CO QAH + MELC LW

Course Outline & Quality Assured Handouts HANDOUT No. 2

paired with MELC- Based Learner’s in EARTH SCIENCE
Worksheet

MELC:
4. Classify rocks into igneous, sedimentary, and metamorphic (S11ES-Ic-6)
5. Identify the minerals important to society (S11ES-Ic-7)

Semester: 1 Week No. 2 Day: 1-4

LESSON 1: CLASSIFICATION OF ROCKS

Rocks are combination of one or more kinds of minerals that can be mixed with other components
likes fossils because it undergoes through a natural cyclic process. Also, rocks are known to be the foundation
of geosphere.
There are three classifications of rocks named as igneous, sedimentary, and metamorphic. They
are being classified based on the processes they undergo to form and the mineral content that they
have.

TOPIC 1: Igneous Rocks

Igneous rock was derived from the Latin word “ignis” meaning fire. It is because all rocks under this
classification came from a crystallization of magma, also known as the molten volcanic materials within or at the
Earth’s surface. Crystallization happens in a cooling down process of materials to form shape.

Two types of igneous rocks:

1. Volcanic rocks (extrusive igneous rocks) - form when magma rises to the top of Earth’s surfaces and
cooling molten materials takes rapidly. These kinds of rocks produce volcanoes, lava flows and tephra.

2. Plutonic rocks (intrusive igneous rocks) - form when magma solidifies beneath the Earth’s surface
and cooling of molten materials takes slowly. These produces plutons remained unexposed not until
erosion happened.

TEXTURE refers to the size, shape, and arrangement of mineral grains and other constituents.

Due to the different rates of cooling of magma, sizes of crystals in mineral also differs. Rapid cooling in
volcanic rocks produces a very small type of crystals. These crystals that are microscopic in size. On the other
hand, slow cooling creates a large and visible crystals in plutonic rock. With this, by just simply looking at the
crystal sizes of a mineral, classifications of rocks could also be determined.

Figure 1: Common Igneous Rocks

Retrieved from: https://engineering.tiu.edu.iq/civil/wp-content/uploads/2019/02/Lec.-GEO-3.pdf

From the illustration, crystal size of minerals can be seen. At the same time, by just looking at the texture,
classifications of rocks could also be determined.
 Magma is composed largely of eight elements listed as: oxygen- 46.6%; silicon -27.7%; aluminum-
8.1%; iron -5.0%; calcium -3.6%, sodium- 2.8%; potassium- 2.6% and magnesium- 2.1%. These materials
constituents mostly with silicate materials. This explains why igneous rocks are composed of silicate materials
that is made out of silicon and oxygen. Also, igneous rocks are considered as primary rocks since most of its
composition belongs to Earth’s crust.
COLOR determines the silica content of minerals. Minerals that are rich in silica materials are light-
colored. An example to these is quartz and feldspar. These minerals belong to granite (plutonic rocks) and
rhyolite (volcanic rocks). On the other hand, minerals that are dark colored is considered poor in silica content
such as pyroxene, feldspar and olivine which can be found in basalt (volcanic rocks) and gabbro (plutonic rocks).

Figure 2: Silica Content Differentiation

Retrieved from: McGraw-Hill Education

From the illustration, it can be observed that as the degree of color changes, silica content also differs.

Bowen’s Reaction Series explains how different igneous rocks formed. For a short background, back in
the early 1900's, N. L. Bowen and others at the Geophysical Laboratories in Washington D.C. began
experimental studies into the order of crystallization of the common silicate minerals from a magma. The
idealized progression which they determined is still accepted as the general model for the evolution of magmas
during the cooling process. Bowen was able to determine that specific minerals form at specific temperatures
as a magma cool.

Figure 3: Bowen’s Reaction Series

Retrieved from: http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry32.html
 TOPIC 2: Sedimentary Rocks

Sedimentary rocks came from the word “sedimentum” meaning settling. It is commonly composed of
sediments that forms into layers. Sediments are the rock and mineral fragments result from weathering in a
parent rock. In horizontal layers of sedimentary rocks, oldest beds are at the bottom and the youngest are at the
top.
Classification of sedimentary rocks are based on the identified composition and process by which it
was formed. Three types of these are:
1. Clastic
2. Chemical
3. Biochemical

CLASTIC/DETRITAL SEDIMENTARY ROCKS

originates from weathering and erosion. It is composed of
particles from microscopic clay to boulders.

The three stages of formation are:

A. Generation - a process of physical and chemical

breakdown of any rock through weathering to form sediments.
Sediments are classified by its grain size. These are the
following arranged from the biggest size of grain to smallest.
• Gravel (>2mm in size)
• Sand (1/16mm to 2mm in size).
• Silt (1/256mm to 1/6mm in size)
• Clay (1/256mm)

B. Transportation - erosion happened where sediments

moved from its origin to another location through streams,
wind, and glaciers. Sizes of grains differ based on the
transport medium that will be in used.

