Rocks - Sedimentary

1. SEDIMENTARY ROCKS

2. Sedimentary Rocks
 Earth’s crust is made mostly of Igneous Rocks.
 But, most rocks on Earth’s surface are sedimentary rocks
(75%).
 All sedimentary rocks form in an aqueous solution.
 Sedimentary is derived from the Latin sedimentum, which
means “settling”.

3. Sedimentary Rocks
 There are 3 main types of Sedimentary Rocks
1. Clastic
2. Chemical
3. Organic
Sedimentary rocks are rocks that
are made of broken-down materials
from other pre-existing rocks on
Earth.
Clastic: Rocks that formed from sediment fragments of other rocks.
Chemical: Rocks formed when dissolved minerals drop out of
solution.
Organic: Rocks that form from the remains of once-living
organisms.

4. Clastic Sedimentary Rock
Formation
1. Weathering
2. Erosion
3. Deposition
4. Compaction
5. Cementation

5. Clastic Sedimentary Rocks:
Weathering
 Rocks on Earth slowly break down under forces of
nature.
 Wind, water, and ice break rock apart.
 This is called weathering (Think of weathering like a
“sand-blaster”).When rock weathers, it breaks into fragments or pieces.
These rock fragments are called sediment.
Sediment includes: gravel, pebbles, sand, slit, and clay.

6. Clastic Sedimentary Rocks:
Sediment
Gravel
Sand From Biggest to Smallest
Silt Based on its size…
Clay
 Sediment is given a name based on it’s size.

7. Clastic Sedimentary Rocks:
EROSION
Strong Currents:
• Carries clay,
silt, sand, and
gravel.
Medium Currents:
• Carries clay,
silt, and sand.
Gentle Currents:
• Carries clay and
silt.
Quiet Currents:
• Carries mostly
clays and muds.
 Erosional currents (moving wind and water) have energy to carry
sediment.
 If the sediment is large, it will need a stronger erosional current to
carry it away.
 As the current slows , the largest sediments begin to drop out first.

8. Clastic Sedimentary Rocks:
EROSION
Strong Currents:
• In the desert when the
wind picks up sand and
sandblasts rock.
Medium Currents:
• At the beach when the
waves come in and erode
the beach, you feel sand
suspended in the water.
Gentle Currents:
• On a hill where gullies
form from water running
over the land.
 These sediments are carried away by wind and
water.
 This process is known as erosion.
 Large sediments – Needs strong current to move
it.

9. Clastic Sedimentary Rocks:
EROSION
LIKE SANDPAPER
• The movement of the sediment acts like
sandpaper to grind it down and smooth it out.
 As these sediments are carried away, they are broken down
more.
 The sediments start out large and jagged.
 As wind and water bounces them around, they get smaller.
 They also become more rounded the further away they are
carried.

10. Clastic Sedimentary Rocks:
Deposition
Deposition occurs when erosional currents slow down!
If the current slows down, there is not enough energy to
continue to carry the sediments in the current.
As a result, the sediments fall out and are deposited.
 Eventually these sediments will be deposited.
 Deposition: when sediments settle out of
moving currents.
Ex. 1
• When a stream
enters a lake.
Ex. 2
• When a river enters
an ocean.

11. Clastic Sedimentary Rocks:
Deposition
 Sediments are usually deposited in layers that
build up.
 Just like when you make “deposits” in a bank.
 Your money “builds-up” over time.
 Or, when you deposit your books in your locker
throughout the day.

12. Clastic Sedimentary Rocks:
Deposition
 After sediment is deposited, clastic
sedimentary rocks form by:
 Compaction
 Cementation

13. Clastic Sedimentary Rocks:
Compaction
 As sediment builds up, it begins to compact.
 Pressure from the layers above push down on the
lower layers.
 Think of a trash compactor.
 Compacting snow to make a snowball.
 If sediments are really
small (Silt or Clay)
they will stick together
forming rock.
 This rock forming
process is known as
Compaction.

14. Clastic Sedimentary Rocks:
Compaction
 Clay sediments compact to make the sedimentary
rock Shale.
 Silt sediments compact to make the sedimentary
rock Siltstone.

