ROCKS

ROCKS

The earth’s crust is composed of rocks. A rock is an aggregate of one or more minerals. Rocks may be hard or soft and in varied colors. For example: Granite is hard, soapstone is soft. Gabbro is black and quartzite can be milky white.

Different kinds of rocks

Many different kinds of rocks are classified into three families based on their mode of formation.

• Igneous Rocks: Igneous rock is formed by the solidification of magma and lava. This is also known as the primary rock.
  • Example: Granite and Basalt etc.
• Sedimentary Rocks: Sedimentary rocks are the result of the deposition of fragments of rocks by exogenous processes. This is also known as the secondary rocks.
  • Example: Sandstone, limestone, shale, etc.
• Metamorphic Rocks: Metamorphic rocks are formed from already-existing rocks that are undergoing recrystallization. Tertiary rocks are another name for metamorphic rocks.
  • Example: Phyllite, schist, gneiss, quartzite, and marble, etc.

IGNEOUS ROCKS

Igneous rocks are also called primary rocks (Parent Rock) because these were first of all the rocks during the formation of the upper crust of the earth on cooling, solidification, and crystallization of hot and liquid magmas after the origin of the earth.

1. Characteristics of Igneous Rocks:
   • Hard Rock: In all, igneous rocks are roughly hard rocks and water percolates with great difficulty along the joints. However, sometimes the rocks become so soft, due to their exposure to environmental conditions for longer duration, that they can be easily dug out by a spade.
     • Example: Basalt.
   • Variant Size: Igneous rocks are granular or crystalline rocks but there are many variations in the size, form, and texture of grains because these properties largely depend upon the rate and place of cooling and solidification of magmas or lavas.
     • Example: When the lavas are quickly cooled down and solidified at the surface of the earth, there is no sufficient time for the development of grains/crystals. Contrary to this, if magmas are cooled and solidified at a very slow rate inside the earth, there is sufficient time for the full development of grains, and thus the resultant igneous rocks are characterized by coarse grains.
   • Layers of Lava: Unlike sedimentary rocks there is no strata in igneous rocks, rather there are layers of lavas.

• Examples: Lava layers near Khandala or along the deeply entrenched valleys of the Koyna river, the Krishna river, the Saraswati river, etc. in and around Mahabaleshwar plateau.
• Effect of Weathering: Since water does not penetrate the rocks easily and hence igneous rocks are less affected by chemical weathering, but coarse-grained igneous rocks are affected by mechanical or physical weathering, and thus the rocks are easily disintegrated and decomposed.
• Absence of Fossils: Since igneous rocks developed when there was no life on the freshly formed earth and because very hot, molten materials killed fossils due to their extremely high temperatures, igneous rocks do not contain any fossils.
• Volcanic Rocks: Mostly associated with volcanic activities, and thus they are also called volcanic rocks that are generally found in the volcanic zones.

1. Classification of Primary Rocks: The igneous rocks are more commonly classified on the basis of the mode of occurrence into two major groups:

• Intrusive Igneous Rocks/Plutonic Rocks: When the rising magmas during a volcanic activity do not reach the earth’s surface, rather they are cooled and solidified below the surface of the earth, the resultant igneous rocks are called intrusive igneous rocks.
  • Example: Granite is the best representative example of this category.
• Extrusive Igneous Rocks: The igneous rocks formed due to cooling and solidification of hot and molten lavas at the earth’s surface are called extrusive igneous rocks.
  • Example: Basalt, Gabbro and obsidian, Granite, breccia, tuff, and pegmatite, etc.

SEDIMENTARY ROCKS

Sedimentary rocks are also known as secondary rocks as they are formed by the sediments of other rocks denuded and deposited by the agents of gradation.

1. Characteristics of Sedimentary Rocks:
   • Porosity: Most of the sedimentary rocks are permeable and porous, but a few of them are also non-porous and impermeable. The porosity of the rocks depends upon the ratio between the voids and the volume of a given rock’s mass.
     • Example: In more compact sedimentary rocks, the porosity is measured at 27% while in a more loosely packed rock, it can go over 47%. While sandstones can have a porosity of 5%, clays have porosity exceeding 50%.
   • Nature: These rocks are generally not crystalline in nature. They are soft and have many layers as they are formed due to the deposition of sediments.