C. Lithification - refers to the processes by which

sediments are transformed into solid sedimentary rocks.
Figure 4: Types of Clastic Sedimentary Rocks
Sediments are deposited when the speed in one direction of Retrieved from: Cochise College
transport medium decreases. Small grains are deposited last
while larger size grains come first. Over period of time, these sediments will undergo compaction and
cementation to form new rock.

CHEMICAL SEDIMENTARY ROCK came results from inorganic chemical reactions. This is a result
of changing physical conditions such as solutions (e.g., fresh water in lakes, groundwater or sea water) and
increased in temperatures. It is a rock made of crystalline minerals.

Figure 5: Types of Chemical Sedimentary Rocks

Retrieved from source: Lyndsay Hauber & Joyce M. McBeth (2018) CC
BY 4.0 after Bradley Deline (2015) CC BY-SA 3.0
 BIOCHEMICAL SEDIMENTARY ROCKS are made by organisms.

This is due to the actions of living

organisms that brought for the extraction
minerals from solution or it can also be from the
remains of dead organisms. It is usually found
near areas lots of biological activity. This is the
sedimentary rock type with the most fossils.
Examples of biochemical
sedimentary rocks are the following:
a. Coquina - made from shells
b. Chalk - made from dead animals
c. Coal - made from dead plants
d. Limestone - made from fossils

Figure 6: Types of Biochemical Sedimentary Rocks

Retrieved from: https://www.slideshare.net/AwaisBakshy/sedimentary-
rocks-bs-1st-year

TOPIC 3: Metamorphic Rocks

Metamorphic Rocks was related from the word metamorphism which means to transform. It is a
kind of rock that originates from pre-existing rocks that are subjected with high temperature/ pressure and
interaction with chemical fluids.

Two kinds of metamorphism:

1. Burial metamorphism - happens when a rock is buried inside the crust because of certain depositions.
Pressure is the primary factor for this.
2. Contact metamorphism - occurs when an igneous rock is heated.

Figure 7: Types of Metamorphic Rocks

Retrieved from: https://gotbooks.miracosta.edu/geology/chapter10.html

Examples of metamorphic rocks are in the illustration. If the sedimentary rock limestone or dolomite is
metamorphosed, it can become the metamorphic rock marble. Same with sandstone that can become quartzite.
And shale turns into slate after it was metamorphosed.

TOPIC 4: Rock Cycle

The rock cycle is an illustration of how rocks are interrelated to each other and the process they undergo
to change. Within this cycle, no rocks remain the same with time, pressure, and other natural forces they
transform. Igneous rock is considered as primary rocks because these are directly form from magma when a
molten rock cooled. So, other rocks such as sedimentary and metamorphic are known as secondary because
they are all formed from igneous rocks.
Imagine that a magma comes out from a volcanic eruption. This magma would turn into solid crystals
after it cools down and that is how an igneous rock formed. As time goes, different weathering agents like wind,
rain, glacier activities act on igneous rocks that will break rocks into debris or small fragments--- called as
sediments. Weathering is a process of breaking rocks into pieces. Erosion is the transportation of small piece
from one place to another. This can happen through water flow, wind, or humans. Deposition, on the other side,
is the process where sediments are added to landform or landmass. Moreover, when the sediments are buried
in layers, with time and pressure, they will cement and compact together that will turn soon into sedimentary
rocks. This process was named as lithification. So, how metamorphic rocks are formed? Think of how a
butterfly grows. They called it as metamorphosis. Same as what occurs in rocks especially when heat is applied.
So, where does the heat come from?
 Heat comes from the inside of
the Earth due to pressure. Try to push
your hands very hard eventually, you
can feel the heat. Rub them then due
to friction, you will also have heat.
Similarly, when tectonic plates move
alone it produces heat. Now, what does
heat do to rocks? It bakes just like how
you bread or cookies are made. Rocks
when applied with heat will transform.
And the cycle continuous.
Take note that this cyclic
process never stands only in one way.
It could overlap to each other depends
on what process happened in the
middle. For example, when
metamorphic breaks and turn into
pieces, this will not go back as molten
rocks rather it will become sediments.
Same with igneous rock when applied
with heat and pressure. It would
immediately turn into metamorphic
rock. It can skip the process of Figure 8: Rock Cycle
Retrieved from: https://o.quizlet.com/7qY6rym7fTJUNthCsacWig_b.png
transformation into sedimentary rocks.

LESSON 2: MINERAL RESOURCES

Most rocks found in the Earth’s crust have metals and other elements at very low concentrations. Gold,
for example, has an average concentration of about 0.005 ppm (parts per million) which is roughly 5 grams of
gold for every 1000 tons of rock. Extracting gold at this concentration will be too high compared to the profit that
will be gained however there are geologic processes that can concentrate minerals and elements in rocks of a
particular area.
There are two types of mineral resources:
1. Metallic mineral deposits – gold, silver, copper, platinum
2. Non-metallic resources – sand, gravel, talc

Mineral resources can be classified according to the mechanism responsible for concentrating the
valuable substance.
1. Magmatic Ore Deposits
- valuable substances are concentrated within an igneous body through magmatic processes such as
crystal fractionation, partial melting and crystal settling.
- magmatic processes can concentrate the ore minerals that contain valuable substances after
accumulating elements that were once widely dispersed and in low concentrations within the magma.

