Petrology

PETROLOGY

PETROLOGY

• 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.

There are three branches of petrology, corresponding to the

three types 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 both)

ROCKSIn geology, a rock is a naturally

occurring solid aggregate of one or more minerals or mineraloids. For

example, the common rock granite is a combination of the quartz, feldspar

and biotite minerals. The Earth's outer solid layer, the lithosphere, is made of

rock.

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.

Igneous rocks

• 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

Igneous Rocks

• 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 basalt.

Igneous Rocks

• Bowen's Reaction Series-The chemical abundance and the rate of cooling of magma typically forms a sequence.- Named after the Canadian petrologist Norman L. Bowen.

Sedimentary Rocks

• 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.

Sedimentary Rocks

• 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 of denudation.

Metamorphic Rocks

• 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".

Metamorphic Rocks

• 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 regions.

Metamorphic Rocks• Two (2) Categories- Foliated E.g.• 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.E.g.• Marble• soapstone• serpentine.

ROCK CYCLE-describes the dynamic transitions through geologic time among the

three main rock types: sedimentary, metamorphic, and

igneous.

WEATHERINGthe breaking down of rocks, soils and minerals as well as artificial materials

through contact with the Earth's atmosphere, biota and waters.

WEATHERING

• 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 minerals.

PHYSICAL WEATHERING

• 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 weathering, disaggregation.

PHYSICAL WEATHERING

• Thermal stress weathering (sometimes called insolation weathering) results from expansion and contraction of rock, caused by temperature changes.

• Thermal stress weathering comprises two main types, thermal shock and thermal fatigue.

THERMAL WEATHERING

• TWO (2) TYPES- Thermal shock occurs when a thermal gradient

causes different parts of an object to expand by different amounts.

- 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 WEATHERING 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 sheets. • 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 surface.

• Intense localized heat can rapidly expand a boulder.

PHYSICAL WEATHERING

• Frost weathering- frost wedging, ice wedging or cryofracturing is

the collective name for several processes where ice is present.

- These processes include frost shattering, frost-wedging 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.

FROST WEATHERING

CHARACTERISTICS• 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 rocks)

• 10% expansion in ice when water freezes.

PHYSICAL WEATHERING

• 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".

PHYSICAL WEATHERING

• 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.

PHYSICAL WEATHERING

• 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 behind.

- These salt crystals expand as they are heated up, exerting pressure on the confining rock.

- May also take place through decomposition.

CHEMICAL WEATHERING

• Chemical weathering - Changes the composition of rocks- Often transforming them when water interacts with minerals to create various chemical reactions. - A gradual and on-going process.- The processes of oxidation and hydrolysis are

most important.

CHEMICAL WEATHERING

Dissolution and carbonation• Rainfall is acidic because atmospheric carbon

dioxide dissolves in the rainwater producing weak carbonic acid.

• 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 it falls.

CHEMICAL WEATHERING• CARBONATION- The process in which atmospheric carbon dioxide leads to solution

weathering. - 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.

CHEMICAL 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.

CHEMICAL WEATERING

• Oxydation- 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.

BIOLOGICAL WEATHERING

• The release of chelating compounds (i.e. organic acids, siderophores) easily affect surrounding rocks and soils, and may lead to podsolization of soils.

• 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.

H

SOIL-are the materials left over after the

rock breaks down combined with organic material.

-In addition many of Earth's landforms and landscapes are the result of

weathering processes combined with erosion and re-deposition.

PHYSICAL PROPERTIES OF SOIL

• Texture• Structure (PEDS)• Density• Porosity• Consistency• Temperature • Colour• Resistivity

TYPES OF SOIL

• Sandy soil - has the largest particles among the different

soil types. - 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

when dry.- Due to the tiny size of its particles and its

tendency to settle together.

TYPES OF SOIL

• Silty 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

• Peaty soil - dark brown or black in color- Soft- easily compressed due to its high water

content- and rich in organic matter.

TYPES OF SOIL

• SALINE 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 - the process by which soil and rock

are removed from the Earth's surface by EXOGENETIC PROCESSES such as

wind or water flow, and then transported and deposited in other

locations

EROSION

• 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

EROSION

• 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 erosion.)

- 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.)

PHYSICAL PROCESSES

- 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).

• Corrosion is the dissolving of rock by carbonic acid in sea water.

PHYSICAL PROCESSES

- Glaciers 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 glacier

PHYSICAL PROCESSES

- Floods- 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, harmful airborne dust, and crop damage—especially after being increased far above natural rates by human activities such as deforestation, urbanization, and agriculture.

PHYSICAL PROCESSES

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 distances. • Saltation is responsible for the majority (50-70%) of wind

erosion, followed by suspension (30-40%), and then surface creep (5-25%)

PHYSICAL PROCESSES

- Gravitational erosionMass 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 runoff.)

• Topography

HUMAN ACTIVITIES THAT INCREASES EROSION RATES

• Agricultural Practices• Deforestation• Roads and Urbanization• Climate Change• Global environmental Aspects- Land Degration- Sedimentation of Aquatic Ecosystems- Airborne Dust Pollution- Tectonic Effects

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 is deteriorating

MONITORING AND MODELLING EROSION

• 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 remediate EROSION?

• What do you think should the Government do to lessen the possibilities of soil erosion in the country?

- Terracing- Wind Breaks- Traditional Planting Methods