Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks: 1)
clastic sedimentary rocks such as breccia, conglomerate, sandstone and shale, that are formed from mechanical weathering debris; 2)
chemical sedimentary rocks such as rock salt and some limestones, that form when dissolved materials precipitate from solution; and, 3)
organic sedimentary rocks such as coal and some limestones which form from the accumulation of
plant or animal debris. Pictures and brief descriptions of some common
sedimentary rock types are shown below.
Breccia is a clastic sedimentary rock that is composed of large
(over two millimeter diameter) angular fragments. The spaces between the
large fragments can be filled with a matrix of smaller particles or a
mineral cement which binds the rock together. The specimen shown above
is about two inches (five centimeters) across.
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Chert is a microcrystalline or cryptocrystalline sedimentary rock material composed of silicon dioxide (SiO
2).
It occurs as nodules and concretionary masses and less frequently as a
layered deposit. It breaks with a conchoidal fracture, often producing
very sharp edges. Early people took advantage of how chert breaks and
used it to fashion cutting tools and weapons. The specimen shown above
is about two inches (five centimeters) across.
Coal is an organic sedimentary rock that forms mainly from plant
debris. The plant debris usually accumulates in a swamp environment.
Coal is combustible and is often mined for use as a fuel. The specimen
shown above is about two inches (five centimeters) across.
Conglomerate is a clastic sedimentary rock that contains large
(greater then two millimeters in diameter) rounded particles. The space
between the pebbles is generally filled with smaller particles and/or a
chemical cement that binds the rock together. The specimen shown above
is about two inches (five centimeters) across.
Iron Ore is a chemical sedimentary rock that forms when iron and
oxygen (and sometimes other substances) combine in solution and deposit
as a sediment. Hematite (shown above) is the most common sedimentary
iron ore mineral. The specimen shown above is about two inches (five
centimeters) across.
Limestone is a rock that is composed primarily of calcium
carbonate. It can form organically from the accumulation of shell,
coral, algal and fecal debris. It can also form chemically from the
precipitation of calcium carbonate from lake or ocean water. Limestone
is used in many ways. Some of the most common are: production of cement,
crushed stone and acid neutralization. The specimen shown above is
about two inches (five centimeters) across.
Rock Salt is a chemical sedimentary rock that forms from the
evaporation of ocean or saline lake waters. It is also known by the
mineral name "halite". It is rarely found at Earth's surface, except in
areas of very arid climate. It is often mined for use in the chemical
industry or for use as a winter highway treatment. Some halite is
processed for use as a seasoning for food. The specimen shown above is
about two inches (five centimeters) across.
Sandstone is a clastic sedimentary rock made up mainly of
sand-size (1/16 to 2 millimeter diameter) weathering debris.
Environments where large amounts of sand can accumulate include beaches,
deserts, flood plains and deltas. The specimen shown above is about two
inches (five centimeters) across.
Shale is a clastic sedimentary rock that is made up of clay-size
(less then 1/256 millimeter in diameter) weathering debris. It typically
breaks into thin flat pieces. The specimen shown above is about two
inches (five centimeters) across.
Siltstone is a clastic sedimentary rock that forms from silt-size
(between 1/256 and 1/16 millimeter diameter) weathering debris. The
specimen shown above is about two inches (five centimeters
الأحد فبراير 25, 2018 3:13 pm من طرف carate
