Transcript How Rocks are Formed: Sedimentary

How Rocks are Formed:
Sedimentary
Most of Earth’s crust is made of
igneous rock, but most of its surface
is covered by sedimentary rocks.
Three Main Kinds of Sedimentary Rock
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Clastic: fragments of other rocks
Chemical: mineral grains that come
from dissolved solutions by
evaporation or chemical action
Organic: remains of plants and
animals
Clastic Rocks
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Clastic sedimentary rocks come from the
weathering of rocks that already exist
Winds, waves, and glaciers pick up and
move the particles
Sediments are deposited when a stream
slows down
Ocean water, lake water, and groundwater
contain natural cements that include
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
Sandstone
Shale
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 animals and plants
The most common are limestone and
coal
Organic limestone is mostly calcite,
shell producing animals die and their
shells pile up and are cemented
Coal
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
The layers are called beds and are separated
by bedding planes
Cross-bedding may develop when beds are
deposited by wind on dunes or by rivers on
deltas or sandbars
Fossils in Sedimentary Rocks
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Animals and plants that die as sediments
pile up are buried
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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Nodule
Nodules are lumps of fine-grained silica called chert
in limestones
Concretions are round masses of calcium carbonate
in shale
Geodes are limestones with hollow spheres of silica
rock, the hollows can be filled with crystals of
quartz or calcite