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Limestone
Shale
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Rocks
There are three types of rocks: Igneous rocks, Metamorphic
rocks and sedimentary rocks. Igneous rock form by the cooling
of magma. Metamorphic rocks form by exposing already existing
sediments to high temperatures or pressure (deep in the
earth's crust). Fossils only occur in sedimentary rocks
which are described below.
Sedimentary rocks
Most sedimentary rocks are formed when erosion products
are deposited in layers, and over time lithifies into rock.
Erosion products are the materials that form during erosion
of rocks by water, ice, wind, etc. This is how conglomerates
(pebble stone), sandstones and shales form. These rocks
are ultimately erosion products of igneous rocks.
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Conglomerate
Sandstone
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Other sedimentary rocks do not originate from erosion
products. For example, peat can compact and lithify into coal.
Limestone forms when little chalk skeletons of sea organisms
sink to the seafloor. Under special circumstances
some sedimentary rocks can form by precipitation of dissolved
chemicals from the seawater. For example gypsum and rocksalt.
Time, high pressure (when sediments are buried deep in
the earth's crust), and high temperature will cause loose
sediments to lithify into rocks. This is the result of compaction
and several chemical processes in the sediment.
When sediments are buried deep in the earth's crust, or exposed
to high temperatures, new types of rocks can be formed as
a result. Fossils usually do not survive this process. The
rocks that are formed are described in the table below:
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Original:
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Rock:
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Slightly metamorphic rock:
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Metamorphic rock:
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--> Higher temperature and pressure-->
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clay
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Shale
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Phylite / Schist
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Metamorphic rocks like gneiss
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sand
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Sandstone
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Quartzite
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chalk
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Limestone
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Marble
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The most abundant sedimentary rocks:
In the table below the most abundant sedimentary rocks
are described. There are several photographs of the described
sediments on this page. The grainsize described, is the average
grainsize.
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Shale
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'Lithified' clay (grains are smaller than 0,004 millimetre).
Breaks usually in thin slices. The rock is soft and
erodes fast. This rock is ultimately an erosion product
of igneous rocks. (See photo)
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Siltstone
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Siltstone consists of grains between 0,004 and 0,0062
millimetre. The grainsize is in between that of shale
and sandstone. This rock is ultimately a erosion product
of igneous rocks.
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Sandstone
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Sandstone consists of grains between 0,062 and 2 millimetre.
This rock is also ultimately an erosion product of igneous
rocks. (See photo)
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Conglomerate
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A conglomerate is a 'pebble stone' with rounded pebbles
in a finer matrix. The pebbles are ranging between 2
millimetre to 4 metres (!). This rock is also ultimately
an erosion product of igneous rocks. (See photo)
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Breccia
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A Breccia is basically the same as a conglomerate,
but the pebbles are angular in stead of rounded.
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Coal
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Coal forms when plant material is compacted. This rock
is black because of the organic material.
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Limestone
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Limestone consists mainly of Calcium-carbonate. This
rock often contains fossils. The sediment forms when
little chalk skeletons of sea organisms sink to the
seafloor.
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Marl
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Marl is a rock consisting of clay and chalk.
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Salt / Gypsum
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This sedimentary rocks form by precipitation of dissolved
chemicals from the seawater. For example: salt sinks
to the seafloor when the salt concentration becomes
to high because of evaporation.
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Ripple marks
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Sedimentary structures
Geologists are always on the lookout for sedimentary structures
to reconstruct the environment during the forming of the deposit.
Fossils can be an important aid!
By studying the layers, structures (like ripple marks,
cross-bedding, etc.) and the sediment itself, a geologist
can make a reconstructing of the former environment during
deposition. The geologist can determine if the sediment is
a deep ocean-, river-, coastal-, or wind-deposit.
When fossils are present in the sediment, they also can
be used for the environment reconstruction. Most of the research
on fossils are on microfossils. The advantage of studying
microfossils in sediments is the abundance of these fossils
in most sediments. Most microfossils can only be seen under
a microscope.
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Burrows
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