Theories of Geography Part 10 – Rocks and Minerals

Rocks & Minerals

Types of Rocks

Minerals are naturally occurring inorganic substances, often with a crystalline structure. They are composed largely of the most abundant elements in the Earth’s crust oxygen & silicon, coupled with metals or the metallic elements of iron, calcium, sodium, potassium, and magnesium.

Rocks are usually composed of two or more minerals. Often, many different minerals are present, but a few rock varieties are made almost entirely of one mineral. Most rock in the Earth’s crust is extremely old, dating back many millions of years, but rock is also being formed at this very hour as active volcanoes emit lava that solidifies on contact with the atmosphere or ocean.

The Great Oxygenation Event or oxygen catastrophe which happened 2400 million years ago in the Proterozoic eon triggered an explosive growth in the diversity of minerals on Earth. The three types of Rocks are Sedimentary, Igneous and Metamorphic.

Igneous rocks

These rocks have crystallized from magma which is made up of various components of pre-existing rocks and has been subjected to melting either at subduction zones or within the Earth’s mantle.

Sedimentary rocks

These rocks are formed through the gradual accumulation of sediment, such as sand on a beach or mud on a river bed. The sediment is buried and then it is compacted as more and more material is deposited on top. In several thousand to Lakhs of years, the sediment becomes so dense that it becomes a rock. This process is known as lithification.

Metamorphic rocks

These rocks once existed as igneous or sedimentary rocks but have been subjected to varying degrees of pressure and heat within the Earth’s crust. The processes involved changes the composition and fabric of the rock and their original nature is often hard to distinguish. Metamorphic rocks are typically found in areas of mountain building. The above three classes of rocks are constantly being transformed from one to another in a continuous process through which the crustal minerals have been recycled during many millions of years of geologic time. The following diagram shows these transformations.

Igneous Rocks

The upper 16 kilometers of the Earth’s crust is made up of 95% Igneous rock, with a thin covering of sedimentary and metamorphic rocks. Igneous rocks are formed when molten rock cools, forming silicate mineral crystals. Felsic minerals are light colored and less dense, and mafic minerals are dark colored and more dense. The igneous rocks are generally hard and water percolates in them not so easily. The most important characteristics of Igneous rocks are as follows:

  • They usually do not occur in distinct beds or strata like sedimentary rocks.
  • Igneous rocks are generally not having any fossils
  • They are generally granular and crystalline.
  • They are less affected by chemical weathering as the water does not percolate in them easily.

Magma as source of Igneous Rocks

The mixture of the Molten Rocks which makes the Igneous rocks is called Magma. Magma in fact is a mixture of molten rocks, volatiles (gas) and other solids. It originated from the partial melting of the lower crust and the upper mantle, mainly at depths of 15-200 kilometers. Most magma is as hot as 700 °C to 1300 °C and is silicate mixtures mostly.

Most igneous rock consists of silicate minerals. These rocks also contain mostly metallic elements. The mineral grains in igneous rocks are very tightly interlocked, and so the rock is normally very strong. Quartz, which is made of silicon dioxide (SiO2), is the most common mineral of all rock classes. It is quite hard and resists chemical breakdown.

The chambers under a volcano where Magma collects are called magma chambers. The magma chambers feed a volcano. Bulks of the igneous rocks are result of the cooling and solidifying of Magma. There are two processes by which Magma cools and solidifies. These are called “plutonic” and “Volcanic Eruption”. When the Molten Magma goes down deep within the earth and gets solidified, it is called Plutonism. On the contrary, the molten Magma can also come out on the surface of earth via a volcanic eruption.

Intrusive and Extrusive Igneous Rocks

Magma that solidifies below the Earth’s surface and remains surrounded by older, pre-existing rock is called intrusive igneous rock. Because intrusive rocks cool slowly, they develop large mineral crystals that are visible to the eye. They are further classified into Plutonic, Hypabyssal, Batholiths and Laccoliths as follows:

  • Plutonic: Generally very large crystal and they were formed due to cooling of magma very deep inside the Earth
  • Hypbyssal / subvolcanic: Consolidated in a zone above the base of Earth’s crust and hence has distinct structural characteristics.
  • Batholiths: They extend to greater depths and larger areas
  • Laccoliths: A sheet intrusion that has been injected between two layers of sedimentary rock.