15. Clastic Sedimentary Rocks:
Cementation
 Larger sediments cannot just be squeezed together to
make rock.
 There needs to be something else that holds the rock
together. Water, carrying dissolved minerals, runs through the pore spaces between sediment.
 Water drains or evaporates, leaving minerals behind.
 Minerals in the water act like glue (quartz and calcite).
 The minerals harden and cement the larger sediment together as rock.
 If sediments are large (Sand and Gravel) they
will not stick together through compaction.

16. Clastic Sedimentary Rocks:
Cementation
This rock forming process is known as cementation.

17. Clastic Sedimentary Rocks:
Cementation
 Sand makes Sandstone
 Sand and Gravel makes Conglomerate
 Large and sharp rock fragments make Breccia

18. Clastic Sedimentary Rocks:
Cementation
 Think of cementation like fruit cake.
Fruit cake has
sediments
• Nuts
• Cherries
• Raisins
• Pineapple
• Cloves
 But those sediments do not stay together on their
own.
 The sediments would be loose if they were not
cemented together with – Batter.

19. Clastic Sedimentary Rocks:
Classification
 Sedimentary rocks are classified by sediment size:
 Clay, Silt, Sand, and Gravel are the major types of
sediment.
 They are compacted and cemented to form sedimentary
rocks.
 Each sediment forms a different type of rock.
Sediment Clay Silt Sand Gravel
Example Shale Siltstone Sandstone
Conglomerate
or Breccia
Size Range
< 0.004
mm
0.004 –
0.063 mm
0.063 –
2.0mm
>2.0 mm

20. Clastic Sedimentary Rocks
 Summary of Clastic Sedimentary Rocks:
 Sedimentary rocks form from pre-existing rocks.
 Rocks on Earth weather and break into
sediments.
 The sediment is eroded and carried away.
 The carried away sediment is later deposited.
 As sediment is deposited, it builds up layer upon
layer.
 The layers compact over time.
 Minerals dissolved in water start to glue sediment
together.

21. Chemical Sedimentary Rocks
 Chemical sedimentary rocks form from
dissolved minerals.
 Water contains dissolved minerals
 When water evaporates, those minerals
precipitate.
 Precipitate means to come out of solution.
 The mineral crystals grow together to make the
rock. Types of Chemical Sedimentary Rocks:
1. Limestone (most common)
2. Rock Salt

22. Chemical Sedimentary Rocks:
Limestone
 Travertine Limestone:
Limestone often found in
caves.
 Limestone
 Forms when Calcium Carbonate precipitates from
solutions.
 The calcium Carbonate is essentially the mineral
Calcite.
 How can we tell if a rock is Limestone?

24. Chemical Sedimentary Rocks:
Rock Salt
 Rock Salt
 Water rich in salt evaporates to leave salt crystals
behind.
 Those crystals are especially the mineral Halite.
 Halite forms the rock, Rock Salt.

25. Organic Sedimentary Rock
 Rocks that come from the remains of organic
matter.
 Types:
 Petrified Wood
 Coal
 Fossiliferous Limestones

26. Petrified Wood
 Petrified wood is a fossil.
 It forms when plant material is
buried by sediment and
protected from decay by oxygen
and organisms.
 Then, groundwater rich in
dissolved solids flows through
the sediment replacing the
original plant material with silica,
calcite, pyrite or another
inorganic material such as opal.

27. COAL
 Coal is formed from vegetation that previously existed
in swampy and marshy soils which prevented their full
decay after their death.
 As their remains piled up and were covered by more
and more deposits, they gradually underwent
compaction and cementation.

28. Chalk and Fossiliferous
Limestones
 Chalk and Fossiliferous Limestones - formed from
the skeletons of marine organisms
 Chalk is a soft, white, porous sedimentary carbonate
rock, a form of limestone composed of the mineral
calcite. Calcite is calcium carbonate or CaCO3

29. Chalk and Fossiliferous
Limestones
 Fossiliferous limestone is any type of limestone, made
mostly of calcium carbonate (CaCO3) in the form of the
minerals calcite or aragonite, that contains an abundance
of fossils or fossil traces.