• Petrology (from the Greek petra, "rock" and
logos, "study") is the branch of geology that studies the
origin, composition, distribution and structure of rocks.
• Petrology utilizes the classical fields of
mineralogy, petrography, optical mineralogy, and
chemical analyses to describe the composition and
texture of rocks. Modern petrologists also include the
principles of geochemistry and geophysics through the
studies of geochemical trends and cycles and the use of
thermodynamic data and experiments to better
understand the origins of rocks.
BRANCHES OF PETROLOGY
• Igneous petrology focuses on the composition
and texture of igneous rocks (rocks such as
granite or basalt which have crystallized from
molten rock or magma). Igneous rocks include
volcanic and plutonic rocks.
BRANCHES OF PETROLOGY
• Sedimentary petrology focuses on the
composition and texture of sedimentary rocks
(rocks such as sandstone, shale, or limestone
which consist of pieces or particles derived
from other rocks or biological or chemical
deposits, and are usually bound together in a
matrix of finer material).
BRANCHES OF PETROLOGY
• Metamorphic petrology focuses on the composition
and texture of metamorphic rocks (rocks such as
slate, marble, gneiss, or schist which started out as
sedimentary or igneous rocks but which have
undergone chemical, mineralogical or textural
changes due to extremes of pressure, temperature or
TYPES OF ROCKS
• Igneous rock
- derived from the Latin word igneus meaning
of fire, from ignis meaning fire
- forms through the cooling and solidification
of magma or lava.
- This magma can be derived from partial melts
of pre-existing rocks in either a planet's
mantle or crust.
• Typically, the melting of rocks is caused by one
or more of three processes:
- an increase in temperature
- a decrease in pressure
- a change in composition
• TWO (2) Categories
- Plutonic or intrusive rocks result when
magma cools and crystallizes slowly within the
Earth's crust. (E.g. granite)
- Volcanic or extrusive rocks result from magma
reaching the surface either as lava or fragmental
ejecta, forming minerals such as pumice or
• Bowen's Reaction Series
-The chemical abundance and the rate of cooling
of magma typically forms a sequence.
- Named after the Canadian petrologist Norman
• Sedimentary rocks
- formed by sedimentation of particles at or near
the Earth's surface and within bodies of water.
- This process causes clastic sediments or organic
particles (detritus) to settle and accumulate, or
for minerals to chemically precipitate (evaporate)
from a solution.
- The particulate matter then undergoes
compaction and cementation during diagenesis.
• Before being deposited, sediment was formed
by weathering and erosion in a source
area, and then transported to the place of
deposition by water, wind, ice, mass
movement or glaciers which are called agents
• Metamorphic rocks
- formed by subjecting any rock type—
sedimentary rock, igneous rock or another older
metamorphic rock—to different temperature
and pressure conditions than those in which the
original rock was formed.
- This process is called metamorphism; meaning
to "change in form".
• Three (3) Major Classes of Metamorphic Rocks
- Contact Metamorphism
intrusion of magma that heats the surrounding rock
causes- Temperature-dominated transformation.
- Pressure metamorphism occurs when sediments are
buried deep under the ground; pressure is dominant
and temperature plays a smaller role. Burial
metamorphism can result in rocks such as jade.
- Regional Metamorphism- both heat and pressure play
a role. This is typically found in mountain-building
• Two (2) Categories
• Schists -composed of lamellar minerals -micas.
• Gneiss- granite gneiss.
• Slates, phyllites, and mylonite.
- Non- Foliated -This branch contains quartzite—a
metamorphosed form of sandstone—and hornfels.
• Two (2) Types of Weathering
- Mechanical or physical weathering involves the
breakdown of rocks and soils through direct
contact with atmospheric conditions, such as
heat, water, ice and pressure.
- Chemical weathering- involves the direct effect
of atmospheric chemicals or biologically
produced chemicals (also known as biological
weathering) in the breakdown of rocks, soils and
• Physical weathering is the class of processes
that causes the disintegration of rocks
without chemical change. The primary process
in physical weathering is abrasion (the process
by which clasts and other particles are
reduced in size).
• - also called mechanical
• Thermal stress weathering (sometimes called
insolation weathering) results from expansion
and contraction of rock, caused by
• Thermal stress weathering comprises two
main types, thermal shock and thermal
• TWO (2) TYPES
- Thermal shock occurs when a thermal gradient
causes different parts of an object to expand by
- Thermal Fatigue is the progressive and localized
structural damage that occurs when a material is
subjected to cyclic loading. The nominal
maximum stress values are less than the ultimate
tensile stress limit, and may be below the yield
stress limit of the material.
