WEATHERING OF ROCKS AND MINERALS

BY: ATEEQ SHAH

ROCKS

Rock are solid, naturally occurring mineral materials Rocks may be consists of one mineral or mixture of two or more

minerals Rock are categorized in three major groups

1. Igneous rocks2. Sedimentary rocks3. Metamorphic rocks

Rocks (mixture of minerals)

Igneous rocks

The rocks formed by cooling and solidification of molten material are called igneous rocks

The word Igneous derived from the Latin word “igneus” means fire

The molten material present beneath earth surface is considered as “Magma” and when this molten material comes at surface due to high pressure and temperature then it is said to be “Lava”

Igneous rocks (core temp)

Igneous Rocks

Igneous rocks are classified into two main groups1. Intrusive igneous rocks (Plutonic Rocks)2. Extrusive igneous rocks (Volcanic Rocks)

3. Intrusive Igneous Rocks: Intrusive igneous rocks are formed by cooling and crystallization of magma These rocks have a coarse texture, because of slow rate of cooling

4. Extrusive Igneous Rocks: Extrusive igneous rocks are formed by cooling and crystallization of Lava These are fine textured rocks due to high rate of cooling

Igneous Rocks

Igneous Rocks Igneous rocks are classified on the basis of Colour and rock texture

The texture of an igneous rock (fine-grained vs coarse-grained) is dependent on the rate of cooling : slow cooling allows large crystals to form, fast cooling yields small crystals. Magmas and their resultant plutonic rock bodies cool and crystallize slowly and are characterized by coarse-grained texture, in which the mineral crystals are visible to the unaided eye. On the other hand, lavas cool quickly at the earth's surface and are characterized by fine-grained texture, in which the crystals are too small to be seen by the naked eyes

Rock texture Light-colored mineralsQuartz Feldspars, Muscovite

Dark-colored mineralsHornblende, augite, biotite

Coarse Granite Diorite Gabbro HornblenditeIntermediate

Rhyolite Andesite Basalt

Fine Felsite Basalt Glass

Igneous Rocks

Igneous Rocks

Igneous

Igneous rocks are classified into two main groups on the basis of chemical composition 1. Mafic2. Felsic

1. Mafic: The term is derived from using the MA from magnesium and the FIC from the Latin word for iron, but mafic magmas also are relatively enriched in calcium and sodium. Mafic minerals are usually dark in color

2. Felsic: The term comes from FEL for feldspar and SIC, which indicates the higher percentage of silica. Felsic minerals are usually light in color

Sedimentary Rocks

The rocks formed from sediments are called sedimentary rocks Sedimentary rocks formed when weathering products released from

older rocks and collected under water and reconsolidate into new rock. Sedimentary rocks cover almost 75 % of the earths land surface Some of the sedimentary rocks are:

Sandstone: Formed from an igneous rock (Granite). Quartz sand weathered from granite and sediments stored in sea shore may become cemented by calcium or iron in the water to become a solid mass called sand stone)

Shale : Clay compacted into shale Limestone Dolomite

Sedimentary Rocks Sandstone

Sedimentary Rocks Limestone

Metamorphic Rocks The rocks produced from pre-existing rocks by the process of

metamorphism The word metamorphism came from Greek words. Meta mean change

and Morph means form. So metamorphism is the changing of rock forms or structure. Metamorphism is the change of minerals or geologic texture (distinct arrangement of minerals) in pre-existing rocks (protoliths), without melting the protoliths (a solid-state change). The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids

When earths plates shift or collide, forces are generated which subject the igneous and metamorphic rocks to tremendous heat and pressure.

These forces may compress and partially remelt the rocks, as well as break the bonds of minerals to form new minerals.

