Earth & Space Science: Session 2
A Closer Look: Sedimentary Rocks
What is sedimentary rock?
Have you ever looked at an outcropping of rock and noticed that it’s made of bands of different colors and/or textures? Those layers that you’re seeing are the telltale sign that you’ve found sedimentary rock. Sedimentary rock covers most of the Earth’s surface. The layers in sedimentary rocks are formed from the weathered products of other rocks (igneous, metamorphic, or sedimentary rocks). These products are rock fragments called sediment. Sediment can range in size from the tiniest particles of clay to boulders. Sedimentary rock forms at or near the Earth's surface at relatively low temperatures and pressures, and usually in water.
How does sedimentary rock form?
Deposition
Sediments that form sedimentary rocks can be transported by
several mechanisms that include wind, ice, and, most commonly,
moving water.
As the energy of water currents decreases — as when a river or
stream empties into a lake or ocean — the sediments settle out
of the water due to gravity. The heavier and larger particles,
such as gravel and sand, settle first. The lighter sediments, such
as silt
and clay, settle last. This laying down of sediments by natural
processes is called deposition.
Compaction, Recrystallization, and Cementation
The deposited sediments gradually accumulate, forming layers.
The weight of the overlying sediment compacts the sediment below.
This compaction squeezes together the layers of sediment, forcing
the grains
together, making the pore spaces around each grain smaller, and
squeezing out some of the water. The remaining water surrounding
the sediments
can contain dissolved minerals, which later recrystallize as
new minerals in the pore spaces. The crystals interlock and connect
the sediment
grains, which essentially "glues" the sediment together.
This forms solid rock. Further compaction and burial can cause
additional recrystallization, making the rock even harder. Calcite
and silica
are common minerals that cement individual sediments together.
Rocks formed in this way are called clastic sedimentary rocks because
the
rock is composed of “clasts” or individual sediments.
Chemical
Processes
Sediments that have precipitated out of solution form chemical
sedimentary rocks. Sedimentary rocks formed by the evaporation
of sea or lake water are called evaporites. As water evaporates,
the concentration
of the salt solids (salt meaning soluble compounds, not just
the familiar sodium chloride commonly known as table salt)
increases and the amount
of water decreases. Halite and gypsum are examples of evaporites.
Biological Processes
Biological processes can form sedimentary rocks in which the
layers are formed from the remains of plants and animals.
In warmer, shallower ocean water, layers of limestone can
form from the accumulation
of the calcium carbonate that composes the shells and skeletons
of coral, shellfish, and other marine organisms. In colder,
deeper waters,
layers of sediment can form from the remains of microorganisms
called foraminiferans and diatoms, which secrete shells of
calcium carbonate
or silica. Rocks formed in this way are called biochemical
sedimentary rocks.
Biological processes also form sedimentary rocks from the accumulation and compaction of dead plant material, typically under wet, acidic conditions in regions with an abundance of growing vegetation. The vegetation proliferates so quickly that new layers of vegetation rapidly bury the dead and decaying plant material. When buried like this, the bacteria use up the oxygen that is available and cannot finish the decomposition of the vegetation. This matter may change to solid carbon in the form of coal, or be converted to hydrocarbons, the source of petroleum oil.
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