Explore the world of rocks and minerals. Learn about igneous, sedimentary, and metamorphic rocks, their formation, classification, and significance in the rock cycle.

Composition of rocks & minerals on earth

• The Earth’s Composition:
  • Outer Layer: Solid form.
  • Interior: Hot and molten form.
  • Crust Composition:
    • 98% composed of 8 elements: Oxygen, Silicon, Aluminium, Iron, Calcium, Sodium, Potassium, Magnesium.
    • The rest include: Titanium, Hydrogen, Phosphorous, Manganese, Sulphur, Carbon, Nickel, and others.
• Minerals:

  

  • Definition: Naturally occurring organic and inorganic substances with an orderly atomic structure, definite chemical composition, and physical properties.
  • Composition:
    • Two or more elements.
    • Single-element minerals: Sulphur, Copper, Silver, Gold, Graphite.
  • Formation: Formed by cooling magma, resulting in crystallization.
  • Types: Coal, petroleum, and natural gas (organic minerals).
• Physical Characteristics of Minerals:
  • External Crystal Form: Cubes, octahedrons, hexagonal prisms, etc.
  • Cleavage: Tendency to break in specific directions.
  • Fracture: Irregular breakage without planes of cleavage.
  • Lustre: Appearance (e.g., metallic, silky, glossy).
  • Colour: Can be characteristic or due to impurities.
  • Streak: Colour of powdered mineral (e.g., Malachite gives green streak).
  • Transparency: Transparent, translucent, or opaque.
  • Structure: Arrangement of crystals (e.g., fine, coarse, fibrous).
  • Hardness: Measured on a scale of 1-10 (e.g., Talc = 1, Diamond = 10).
  • Specific Gravity: Ratio of weight in air to weight in water.
• Metallic Minerals:
  • Precious Metals: Gold, Silver, Platinum.
  • Ferrous Metals: Iron and its alloys (e.g., Steel).
  • Non-ferrous Metals: Copper, Lead, Zinc, Tin, Aluminium.
• Non-Metallic Minerals:
  • Examples: Sulphur, Phosphates, Nitrates.
  • Cement: A mixture of non-metallic minerals.
• Rocks:
  • Definition: Aggregates of one or more minerals.
  • Types:
    • Igneous Rocks: Formed from magma and lava (e.g., Granite, Basalt).
    • Sedimentary Rocks: Formed by deposition and lithification (e.g., Sandstone, Shale).
    • Metamorphic Rocks: Formed through recrystallization under pressure and temperature (e.g., Marble, Slate).
  • Rock Cycle: Continuous transformation of rocks through processes like subduction and recrystallization.

Non-Metallic Minerals Overview

• Non-Metallic Minerals
  • These minerals do not contain metal content.
  • Examples include:
    • Sulphur
    • Phosphates
    • Nitrates
  • Cement is a mixture of non-metallic minerals.
  • Additional characteristics:
    • Non-metallic minerals are often used in construction and industrial applications.
    • They may appear in various forms, such as solid (e.g., coal), liquid, or gaseous (e.g., petroleum and natural gas).
• Rocks
  • The earth’s crust is primarily composed of rocks.
  • A rock is defined as an aggregate of one or more minerals.
  • Rocks may vary in:
    • Hardness: For example, granite is hard while soapstone is soft.
    • Color: Examples include:
      • Gabbro: Black
      • Quartzite: Milky white
  • Most common minerals found in rocks:
    • Feldspar
    • Quartz
  • Study of Rocks:
    • The scientific study of rocks is called Petrology.
    • A Petrologist studies:
      • Mineral composition
      • Texture
      • Structure
      • Origin
      • Occurrence
      • Alteration
      • Relationships with other rocks

Rocks : Igneous Rocks

• Igneous Rocks

  

  • Igneous rocks form out of magma and lava from the earth’s interior.
  • They are referred to as primary rocks because they are the first rocks formed.
  • The term igneous comes from the Latin word "Ignis", meaning "Fire".
  • Formation:
    • When magma cools and solidifies, it forms igneous rocks.
    • The cooling process can occur either:
      • Within the earth's crust: Producing coarse-grained rocks (e.g., granite).
      • On the surface: Producing fine-grained or glassy rocks (e.g., basalt).

    

  • Classification by texture:
    • Texture depends on:
      • Cooling rate: Slow cooling results in large mineral grains; rapid cooling results in fine grains.
      • Arrangement of grains: Determines the overall texture.
    • Types of igneous rocks:
      • Granite: Coarse-grained and commonly found in the continental crust.
      • Gabbro: Dark, coarse-grained rock formed at great depths.
      • Basalt: Fine-grained rock, often forming oceanic crust.
      • Pegmatite: Contains very large crystals due to extremely slow cooling.
      • Volcanic breccia and tuff: Formed from explosive volcanic activity.

