Sedimentary Rocks - TeacherWeb

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22 févr. 2014 (il y a 3 années et 3 mois)

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Sedimentary Rocks

What is a sedimentary rock?

Sedimentary rocks are formed from the
compaction and cementation of
sediment
.

The processes of
weathering

and
erosion

break
down rocks to produce inorganic sediment (pieces
of preexisting rocks).

These rock fragments can become a sedimentary
rock if sufficient burial and compaction of the
sediment occurs.

All sedimentary rocks are made from pieces of
other rocks.

How is sediment transported?


Sediment is transported by
a transporting agent.
Transporting agents
include:

Water

Wind

Glaciers

Mass Wasting (gravity)

How does sediment become a
sedimentary rock?


Sediment can become
sedimentary rock through
one (or more) of the
following processes:

Compression

Cementation

Chemical Processes

Biological Processes

**The scientific term for
sedimentary rock
formation is
lithification.

Rock Formation by Compression

Since most sediment is deposited in
basins

(areas of lower elevation), it is very common
for sediment to get buried below other
sediment. This puts tremendous amounts of
pressure on the buried rock fragments.

Over time the sediment may be exposed to
compression

(squeezing) due to pressure of
overlaying sediment or water. If sufficient
pressure is applied to the sediment it can
form a sedimentary rock.

An example of a rock formed by
compression is sandstone. How does
sandstone differ from siltstone?

Rock Formation by Compression

Since most sediment is deposited in
basins

(areas of lower elevation), it is very common
for sediment to get buried below other
sediment. This puts tremendous amounts of
pressure on the buried rock fragments.

Over time the sediment may be exposed to
compression

(squeezing) due to pressure of
overlaying sediment or water. If sufficient
pressure is applied to the sediment it can
form a sedimentary rock.

An example of a rock formed by
compression is sandstone. How does
sandstone differ from siltstone?

Answer: Sandstone has a larger grain size than
siltstone.


Rock Formation by Cementation

Some minerals that are soluble in water
can eventually become cementing agents
in a process called cementation. You
can think of these minerals as glue that
holds the sediment together.

Cementation occurs when sediments are
combined with mineral cements that
precipitate

out of solution (ground
water) and “glue”
clasts

of rock together.
The cementing agent will often
determine the color of the rock.


The processes of compression and
cementation often work together to form
a sedimentary rock.

Rock Formation by Chemical
Processes

Some sediment is created through chemical weathering when soluble minerals
are dissolved by water to form a solution. Sea water is a common example of
this type of solution.

Often, evaporation of water causes the mineral
precipitation

that results in the
formation of a chemically formed sedimentary rock. When water evaporates
the soluble materials are left behind as chemical deposits. The rocks that form
from this process are called
evaporites
.

If an evaporite is made of only one type of mineral it is referred to as a
monomineralic rock.


Rock Formation by Biologic
Processes

The terms
biologic

and
organic
refer to
living things. Therefore, rocks formed
from biologic processes must involve
things that were once living.

Organic sediments

are the remains of
any living thing (plants or animals).
These sediments form
bioclastic

sedimentary rocks.

The most common examples of
bioclastic rocks are limestone and coal.
Briefly describe how these two
sedimentary rocks are formed. (See p. 7
of your ESRT).

How are sedimentary rocks
classified?

Sedimentary rocks are classified as
clastic (fragmental), chemical
(crystalline), or organic
(bioclastic) depending on how
they were formed.


Clastic sedimentary rocks are
classified on the basis of grain
size.

Organic or chemically formed
sedimentary rocks are identified
through composition and
texture.


Fossils and Sedimentary Rocks

**Fossils are found
exclusively in
sedimentary rocks.


Give one reason why
you would not expect
to find fossils in:

Metamorphic rocks

Igneous rocks

Identification of Sedimentary Rocks

Turn to page 7 of your reference table.