Thermal stress weathering
• an important mechanism in deserts
• a large diurnal temperature range (hot in the day and cold at night)
• The repeated heating and cooling exerts stress on the outer layers
of rocks, which can cause their outer layers to peel off in thin
• The process of peeling off is also called exfoliation. Although
temperature changes are the principal driver, moisture can enhance
thermal expansion in rock.
• Forest fires and range fires are also known to cause significant
weathering of rocks and boulders exposed along the ground
• Intense localized heat can rapidly expand a boulder.
• Frost weathering
- frost wedging, ice wedging or cryofracturing is
the collective name for several processes where
ice is present.
- These processes include frost shattering, frostwedging and freeze-thaw weathering.
- Severe frost shattering produces huge piles of
rock fragments called scree which may be located
at the foot of mountain areas or along slopes.
• Common in mountain areas where the temperature is
around the freezing point of water.
• Certain frost-susceptible soils expand or heave upon
freezing as a result of water migrating via capillary
action to grow ice lenses near the freezing front.
• The ice accumulations grow larger as they attract liquid
water from the surrounding pores.(Pores – spaces on
• 10% expansion in ice when water freezes.
• Pressure Release
- also known as unloading, overlying materials
(not necessarily rocks) are removed (by
erosion, or other processes), which causes
underlying rocks to expand and fracture parallel
to the surface.
- Over time, sheets of rock break away from the
exposed rocks along the fractures, a process
known as exfoliation. Exfoliation due to pressure
release is also known as "sheeting".
• Hydraulic action occurs when water (generally
from powerful waves) rushes rapidly into
cracks in the rock face, thus trapping a layer
of air at the bottom of the crack, compressing
it and weakening the rock. When the wave
retreats, the trapped air is suddenly released
with explosive force.
• Salt crystallization
- otherwise known as haloclasty
- causes disintegration of rocks when saline
solutions seep into cracks and joints in the
rocks and evaporate, leaving salt crystals
- These salt crystals expand as they are heated
up, exerting pressure on the confining rock.
- May also take place through decomposition.
• Chemical weathering
- Changes the composition of rocks
- Often transforming them when water interacts
with minerals to create various chemical
- A gradual and on-going process.
- The processes of oxidation and hydrolysis are
Dissolution and carbonation
• Rainfall is acidic because atmospheric carbon
dioxide dissolves in the rainwater producing weak
• Acid rain occurs when gases such as sulfur
dioxide and nitrogen oxides are present in the
atmosphere. Sulfur dioxide(SO2), comes from
volcanic eruptions or from fossil fuels, can
become sulfuric acid within rainwater, which can
cause solution weathering to the rocks on which
- The process in which atmospheric carbon dioxide leads to solution
- Carbonation occurs on rocks which contain calcium carbonate, such
as limestone and chalk.
- This takes place when rain combines with carbon dioxide or an
organic acid to form a weak carbonic acid which reacts with
calcium carbonate (the limestone) and forms calcium bicarbonate.
- This process speeds up with a decrease in temperature, not
because low temperatures generally drive reactions faster, but
because colder water holds more dissolved carbon dioxide gas.
- Carbonation is therefore a large feature of glacial weathering.
• Mineral hydration is a form of chemical
weathering that involves the rigid attachment
of H+ and OH- ions to the atoms and
molecules of a mineral.
• Hydrolysis is a chemical weathering process
affecting silicate and carbonate minerals. In
such reactions, pure water ionizes slightly and
reacts with silicate minerals.
- This process is better known as
'rusting', though it is distinct from the rusting
of metallic iron.
- Many other metallic ores and minerals oxidize
and hydrate to produce colored
deposits, such as chalcopyrites or CuFeS2
oxidizing to copper hydroxide and iron oxides.
• The release of chelating compounds (i.e. organic
acids, siderophores) easily affect surrounding
rocks and soils, and may lead to podsolization of
• Acidifying molecules (i.e. protons, organic acids)
by plants so as to break down aluminium and iron
containing compounds in the soils beneath them.
• Decaying remains of dead plants in soil may form
organic acids which, when dissolved in
water, cause chemical weathering.
PHYSICAL PROPERTIES OF SOIL
TYPES OF SOIL
• Sandy soil
- has the largest particles among the different
- dry and gritty to the touch
- Water drains rapidly,.
TYPES OF SOIL
• Clay soil
- has the smallest particles among the three so
it has good water storage qualities.
- sticky to the touch when wet, but smooth
- Due to the tiny size of its particles and its
tendency to settle together.