Sedimentary Rocks

• Sedimentary Rocks

  

  • The word ‘sedimentary’ is derived from the Latin word sedimentum, meaning ‘settling’.
  • Formation process:
    • Rocks (igneous, sedimentary, and metamorphic) on the surface are exposed to denudational agents.
    • These agents break rocks into fragments of various sizes.
    • The fragments are:
      • Transported by external forces such as water, wind, and glaciers.
      • Deposited in layers over time.
      • Through compaction and lithification (the process of turning sediments into rock), sedimentary rocks are formed.
  • Key characteristics:

    

    • Many sedimentary rocks display distinct layers, with varying thickness, even after lithification.
    • Common examples with visible layers: sandstone and shale.
  • Classification of sedimentary rocks:
    • Mechanically formed: Created from physical processes. Examples include:
      • Sandstone
      • Conglomerate
      • Limestone
      • Shale
      • Loess
    • Organically formed: Derived from organic matter. Examples include:
      • Geyserite
      • Chalk
      • Limestone (organic type)
      • Coal
    • Chemically formed: Formed through chemical precipitation. Examples include:
      • Chert
      • Halite
      • Potash
      • Chemical limestone

Metamorphic Rocks

• Metamorphic Rocks
  • The word ‘metamorphic’ means ‘change of form’.
  • Formation process:

    

    • Formed due to pressure, volume, and temperature (PVT) changes.
    • Occurs under the following conditions:
      • Rocks are forced to lower levels by tectonic processes.
      • Molten magma rising through the crust comes into contact with crustal rocks.
      • Rocks are subjected to high pressure from overlying rocks.
    • Undergoes recrystallisation and reorganisation of materials within the original rocks.
  • Types of metamorphism:

    

    • Dynamic Metamorphism:
      • Involves mechanical disruption and reorganisation of minerals.
      • Occurs without significant chemical changes.
    • Thermal Metamorphism:
      • Involves chemical alteration and recrystallisation due to high temperatures.
      • Two types of thermal metamorphism:
        • Contact Metamorphism:
          • Occurs when rocks are in contact with hot magma or lava.
          • Rocks recrystallise, and new materials may form from magma or lava.
        • Regional Metamorphism:
          • Occurs due to tectonic shearing combined with high temperature or pressure.
          • Causes widespread recrystallisation.
  • Key features:
    • In some rocks, grains or minerals arrange in layers (foliation) or lines (lineation).
    • Sometimes, minerals are arranged in alternating bands of light and dark shades, known as banding.
  • Classification:

    

    • Foliated Rocks: Display a layered structure. Examples: gneissoid, slate, schist.
    • Non-Foliated Rocks: Do not exhibit layers. Examples: marble, quartzite.
  • Examples of metamorphic rocks:
    • Gneissoid
    • Granite
    • Syenite
    • Slate
    • Schist
    • Marble
    • Quartzite

Rock Cycle

The rock cycle is a continuous process through which old rocks are transformed into new ones. Rocks do not remain in their original form permanently and undergo various changes over time.

Key Processes in the Rock Cycle:

• Igneous Rocks:
  • Known as primary rocks.
  • Formed from the cooling and solidification of molten magma.
  • Can transform into metamorphic rocks under pressure and heat.
• Sedimentary Rocks:
  • Formed from fragments of igneous and metamorphic rocks through processes like weathering, erosion, deposition, and lithification.
  • These rocks can themselves break into fragments and reform into new sedimentary rocks.



• Metamorphic Rocks:
  • Formed when existing rocks undergo recrystallisation and reorganisation due to pressure, temperature, or tectonic forces.
  • Can break down into fragments to form sedimentary rocks or melt into magma to form igneous rocks.
• Subduction Process:
  • The crustal rocks (igneous, sedimentary, and metamorphic) may be carried down into the mantle through subduction.
  • Increased temperature in the earth's interior melts these rocks into molten magma, which forms the source for new igneous rocks.

Summary of the Rock Cycle:



• Igneous rocks → Metamorphic rocks
• Igneous & Metamorphic rocks → Sedimentary rocks
• Sedimentary rocks → Fragments → New sedimentary rocks
• All rock types → Subduction → Molten magma → New igneous rocks

The rock cycle demonstrates how rocks continually change forms, showcasing the dynamic nature of Earth’s geology.