Rocks – Introduction:

Rocks are naturally occurring solid masses. These are aggregates of minerals like feldspars, micas, amphiboles, pyroxenes, olivine, quartz, calcite, dolomite, clays, and gypsum. Some rocks also contain mineraloids, which are rigid, mineral-like substances, like volcanic glass, lacking crystalline structure. Rocks form the outer solid layer, the crus, and most of its interior, except the liquid core of the earth.

The scientific study of the character and origin of rocks is called petrology, under geology.

On the basis of origin and mode of formation, rocks may be classified into:

  1. Igneous
  2. Sedimentary
  3. metamorphic
The igneous rocks 

(derived from the Latin word igneus, meaning of fire, from ignis meaning fire)

Igneous rocks are formed when magma cools in Earth’s crust i.e. cooling and solidification of silicate minerals (magma). These rocks include volcanic and plutonic rocks.

igneous rocks

Igneous rocks

The Igneous rocks are divided into two main categories:

  1. Plutonic or Intrusive: when magma cools and gets crystallized slowly with the crust of the earth.  Types of Intrusive rocks:
    • Batholith: Large intrusive mass of igneous rocks (granite), exposure in larger than 100 sq. km area. the rock is formed from cooled magma in the deep earth’s crust. These rocks are found in the Aswan Granite Batholith in Egypt.
    • Laccolith: an intrusive, dome-like mass of igneous rock that arches the overlaying sediment possesses a flat floor. Laccoliths are found in The Rocky Mountains in North America.
    • Sill: it is a tubular body of intrusive rock. Sill found in the west bank of Hudson River (opposite New York city) Palisades sill
    • Dike (Dyke): Tubular intrusive rock that occurs around strata or other structural features of the surrounding rock.
      • Channel Island
      • The world-famous dikes – The Spanish peak region
    • Phacolith: it is a concave- convex-shaped igneous rock. These rocks were found near Bayalan, Ajmer district, Rajasthan in India
    • Stock: It is a small, rough, circular intrusive body, less than 100 sq. km. surface exposure. Examples of stock are:
      • The Alta and Clayton peak stocks, Utah
      • Hellroaring creek and salal creek stocks, Canada.
    • Zeolites: a large group of silicates consisting of true water of crystallization. These occur in cavities in basic volcanic rocks. Zeolites are thought to be the product of hydrothermal activity. It is found in:
      • China
      • Japan
      • Jordan
      • Turkey
      • The United State of America
    • Lopoliths: The magma intruded and solidified in a concave shallow basin with a central depressed body. Some examples are:
      • The Sudbury igneous complex of Ontario
      • the Bushveld igneous complex of South Africa.
      • The Great Dyke in Zimbabwe
      • The Humboldt lopolith of Nevada.
  1. Volcanic or Extrusive: Minerals like pumice or basalt Rocks are formed by magma after reaching the surface either as lava or fragmental ejecta (above the earth’s surface). They can be divided as:
    • Explosive Rocks: These types of igneous rocks are formed by Deposits of volcanic materials like lapilli, bombs, ash, dust. Examples of Explosive rocks are:
      • Long Valley in California
      • Valles in New Mexico
    • Quite Type: when the lava comes from the fissure and minor cracks and after cooling it takes shape of basaltic igneous rocks. almost all lava plateaus found in the world are the result of the fissure eruption. Example of a Quite:
      • Columbia plateau of Washington states and Oregon (USA).
      • Peninsular of India

Characteristics of Igneous Rocks:

  • These are solidified forms of magma, which do not occur in distinct beds or strata.
  • The rocks are mostly not fossiliferous.
  • The igneous rocks are generally granular and crystalline.
  • These rocks are usually hard and water draining in them is very difficult at the joints.
  • The rocks are mostly related to volcanic activities.
  • mostly consisting of silicate minerals.
  • Minerals found in igneous rocks are such as- copper, iron ore, gold, silver, zinc, lead, manganese, mica, etc.

Examples of Igneous rock with countries in which they found is given below:

igneous rock table

Igneous rocks with examples, places where they found

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