Rocks - Sedimentary

SEDIMENTARY ROCKS

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”.

Sedimentary Rocks

There are 3 main types of Sedimentary Rocks1. Clastic2. Chemical3. 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.

Clastic Sedimentary Rock Formation1. Weathering2. Erosion3. Deposition4. Compaction5. Cementation

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.

Clastic Sedimentary Rocks: Sediment

GravelSand From Biggest to SmallestSilt Based on its size…Clay

Sediment is given a name based on it’s size.

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.


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.


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.

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.


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.


After sediment is deposited, clastic sedimentary rocks form by: Compaction Cementation

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.

Clastic Sedimentary Rocks: Compaction

Clay sediments compact to make the sedimentary rock Shale.

Silt sediments compact to make the sedimentary rock Siltstone.

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.


This rock forming process is known as cementation.


Sand makes Sandstone Sand and Gravel makes Conglomerate Large and sharp rock fragments make Breccia


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.

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 SandstoneConglomer

ate or Breccia

Size Range < 0.004 mm

0.004 – 0.063 mm

0.063 – 2.0mm

>2.0 mm

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. Finally, a clastic sedimentary rock is formed.

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

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?

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.

Organic Sedimentary Rock Rocks that come from the remains of

organic matter. Types:

Petrified Wood Coal Fossiliferous Limestones

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.

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.

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

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.

Rocks - Sedimentary

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