Clastic sedimentary rocks are composed of discrete fragments or clasts of materials derived from other rocks. They are composed largely of
quartz with other common minerals including
feldspar,
amphiboles,
clay minerals, and sometimes more exotic
igneous and
metamorphic minerals.
Clastic sedimentary rocks, such as breccia or sandstone, were formed from rocks that have been broken down into fragments by weathering, which then have been transported and deposited elsewhere.
Clastic sedimentary rocks may be regarded as falling along a scale of
grain size, with
shale being the finest with particles less than 0.002 mm,
siltstone being a little bigger with particles between 0.002 to 0.063 mm, and
sandstone being coarser still with grains 0.063 to 2 mm, and
conglomerates and
breccias being more coarse with grains 2 to 263 mm. Breccia has sharper particles, while conglomerate is categorized by its rounded particles. Particles bigger than 263 mm are termed
blocks (angular) or
boulders (rounded).
Lutite,
Arenite and
Rudite are general terms for sedimentary rock with clay/silt-, sand- or conglomerate/breccia-sized particles.
The classification of clastic sedimentary rocks is complex because there are many variables involved. Particle size (both the average size and range of sizes of the particles), composition of the particles, the cement, and the matrix (the name given to the smaller particles present in the spaces between larger grains) must all be taken into consideration.
Shales, which consist mostly of clay minerals, are generally further classified on the basis of composition and bedding.
Coarser clastic sedimentary rocks are classified according to their particle size and composition. Orthoquartzite is a very pure quartz sandstone;
arkose is a sandstone with quartz and abundant feldspar;
greywacke is a sandstone with quartz, clay, feldspar, and metamorphic rock fragments present, which was formed from the sediments carried by turbidity currents.
All rocks disintegrate when exposed to mechanical and chemical
weathering at the Earth's surface.
Mechanical weathering is the breakdown of rock into particles without producing changes in the chemical composition of the minerals in the rock. Ice is the most important agent of mechanical weathering. Water percolates into cracks and fissures within the rock, freezes, and expands. The force exerted by the expansion is sufficient to widen cracks and break off pieces of rock. Heating and cooling of the rock, and the resulting expansion and contraction, also aids the process. Mechanical weathering contributes further to the breakdown of rock by increasing the surface area exposed to chemical agents.
Chemical weathering is the breakdown of rock by chemical reaction. In this process the minerals within the rock are changed into particles that can be easily carried away. Air and water are both involved in many complex chemical reactions. The minerals in igneous rocks may be unstable under normal atmospheric conditions, those formed at higher temperatures being more readily attacked than those which formed at lower temperatures. Igneous rocks are commonly attacked by water, particularly acid or alkaline solutions, and all of the common igneous rock forming minerals (with the exception of quartz which is very resistant) are changed in this way into clay minerals and chemicals in solution.
Rock particles in the form of clay, silt, sand, and gravel, are transported by the agents of erosion (usually water, and less frequently by ice and wind) to new locations and redeposited in layers, generally at a lower elevation.
These agents reduce the size of the particles, sort them by size, and then deposit them in new locations. The sediments dropped by streams and rivers form alluvial fans, flood plains, deltas, and on the bottom of lakes and the sea floor. The wind may move large amounts of sand and other smaller particles. Glaciers transport and deposit great quantities of usually unsorted rock material as
till.
These deposited particles eventually become compacted and cemented together, forming clastic sedimentary rocks. Such rocks contain inert minerals which are resistant to mechanical and chemical breakdown such as quartz,
zircon,
rutile, and
magnetite. Quartz is one of the most mechanically and chemically resistant minerals.
Organic
Organic sedimentary rocks contain materials generated by living organisms, and include carbonate minerals created by organisms, such as
corals,
mollusks, and
foraminifera, which cover the
ocean floor with layers of
calcite which can later form
limestone. Other examples include
stromatolites, the
flint nodules found in
chalk (which is itself a biochemical sedimentary rock, a form of limestone), and coal and
oil shale (derived from the remains of tropical plants and subjected to pressure).
Chemical
Chemical sedimentary rocks form when mineral solutions, such as
sea water, evaporate. Examples include the
evaporite minerals
halite (rock salt) and
gypsum.
Economic and scientific relevance
Sedimentary rocks are economically important in that they can easily be used as construction material because they're soft and easy to cut. For example, the White House in Washington DC is made of sandstone. In addition, sedimentary rocks often form
porous and
permeable reservoirs in
sedimentary basins in which
petroleum and other
hydrocarbons can be found (see
Bituminous rocks).
It is believed that the relatively low levels of
carbon dioxide in the
Earth's
atmosphere, in comparison to that of
Venus, is because of large amounts of carbon being trapped in limestone and
dolomite sedimentary layers. The flux of carbon from eroded sediments to marine deposits is part of the
carbon cycle.
The shape of the particles in sedimentary rocks has an important effect on the ability of
micro-organisms to colonize them. This interaction is studied in the science of
geomicrobiology. One measure of the shape of these particles is the roundness factor, also known as the Krumbein number after the geologist
W. C. Krumbein.
Further Information
Get more info on 'Sedimentary Rock'.
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