TYPES OF SOIL
smaller particles than sandy soil
smooth to the touch.
When moistened, it’s soapy slick.
When you roll it between your fingers, dirt is
left on your skin.
TYPES OF SOIL
dark brown or black in color
easily compressed due to its high water
- and rich in organic matter.
TYPES OF SOIL
soil in extremely dry regions
brackish because of its high salt content.
can cause damage to and stall plant
growth, impede germination, and cause
difficulties in irrigation.
• Erosion is a natural process
• Human activities have increased by 10-40 times the
rate at which erosion is occurring globally.
• Excessive erosion causes problems such as
desertification, decreases in agricultural productivity
due to land degradation, sedimentation of
waterways, and ecological collapse due to loss of the
nutrient rich upper soil layers.
• Water and wind erosion
- two primary causes of land degradation
- they are responsible for 84% of degraded acreage
• PHYSICAL PROCESSES
- Rainfall (There are four primary types of erosion
that occur as a direct result of rainfall—splash
erosion, sheet erosion, rill erosion, and gully
- Rivers and Streams (Bank erosion is the wearing
away of the banks of a stream or river. Thermal
erosion is the result of melting and weakening
permafrost due to moving water.)
- Coastal Erosion
• Hydraulic action takes place when air in a joint is
suddenly compressed by a wave closing the entrance
of the joint. This then cracks it.
• Wave pounding is when the sheer energy of the wave
hitting the cliff or rock breaks pieces off.
• Abrasion or corrasion is caused by waves launching
seaload at the cliff. It is the most effective and rapid
form of shoreline erosion (not to be confused with
• Corrosion is the dissolving of rock by carbonic acid in
Three (3) different processes:
• abrasion/scouring -debris in the basal ice scrapes
along the bed, similar to sandpaper on wood.
• plucking - cause pieces of bedrock to crack off.
• ice thrusting - the glacier freezes to its bed, then
as it surges forward, it moves large sheets of
frozen sediment at the base along with the
- Freezing and Thawing (Cold weather causes
water trapped in tiny rock cracks to freeze and
expand, breaking the rock into several pieces.)
- Wind erosion is a major geomorphological
force, especially in arid and semi-arid regions. It
is also a major source of land
degradation, evaporation, desertification, harmf
ul airborne dust, and crop damage—especially
after being increased far above natural rates by
human activities such as
deforestation, urbanization, and agriculture.
Two (2) Varieties of WIND EROSION
• Deflation is divided into three categories:
(1) surface creep, where larger, heavier particles slide or roll
along the ground;
(2) saltation, where particles are lifted a short height into the
air, and bounce and saltate across the surface of the soil; and
(3) suspension, where very small and light particles are lifted
into the air by the wind, and are often carried for long
• Saltation is responsible for the majority (50-70%) of wind
erosion, followed by suspension (30-40%), and then surface
- Gravitational erosion
Mass movement is the downward and outward
movement of rock and sediments on a sloped
surface, mainly due to the force of gravity.
Slumping happens on steep hillsides, occurring along
distinct fracture zones, often within materials like clay
that, once released, may move quite rapidly downhill.
Surface creep is the slow movement of soil and rock
debris by gravity which is usually not perceptible except
through extended observation.
- Exfoliation is a type of erosion that occurs when a rock
is rapidly heated up by the sun.
FACTORS AFFECTING SOIL EROTION
• Precipitation and wind speed (climatic factors)
• Soil structure and Composition
• Vegetative cover (It increases the permeability
of the soil to rainwater, thus decreasing
HUMAN ACTIVITIES THAT INCREASES
Roads and Urbanization
Global environmental Aspects
Sedimentation of Aquatic Ecosystems
Airborne Dust Pollution
MEASURING OF EROSION
• MEM(micro-erosion meter ) has proved
helpful in measuring bedrock erosion in
various ecosystems around the world. It can
measure both terrestrial and oceanic erosion.
• TMEM(traversing micro-erosion meter ) can
be used to track the expanding and
contracting of volatile rock formations and can
give a reading of how quickly a rock formation
MONITORING AND MODELLING
• Erosion models
- non-linear(difficult to work with numerically, and
impossible to scale up to making predictions
about large areas from data collected by
sampling smaller plots)
- The most commonly used model for predicting
soil loss from water erosion is the Universal Soil
Loss Equation (USLE), which estimates the
average annual soil loss A as:
A = RKLSCP
PREVENTION AND REMEDIATION
• As a student how can you help prevent and
• What do you think should the Government do
to lessen the possibilities of soil erosion in the
- Wind Breaks
- Traditional Planting Methods