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Q.Describe the characteristics and types of primary rocks.

UPSC Mains 2022•Geography

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Introduction

Primary rocks, commonly known as igneous rocks, are formed directly from the cooling and solidification of molten magma or lava. They represent the foundational rock type from which other rocks are derived, and their characteristics depend heavily on the composition of the molten material and the environment in which they cool.

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Characteristics of Primary Rocks

  • Identification: They are identified by their distinct texture, density, color, and mineral composition. Their texture is determined by the shape, size, cooling duration, and crystalline arrangement within the rock.
  • Absence of Fossils: Due to the intense heat of molten magma/lava during formation, fossils are not preserved in these rocks.
  • Chemical and Physical Properties: They generally do not react with acids and tend to have a fine-grained, smooth texture.
  • Hardness: They are typically hard and compact, making water percolation highly difficult.

Types of Igneous Rocks

  • Intrusive Igneous Rocks: Formed when magma cools slowly deep beneath the Earth's surface, allowing large crystals to develop. Examples include granite and dolerite.
  • Extrusive Igneous Rocks: Formed when lava reaches the Earth's surface and cools rapidly, resulting in fine-grained textures. Also known as volcanic rocks, examples include basalt and gabbro.

Conclusion

In conclusion, primary rocks serve as the fundamental building blocks of the Earth's crust, offering invaluable insights into the planet's geological evolution and internal processes.

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