If the magma reaches the surface and emerges as lava, it forms extrusive igneous rock. Extrusive igneous rocks cool very rapidly on the land surface or ocean bottom and thus show crystals of only microscopic size. We note here that Granite typically accumulates in batholiths. A single batholith sometimes extends down several kilometers and may occupy an area of several thousand square kilometers.

Felsic Rocks and Mafic Rocks

Whatever may be the process of cooling and solidifying, the magma while converting into a rock, undergoes numerous chemical and physical changes.  Accordingly, there are two major types of Igneous rocks are produced viz. Felsic Rocks and Mafic Rocks. Felsic rocks are rich in silicon, oxygen, aluminium, sodium, and potassium, while the mafic rocks are rich in magnesium and iron. If the rock is highly dominated by Magnesium and Iron, it is called Ultramafic.

Examples of Igneous Rocks

  • Granite: Intrusive (batholith generally), Felsic, igneous rock. Worldwide average chemical composition of Igneous Rocks has SiO2 — 72.04% & Al2O3 — 14.42%
  • Diorite: intermediate intrusive igneous rock
  • Gabbro: Mafic igneous rocks equivalent to basalt. Peridotite, Rhyolite, Andesite, Basalt, Komatiite, Diabase etc.

Sedimentary Rocks

Sedimentary rocks are made from layers, or strata, of mineral particles found in other rocks that have been weathered and from newly formed organic matter.  Sedimentary rocks are important because they preserve a record of ancient landscapes, climates, and mountain ranges, as well as the history of the erosion of Earth. In addition, fossils are found in abundance in sedimentary rocks younger than 600 million years and provide evidence of the evolution of life through time. Earth’s geologic time scale was worked out using this record of sedimentary rocks and fossils.

Salient Features of Sedimentary Rocks

Sedimentary rocks form at Earth’s surface by the hydrologic system. Their origin involves the weathering of pre-existing rock, transportation of the material away from the original site, deposition of the eroded material in the sea or in some other sedimentary environment, followed by compaction and cementation. Some common features are:

  • They contain strata or layers. The layers are rarely horizontal and generally tilted due to lateral compressive and tensile forces.
  • They are formed of sediments derived from the older rocks, plants and animals remain.
  • Most part (around 75 percent) of the surface area of the globe is covered by Sedimentary Rocks.
  • Most of the sedimentary rocks are permeable and porous.
  • Sedimentary rocks are generally characterized by different sizes of joints, generally perpendicular to the bedding plains.

Types of Sedimentary Rocks

When rock minerals are weathered, their chemical composition is changed, weakening the solid rock. The rock breaks up into particles of many sizes. When these particles are transported in a fluid such as air, water, or glacial ice, we call them sediment. There are three major classes of sediment: clastic sediment, chemically precipitated sediment, and organic sediment.

On this basis, three main types of sedimentary rocks are recognized viz. clastic rocks, organic rocks and chemically precipitated rocks.

  • Clastic: Made up of discrete fragments or clasts of materials derived from other minerals, largely of quartz and others such as feldspar, amphiboles, clay minerals
  • Organic : They contain the materials which are generated by living organisms such as corals, mollusks, and foraminifera, which cover the ocean floor with layers of calcium carbonate, which can later form limestone.
  • Chemical: Formed by the Chemical & Biological Processes like limestone, rock salt, gypsum and dolostone

Clastic Sedimentary Rocks

Clastic sediment is made up of inorganic rock and mineral fragments, called clasts. These can come from igneous, sedimentary, or metamorphic rocks, and so they can include a very wide range of minerals. Quartz and feldspar usually dominate clastic sediment.  When layers of clastic sediment build up, the lower strata are pushed down by the weight of the sediments above them. This pressure compacts the sediments, squeezing out excess water. Dissolved minerals recrystallize in the spaces between mineral particles in a process called cementation, thus giving rise to the Clastic Sedimentary Rocks.

Due to the mechanical process, the clastic sedimentary rocks are also sometimes called mechanically formed Sedimentary Rocks.

Common examples of clastic / mechanically formed sedimentary rocks include Sandstone {cemented sand grains}, Siltstone {Cemented silt particles}, Congomerate {sandstone containing pebbles of hard rocks}, Mudstone {mainly silt and clay}, Claystone {mainly clay} and shale {clay and mud rock which breaks easily into flat flakes and plates}.

