Sedimentary rock is one of the three main rock groups (along with igneous
and metamorphic rocks) and is formed in three main ways—by the deposition of the
weathered remains of other rocks (known as clastic sedimentary rocks); by the
deposition of the results of biogenic activity; and by precipitation from
solution. Sedimentary rocks include common types such as chalk, limestone,
sandstone, and shale.
Sedimentary rock is one of the three main rock groups (along with igneous
and metamorphic rocks) and is formed in three main ways—by the deposition of the
weathered remains of other rocks (known as clastic sedimentary rocks); by the
deposition of the results of biogenic activity; and by precipitation from
solution. Sedimentary rocks include common types such as chalk, limestone,
sandstone, and shale.
Formation
Sedimentary rocks are formed from overburden pressure as particles of
sediment are deposited out of air, ice, 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 undergone by a sediment after its initial deposition and
during and after its lithification, exclusive of surface alteration
(weathering).
Sedimentary rocks contain important information about the history of the
Earth. They contain fossils, the preserved remains of ancient plants and
animals. 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 do not destroy fossil remnants.
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 five percent 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.
Clastic sedimentary rocks
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 feldspars, amphiboles, clay minerals, and
sometimes more exotic igneous and metamorphic minerals.
Clastic sedimentary rocks may be regarded as falling along a scale of grain
size, with shale being the finest with particles less than 0.004 mm, siltstone
being intermediate with particles between 0.004 to 0.06 mm, and sandstone being
coarser still with grains 0.06 tp 0.2 mm, and conglomerates and breccias being
the coarsest with grains 2 to 256 mm.
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.
Courser 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.
All rocks disintegrate slowly as a result of mechanical weathering and
chemical weathering.
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.
Biogenic sedimentary rocks
Biogenic sedimentary rocks contain materials generated by living organisms,
and include carbonate minerals created by organisms, such as corals, molluscs,
and foraminifera, which cover the ocean floor with layers of calcite which can
later form limestone. Other examples include stromatolites, and the flint
nodules found in chalk (which is itself a biogenic sedimentary rock, a form of
limestone).
Precipitate sedimentary rocks
Precipitate sedimentary rocks form when mineral solutions, such as sea
water, evaporate. Examples include the evaporite minerals halite and gypsum.
Other information
Sedimentary rocks are economically important in that they can be used as
construction material. In addition, sedimentary rocks often form porous and
permeable reservoirs in sedimentary basins in which petroleum and other
hydrocarbons can be found.
It is believed that the relatively low levels of carbon dioxide in the
Earth's atmosphere, in comparison to that of Venus, is due to large amounts of
carbon being trapped in limestone and dolomite sedimentary layers. The flux of
carbon from eroded sediments to marine deposits is known as 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.