• Found largely on the surface of the earth: Sedimentary rocks are largely found on the Earth’s surface. They cover 75% of the Earth’s area.
• Stratified rocks: Due to successive depositions, they have a layered or stratified structure and hence are also called as Stratified Rocks.
• Contain organic substance: These rocks may have the presence of the remains of plants and animals in between various layers.
• Sphericity: The sphericity of the sedimentary rocks varies according to their course of the journey during the process of transportation. In general, grains become more rounded the further they are from their source rock.
• Further, the sedimentary rocks are from their original rock, the more rounded they are, and their sphericity increases. Parallely, sediments that are carried through longer distances have more sorted debris rather than those that have traveled relatively shorter distances.
• Composition: Sedimentary rocks may be well consolidated, poorly consolidated, and even unconsolidated. The composition of the rocks depends upon the nature of cementing elements and rock-forming minerals.

1. Classification of Sedimentary Rocks: Sedimentary rocks can be classified under three categories with respect to their origin & composition:

• Mechanically formed/Clastic Sedimentary Rocks: They are formed by the consolidation of sediments under excessive pressure and cementation.
  • Example: Conglomerate, Breccia, Sandstone, Shale, etc.
• Organically/Biologically formed Sedimentary Rocks: The consolidation of organic matters derived from plants and animals forms this type of rocks.
  • Example: Coal, limestone, chalk, chert, etc.
• Chemically formed Sedimentary Rocks: They are formed by various chemical reactions.
  • Example: Gypsum, rock salt, limestone, etc.

METAMORPHIC ROCKS

These rocks are formed due to complete alteration in the appearance and constitution of pre-existing rocks due to changes in mineral composition and texture through temperature and pressure. These rocks are the hardest rocks and do not contain fossils.

Examples of metamorphic rocks: Gneiss, granite, syenite, slate, schist, marble, quartzite, etc.

1. Characteristics of Metamorphic rocks
   • Non-Foliated: Occurs along with the tectonic plate faults where the rocks get rubbed with each other, resulting in the grain size reduction. Transformation of these rocks is classified as one which could not form non-foliated rock and is of low grade.
   • Harder in nature: They are harder and more compact than the original igneous rocks.
     • Example: Marble is harder than limestone.
   • Impermeable: Most of the rocks are impermeable as they do not allow water to percolate through the rocks.
   • Do not contain fossil fuel: They do not have fossil fuels as fossil fuels are destroyed during the formation of igneous rocks.
   • Formation of metals and stones: Circulation of rock results in a huge number of minerals, water, and chemical reactions which result in different kinds of precious metals and stones.
2. Classification of Metamorphic Rocks: On the basis of the agency of metamorphism, metamorphic rocks can be of two types:

• Dynamic Metamorphism: When the metamorphism happens without any appreciable chemical change, it is called Dynamic Metamorphism.
  • Example: Under high pressure, granite is converted into gneiss; clay and shale are transformed into schist.
• Thermal Metamorphism: If metamorphism happened due to the influence of heat, it is called Thermal Metamorphism.
  • Example: As a result of thermal metamorphism, sandstone changes into quartzite and limestone into marble.
  • It has two types: Contact Metamorphism and Regional Metamorphism.
    • Contact Metamorphism: When the reorganization occurs due to direct contact with the hot magma, it is called Contact Metamorphism.
    • Regional Metamorphism: If the rocks undergo reorganization due to tremendous heat/pressure formed as a result of tectonic shearing, it is called Regional Metamorphism.

ROCKS CYCLE

• Rock Cycle: Rocks do not remain the same; they transform, and this process is called the Rock Cycle.
• Primary material: Igneous rocks are the primary rocks; metamorphic and sedimentary rocks are other forms of these primary rocks.
• Formation of rocks: The igneous and metamorphic rocks can be fragmented and turn into sedimentary rocks. Sedimentary rocks also fragment further to become new sedimentary rocks.
• Continuation of cycle: The crustal rocks such as Metamorphic and Igneous rocks can subduct due to various movements, move slowly to the mantle, and due to high temperature, they melt down to lava, continuing the rock cycle.
• Example: 
  1. Clay Slate
  2. Limestone Marble
  3. Sandstone Quartzite
  4. Granite Gneiss
  5. Shale Schist
  6. Coal Graphite (one which is in our pencil)