Chemically Precipitated and Organic Sedimentary Rocks

Chemically precipitated sediment is made of solid inorganic mineral compounds that precipitate from water solutions or are formed by organisms living in water. One of the most common sedimentary rocks formed by chemical precipitation is limestone. The third class of sediment is organic sediment. This is made up of the tissues of plants and animals. Peat is an example of organic sediment. This soft, fibrous, brown or black substance accumulates in bogs and marshes where the water stops the plant or animal remains from decaying.

Examples of Chemically precipitated rocks are Limestone {Calcium Carbonate, formed by precipitation on sea or lake floors}, Dolomite {Magnesium and Calcium Carbontes}, Chert {a microcrystalline form of silica} and Evaporites {minerals formed by evaporation of salty solutions in shallow inland lakes or coastal lagoons}.


Limestone is by far the most abundant chemically precipitated rock. It is composed principally of calcium carbonate (CaCO3 or calcite) and originates by both inorganic chemical and biochemical processes. Limestone has a great variety of rock textures such as skeletal limestone, oolitic limestone, and microcrystalline limestone. Marine sediments form largely by biochemical precipitation.

Carbonate sediments dominate at shallow depths and in warm near-shore waters. Elsewhere, siliceous sediment, which eventually forms chert, is typical in deeper water.

Skeletal Limestone

Some marine invertebrate animals construct their shells or hard parts by extracting calcium and carbonate ions from seawater. Corals, clams, algae, snails, and many other marine organisms construct their skeletons of calcium carbonate. After the organisms die, the shells accumulate on the seafloor. Over a long period of time, they build up a deposit of limestone with a texture consisting of shells and shell fragments. These particles may then be cemented together as more calcite precipitates between the grains. This type of limestone, composed mostly of skeletal debris, can be several hundred meters thick and can extend over thousands of square kilometers. Chalk is a skeletal limestone in which the skeletal fragments are remains of microscopic plants and animals.

Oolitic Limestone

Other limestones are composed of small semi spherical grains of calcium carbonate known as oolites. Oolites form where small fragments of shells or other tiny grains become coated with successive thin layers of CaCO3 as they are rolled along the seafloor by waves and currents.

Microcrystalline limestone

A third important type of limestone forms in quiet waters where calcium carbonate is precipitated by algae as tiny, needle like crystals that accumulate on the seafloor as limy mud. Soon after deposition, the grains commonly are modified by compaction and recrystallization. Some kinds of algae produce calcium carbonate particles that accumulate to form limestone. These are found near the Kuril Islands of the north Pacific. Diatoms are the shells of tiny single-celled algae that are made of silica. Some deepmarine sediments are dominated by diatoms. Some accumulations convert to chert.

Dolostone / Dolomite

Dolostone or dolomite rock is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite. Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg(CO3)2. It is similar to limestone in general appearance, but reacts with acid only when powdered. Dolostone is commonly dull brownish yellow or light gray.


Chert is a common rock composed of microcrystalline quartz. In a hand specimen, it is hard, dense, and typically breaks like glass, but under a high-power microscope, it has a fibrous or granular texture. A distinctive type of deep-marine chert develops from deposits of siliceous shells of microscopic organisms, such as radiolaria and diatoms.

Rock salt

Rock salt is made of the mineral halite (NaCl). It crystallizes when evaporation concentrates sodium and chlorine ions to the point that salt is stable in the residual brine. Strong evaporation creates saline lakes in closed desert basins (for example, the Great Salt Lake and the Dead Sea). Enhanced evaporation also occurs in restricted bays along the shore of the ocean.


Gypsum, CaSO4•2H2O too originates from evaporation. It collects in layers as calcium sulphate is precipitated from water.


Coal is an important biochemical precipitate. It forms by the decomposition of organic material buried within sedimentary rocks. Lush vegetation may form in an ancient swamp and then be converted by burial into coal. The coal beds on the left are interlayered with sandstone. The accumulation of partially decayed vegetation is called Peat.  Peat is a compound of hydrogen, carbon, and oxygen. They formed from plant remains that built up over millions of years and were compacted under thick layers of inorganic clastic sediment. Hydrocarbons can be solid (peat and coal), liquid (petroleum), or gas (natural gas). Coal is the only hydrocarbon that is a rock. We often find natural gas and petroleum in open interconnected pores in a thick sedimentary rock layer, such as in porous sandstone.