Examples:
• Crystal settling – as magma cools down, heavier minerals tend to crystallize early and settle at the
lower part of the magma chamber. From a basaltic magma, chromite (FeCr2O4), magnetite (Fe3O4) and
platinum (Pt) can be concentrated through crystal settling

• Fractional crystallization – the residual melt has high percentage of water and volatile substances that
are favorable for the formation of pegmatites. Pegmatites are enriched in Lithium, Gold, Boron, rare
elements and some other heavy metals. Fractional crystallization of granitic magmas can concentrate
rare earth elements (such as cesium and uranium) and heavy metals. This can also form pegmatites
(large crystals of quartz, feldspars and muscovite) which may contain semi-precious gems such as beryl,
topaz, and tourmaline

2. Hydrothermal Ore Deposits

- concentration of valuable substances by hot aqueous (water-rich) fluids flowing through fractures and
pore spaces in rocks.
- hydrothermal solutions are hot, residual watery fluids derived during the later stages of magma
crystallization and may contain large amounts of dissolved metals. These can also originate from the
ground water circulating at depth that is being heated up by a cooling and solidifying igneous body or
along depths with known geothermal gradient.
 - Such hot water can dissolve valuable substances (at low concentrations) from rocks. As the metal
enriched hot waters move into cooler areas in the crust, the dissolved substances may start to
precipitate.

There are numerous hydrothermal mineral deposits as compared to the different types of deposits:
• Vein type deposits - a fairly well-defined zone of mineralization, usually inclined and discordant and
typically narrow. Most vein deposits occur in fault or fissure openings or in shear zones within the country
rock. Sometimes referred to as (metalliferous) lode deposits, many of the most productive deposits of
gold, silver, copper, lead, zinc, and mercury occur as hydrothermal vein deposits.

• Disseminated deposits - Deposits in which the ore minerals are distributed as minute masses (very low
concentration) through large volumes of rocks. This occurrence is common for porphyry copper deposits.

• Massive sulfide deposit (at oceanic

spreading centers) - Precipitation of metals
as sulfide minerals such as sphalerite (ZnS)
and chalcopyrite (CuFeS2) occur when hot
fluids that circulated above magma chambers
at oceanic ridges that may contain sulfur,
copper and zinc come in contact with cold
groundwater or seawater as it migrates
towards the seafloor.

• Stratabound ore deposits (in lake or

oceanic sediment) – This deposit is formed
when the dissolved minerals in a hydrothermal
fluid precipitate in the pore spaces of
unconsolidated sediments on the bottom of a
lake or ocean. Such minerals may contain
economic concentrations of lead, zinc and
copper, usually in sulfide form like galena
(PbS), sphalerite (ZnS) and chalcopyrite Figure 9: Various processes forming ore deposits
(CuFeS2). Retrieved from: Essentials of Geology by Stephen Marshak, 4th ed.,
2013, p377
3. Sedimentary Ore Deposits – some valuable substances are concentrated by chemical precipitation
coming from lakes or seawater.

4. Placer Ore Deposits

- Deposits formed by the concentration of valuable substances through gravity separation during
sedimentary processes.
- Usually aided by flowing surface waters either in streams or along coastlines.
- Concentration would be according to the specific gravity of substances, wherein the heavy minerals
are mechanically concentrated by water currents and the less-dense particles remain suspended and
are carried further downstream.
- Usually involves heavy minerals that are resistant to transportation and weathering.
- Common deposits are gold and other heavy minerals such as platinum, diamonds and tin;
- The source rock for a placer deposit may become an important ore body if located.

5. Residual Ore Deposit

- A type of deposit that results from the accumulation of valuable materials through chemical weathering
processes.
- During the process, the volume of the original rock is reduced by leaching.
- Important factors for the formation of residual deposit include parent rock composition, climate (tropical
and sub-tropical: must be favorable for chemical decay) and relief (must not be high to allow
accumulation)
- Common deposits are bauxites and nickeliferous laterites. Bauxite, the principal ore of aluminum, is
derived when aluminum-rich source rocks undergo intense chemical weathering brought by prolonged
rains in the tropics, leaching the common elements that include silicon, sodium and calcium through
leaching.
- Nickeliferous laterites or nickel laterites are residual ore deposits derived from the laterization of olivine-
rich ultramafic rocks such as dunite and peridotite. Like in the formation of bauxite, the leaching of
nickel-rich ultramafic rocks dissolves common elements, leaving the insoluble nickel, magnesium and
iron oxide mixed in the soil.
- Secondary Enrichment Deposits are derived when a certain mineral deposit becomes enriched due to
weathering.
 REFERENCES
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