Sedimentary Rocks
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"Sedimentary" means
"settled" or "layered".
- How they are made:
- Formed from sediment. Sediment is
bits of broken down rock, minerals, and even remains of plants and
animals.
- Sediment accumulates (piles up)
over a long time, usually under water (like in oceans,
lakes, rivers).
- The weight of the layers on top presses
down on the layers below (compaction).
- Water and minerals act like cement
to glue the sediment particles together (cementation).
- This whole process turns loose sediment
into solid sedimentary rock.
- Characteristics of Sedimentary Rocks:
- Layered (Stratified):
The most important feature! They form in layers called strata.
You can often see these layers.
- Layers vary:
Strata can be thin (inches) or thick (feet).
- Texture:
Can be coarse (rough, like sandstone with big grains) or fine-grained
(smooth, like clay with tiny particles).
- Hardness:
Can be soft (like chalk) or hard (like some sandstones).
- Made of different materials:
Sediment can be brought by:
- Streams & Rivers
- Glaciers
(ice)
- Winds
- Even animals!
(shells, coral)
- Non-crystalline:
Usually not made of crystals (unlike igneous rocks).
- Often contain Fossils:
Because they form from sediment that piles up slowly, they can trap and
preserve fossils of animals, plants, and tiny organisms. This is a
key feature!
- Most varied:
Sedimentary rocks are the most varied type of rock because they
can be made from so many different things and in different ways.
- Classification of Sedimentary Rocks:
We can group them in different ways, but the paragraph talks about types
based on how they are formed:
- Clastic (Mechanically Formed)
Sedimentary Rocks:
- "Clastic" means
"broken pieces".
- Made from:
Bits of broken down rocks and minerals that are carried and deposited.
- Examples:
- Sandstone:
Made of sand grains (often quartz from granite).
- Texture, color, and what's in it
can vary a lot.
- Used for building, grindstones
(for sharpening).
- "Grit" is a coarse
type of sandstone.
- Conglomerate:
Like sandstone, but made of larger, rounded pebbles cemented
together. Think of "concrete" but natural.
- Breccia:
Like conglomerate, but made of angular (sharp-edged) rock fragments
cemented together.
- Clay:
Very fine sediment. Used for brick-making.
- Shale (Mudstone):
Also very fine sediment, like hardened mud or clay.
- Sand and Gravel:
Loose sediment, not yet cemented into solid rock.
- Organically Formed Sedimentary Rocks:
- Made from:
Remains of living things.
- Examples:
- Calcareous Rocks (Limestone &
Chalk):
- Made from shells and
skeletons of sea creatures like corals and shellfish.
- "Calcareous" means
made of calcium carbonate (like shells).
- Limestone:
Harder, used for building, cement.
- Chalk:
Softer, used for writing, etc.
- Carbonaceous Rocks (Coal, Peat,
Lignite):
- Made from plant matter
from old swamps and forests.
- Plant stuff gets buried, compressed
by layers on top.
- Over time, turns into carbon-rich
materials:
- Peat:
Partly decayed plant matter (early stage of coal).
- Lignite (Brown Coal):
Softer, lower grade coal.
- Coal:
Hard, black, used for fuel. Very important economic value (for
energy).
- Chemically Formed Sedimentary Rocks:
- Made from:
Chemicals that precipitate (come out of) solutions (liquids).
- Examples:
- Rock Salt:
Forms from dried up seas or lakes. When salty water evaporates,
salt minerals are left behind and form layers.
- Gypsum (Calcium Sulphate):
Forms from evaporation of salty lakes with high salt content,
like the Dead Sea.
- Potash & Nitrates:
Can also form in similar chemical ways.
In Simple Words: Sedimentary
rocks are "layered rocks" made from sediment that settles down and
gets cemented together over time, usually underwater. They often have fossils.
There are three main types: clastic (made of rock bits like sandstone), organic
(made of living things like limestone and coal), and chemical (made from
chemicals in water like rock salt).