Metamorphic Rocks

The mountain-building processes of the Earth’s crust involve tremendous pressures and high temperatures. These extreme conditions alter igneous or sedimentary rocks, transforming them into metamorphic rock. Thus, metamorphic rocks are formed from the pre-existing rocks within the Earth’s crust by changes in temperature and pressure and by chemical action of fluid. This means that Both the Igneous and Sedimentary socks undergo profound physical and chemical changes under the increased pressure and temperature. The process is called “metamorphism”.

Some metamorphic Rocks are Schist, Gneiss, Slate, Quartzite, Marble and  Granite.

There are two basic types of metamorphic rocks:

  • Foliated metamorphic rocks such as gneiss, phyllite, schist and slate which have a layered or banded appearance that is produced by exposure to heat and directed pressure. This is called Foliation.
  • Non-foliated metamorphic rocks such as marble and quartzite which do not have a layered or banded appearance. In the surface environment, rocks weather into sediment. In the deep environment, heat and pressure transform sediment into rock that is eventually exposed at the surface.

Weathering & Mass Wasting

Weathering There are two types of the processes that affect the landforms viz. Exogenic and Endogenic.

  • Endogenic are the processes that occur within the earth’s surface such as Plate tectonics, earthquakes, volcanoes etc.
  • Exogenic are the processes that occur on or near the earth’s surface. The tidal force is Exogenic. The radiation from Sun is also Exogenic.

Further, there are 3 Exogenic geological processes which refer to the process of disaggregation which lead to the reduction in the elevation & relief of the landforms and landscapes such as rocks and mountains. These 3 important phenomena are weathering, mass wasting and erosion. These all together are called “Degradation” or “Denudation“.

Endogenic processes uplift and expose continental crust to the Exogenic denudation. Exogenic denudation works in opposition and reduces landscapes to sea level.

Weathering is the breaking down of Rocks, soils and minerals through “direct Contact” with the atmosphere of the earth. It occurs in situ, means there is no movement involved.


This is distinct from erosion which involves the movement of rocks and minerals such as water, ice, wind and gravity.

Mass wasting

Mass wasting involves the movement of the rocks and particles across a slope due to gravity.


Weathering refers to the combined action of all processes that cause rock to disintegrate physically and decompose chemically because of exposure near the Earth’s surface. Weathering produces regolith. Weathering also creates a number of distinctive landforms. Regolith is a surface layer of weathered rock particles that lies above solid, unaltered rock. Weathering is the in situ disintegration and breakdown of rocks, soils and minerals.

Types of Weathering

There are three types of weathering viz. Mechanical or Physical Weathering, Chemical Weathering & Biological Weathering.

Physical Weathering

Physical Weathering can be caused by thermal changes, Frost Action, Pressure Release, Hydraulic action and Haloclasty. These terms have been discussed below:

Thermal Changes

Repeated changes in the temperature (heating and cooling) exert the stress on the outer layers of the rocks which is called as Thermal Stress. The rocks expand when there is a rise in the temperature and contract when there is a fall in the temperature. In deserts, the phenomena are more common as there is large diurnal temperature range. The Forest fires can raise the temperature suddenly and this leads to thermal shock.

Thermal Expansion versus Thermal Contraction

Please note that all materials respond by changing volumes because of temperature. Most materials expand when there is a rise in temperature. But there are some rare example which contract when temperature increases and expand when temperature decreases.  This is called Thermal contraction. The coefficient of thermal expansion is positive for the material which expand when there is a rise in temperature. If the coefficient of thermal expansion becomes zero, there is no expansion or contraction. At negative coefficient, the material contracts when there is a rise in temperature. Best example is water. Water when cooled till 4°C, the coefficient of thermal expansion decreases and become zero at 4°C. After that, when temperature is further reduced, it expands. So, at 4°C, water has maximum density. After that, density is reduced and this is the reason why ice floats and the water bodies are able to retain a temperature of 4°C at sub zero weathers. Similarly, Pure Silicon has a negative coefficient of thermal expansion between -255°C to -153°C.

Frost Action

One of the most important physical weathering processes in cold climates is frost action. As water in the pore spaces of rocks freezes and thaws repeatedly, expansion can break even extremely hard rocks into smaller fragments. Water penetrates fractures in bedrock. These fractures, called joints, are created when rocks are exposed to heat and pressure, then cool and contract. Joints typically occur in parallel and intersecting planes, creating natural surfaces of weakness in the rock. Frost action then causes joint-block separation. Water invades sedimentary rocks along their stratification planes, or bedding planes.

