Transcript How Rocks are Formed: Sedimentary

Sedimentary Rocks
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Sedimentary rocks form when
sediments harden into rocks
3 main kinds clastic, chemical and
organic
Most of Earth’s crust is covered by
sedimentary rocks
Three Main Kinds of Sedimentary Rock
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Clastic:cemented together
fragments of other rocks
Chemical:mineral grains that are
removed from a solution by
evaporation or chemical action
Organic: remains of plants and
animals
Clastic Rocks
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Clastic sedimentary rocks are made up of
weathered rocks that already exist
Rocks are weathered and transported by
rivers, winds, waves, and glaciers
Sediments are deposited when the
transport system loses energy ie: stream
Ocean water, lake water, and groundwater
contain natural cements: silica(SiO2),
calcite (CaCO3), and iron oxide(FeO)
These dissolved minerals settle into the
spaces between the sand grains or
pebbles, binding them together into rock
The pressure of overlying sediments can
also make fine sediments stick together
Clastic Rocks: Conglomerate
• Coarse, large grains
• Deposited in high
energy system (rough
water)
• Cemented mixture of
rounded pebbles and
sand grains
• Pebbles can be any
durable rock material
Clastic rocks: Sandstone
• Medium grained
• Quartz grains (7
on Moh’s scale of
hardness)
• Porous (small
holes)
• Permeable (water
can pass through)
Clastic rocks: Shale
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Fine grained
clay minerals
Impermeable
Smooth, soft and
easily broken
Found in very low energy
environments
Sorting of Sediments
Chemical Sedimentary Rocks
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Dissolved minerals fall out of solution
Evaporation or the combining of dissolved
ions to form new minerals.
The most common are limestone, rock
salt, and rock gypsum
Limestones are formed from tiny grains of
calcite
Rock salt is the natural form of table salt,
it is almost pure halite
Rock gypsum occurs in layers and is
almost pure gypsum
Chemical Sedimentary Rocks
Rock Salt
Limestone
Organic Sedimentary Rocks
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Organic sediments come from the
remains of plants and animals
The most common are shell limestone
and coal
shell limestone is mostly calcite, shell
producing animals die and their shells
pile up and are cemented together
Sedimentary Features: Stratification
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Stratification is the arrangement of rocks in
visible layers
When there is a change in the type of
sediments being laid down, new rock layers
are formed
Change results from new source sediments or
energy change
The layers are called beds and are separated
by bedding planes
Sedimentary Features: Cross Bedding
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Cross-bedding develops when beds are
deposited at an angle:
wind on dunes, rivers on deltas or
sandbars
Fossils in Sedimentary Rocks
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Animals and plants that die and are
buried, as sediments pile up= fossils
The hard parts may remain as fossils
when the sediments turn to rock
Fossils are the remains, impressions, or
any other evidence of plants and animals
preserved in rock
Ripple Marks and Mud Cracks
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Many sandstones show ripple marks that
are formed by the action of winds,
streams, waves or currents on sand.
Mud cracks develop when deposits of wet
clay dry and contract
Mud cracks are later filled with different
materials, and the clay becomes shale
rock.
Nodules, Concretions, Geodes
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Nodules are lumps of fine-grained silica called
chert, solid replacement bodies
Concretions are round masses of mineral
precipitation that form around some kind of nucleus
A Geode is a hollow nodule of silica rock, filled with
crystals of quartz or calcite
Geode
Nodule