Examples Marble schist

Metamorphic Rocks Marble

Metamorphic Rocks Schist

Some of the important sedimentary and metamorphic rocks

Dominant Mineral Types of RockSedimentary Metamorphic

Calcite Limestone MarbleDolomite Dolomite MarbleQuartz Sandstone QuartziteClays shale SlateVariable-Silicates Conglomerate GneissVariable-Silicates Schist

Rock Cycle

WEATHERING

Weathering

Weathering is the disintegration (physical breakdown) and decomposition (chemical breakdown) of rocks and minerals

It is a continues process. Rocks Soil particles further on During physical weathering, particles size continues to decrease and

constitutes dissolve in the weathering solution Secondary minerals are formed by chemical alteration of primary minerals Weathering occurs due to environment changes and biological activities. On the basis of chemical and physical breakdown, weathering is classified

into three main groups:1. Physical weathering 2. Chemical weathering3. Biological weathering

1. Physical weathering

The physical breakdown of rocks and minerals is called physical weathering.

In physical weathering, rocks are decreased in size but the chemical composition remains same.

Physical weathering is predominate in arid and cold areas. Factors responsible for physical weathering are:

1. Temperature2. Abrasion by water, ice and wind3. Plants and Animals

1. Temperature

Minerals have the tendency to expand and contract in varying temperature

Rocks exposed to sunlight heat up during the day and cool down at night, cause alternate expansion and contraction of the minerals present in the rock.

This expansion and contraction of different minerals in a rock set up stresses that cause the rock to crack apart.

Sometimes inner temperature of rocks in lower than that of outer layer, which may destroy the surface of the rock only. This phenomenon of peeling away of rock surface is called exfoliation

Exfoliation

2. Abrasion by water, ice and wind water

After rain, flowing water carries and shift sediments Water loaded with sediments have a great cutting power The rounding of riverbed rocks is a clear evidence of abrasion by

water

Wind Windblown dust and sand also affect the rocks by abrasion In arid areas, rounded rocks may be formed by abrasion due to

windblown Ice

In glacial areas, huge moving ice masses embedded with soil and rock fragments grind down rocks in their path

Abrasion

3. Plants and Animals

Plants and animals have a little contribution in physical weathering

Roots of the small and higher plants may exert pressure on rocks to become apart

Narrow spaces produced for the entrance of water. Water on cooling expanded and exert huge pressure to crack down the rocks

Animals like ants, earthworms and rodents slowly disintegrate the rocks

Human may accelerate the intensity of physical weathering by ploughing and cultivation

Plants on rocks

2. Chemical weathering The decomposition of rocks and minerals is called chemical weathering Chemical weathering is more severe in warm and humid regions It is accelerated by the presence of water, oxygen and organic acids

produced by plants and microorganisms In chemical weathering, the chemical composition of rocks and minerals is

slightly or completely destroyed and altered to form new minerals (secondary minerals)

Secondary minerals are formed due to chemical weathering Processes involved in chemical weathering are:

1. Hydrolysis2. Hydration 3. oxidation4. Dissolution5. Acidification

1. Hydrolysis

Hydrolysis is the process of breakdown of water molecule into H and OH ions

On hydrolysis, H ion added to the mineral composition by replacing another atom

Example1. K-feldspar (Microcline)

KAlSi3O8 + H2O HAlSi3O8 + KOH (Soluble )2. Acid Silicate clay

2HAlSi3O3 + 11H2O Al2O3 + 6H4SiO4 (Silicic Acid)

2. Hydration

The addition of water in a mineral constitute is called hydration Hydrated minerals are more prone to decomposition due to their

increased volume which make it more softerExample:-HematiteFe2O3 + 3H2O Fe2O3.3H2O (limonite)

3. Oxidation

Oxidation is a process of : Addition of Oxygen Removal of electron Removal of HydrogenExample: Iron Oxide 4FeO + O2 + 2H2O 4FeOOH (Geothite)

Oxygen is added to the mineral constitute

4. Reduction

Reduction is : The removal of oxygen Addition of Hydrogen Addition of electron

During reduction, Hydrogen is added to the mineral composition and alter it to form secondary minerals

Example:Olivine3MgFeSiO4 + 2H2O H4Mg3Si2O9 (Serpentine) + SiO2 + FeO

5. Dissolution

The process of hydrating of cations and anions in the minerals to dissociate from each other. This process of dissociation is called dissolution

Example:CaSO4. 2H2O + 2H2O Ca + SO4 + 4H2O