Pressure Release or exfoliation

This refers to the release of the pressure from unloading of existing rock on the rocks that lie beneath it due to other processes such as erosion. The igneous rocks are formed deep in earth and when the rocks above them get removed, the igneous rocks expose and the pressure is released. This causes their outermost surfaces to expand. This expansion leads to weathering.

Hydraulic Action

This phenomenon takes place due to very high powered water waves. When water rushes into cracks in the rocks with a very fast speed, the trap of air in the cracks get compressed and thus weakens the rocks. When water retreats, the trapped air is suddenly released with explosive force.

Salt Crystallization or Haloclasty

This refers to the process in which the rocks are denudated due to salt formation. This is a two step process. The first step is started when saline water seeps into cracks and evaporates depositing salt crystals. In the second step, when the rocks are heated up, the crystals expand putting pressure on the surrounding rock. Over the period of time, it splinters the stone into fragments.

Biological Weathering

Biological Weathering refers to the contribution made by the organisms such as Lichens and mosses, which grow on essentially bare rock surfaces and create a more humid chemical microenvironment. Biological weathering is both physical as well as chemical breakdown of the surface micro layer of the rock. The animals such as earthworms and other annelids, moles, rabbits all contribute to the biological weathering.

Chemical Weathering

Chemical weathering refers to the changes in the chemical composition of the rocks and generally refers to the chemical reactions of water with minerals.


Hydration means absorption of water by some kinds of rock, leading to expansions and disintegrations. When water molecules bind with the mineral molecules, it is called Mineral Hydration.


The chemical breakdown of the rocks caused by rainwater is called Hydrolysis. The result may be secondary minerals with different chemical structure.


Oxidation or rusting occurs when atmospheric oxygen reacts with the minerals such as Iron Ores. This leads to decomposition of the rocks.


This refers to dissolving of the minerals in water.


Carbonation refers to the chemical weathering in which Carbon dioxide attacks the rocks after it makes weak acid reacting the water.  The rocks are generally made up of calcium carbonate such as Limestone and Chalk.

Mass wasting

In Mass Wasting, the gravitational force of the earth acts directly on the loose material and the unstable slopes result the slide of the rocks and rock debris. This is known as Mass movement. This movement may be slow or fast depending upon the slope angle.  The steepest angle that cohesion less slope can maintain without losing its stability is known as its Critical angle of repose. Thus, mass wasting is spontaneous movement of soil, regolith, and rock under the influence of gravity. There are many forms of mass wasting, depending on the speed of the motion and the amount of water involved. Mass wasting is of following types:

Creeps It is a long term process which refers to the small movements of soil or rock in different directions over time, directed by gravity. The speed is so slow that naked eye is not able to show the movement.

Landslides It includes the rock slides, slumps (short distance moving of rocks) & sturzstroms (more horizontal movement when compared to its initial vertical drop). Landslides are most common type of mass wasting.

Flows Flows refer to the movement of the soil, dust, rock particles and bigger pabbles resembling the fluid behavior. Examples of the flows are avalanches, mudflows, debris flows, earth flow, lahars and sturzstroms. The water and air may contribute to the fluid like behaviour.

Topples When rocks break away and fall from a slope , it is called Topples.

Slump Slump refers to slipping of the rock material.

Falls Rocks fell from the steep slopes such as a cliff face, and the movement may be contributed by the earthquakes, rain, plant-root wedging, expanding ice, among other things.

Induced Mass wasting Human activities can induce mass wasting processes by creating unstable piles of waste soil and rock and by removing the underlying support of natural masses of soil, regolith, and bedrock. Mass movements produced by human activities are called induced mass wasting.


Erosions refer to the earth-sculpting processes in which the debris produced by weathering is “transported”. So it’s a kind of weathering in which the soils break up and get carried away. The agents of erosion are Rainwater, River water, ice, wind, sea waves, and underground water. Erosion is a very important topic physical and well as human geography. Apart from the transport by wind, water, or ice; erosion also involves the down-slope creep of soil and erosion by the living organisms, such as burrowing animals, in the case of bioerosion, and human land use.

January 13, 2018

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