Question

Which of the following is the correct process for the formation of Igneous rocks ?

• A. Intense pressure on existing rocks.
• B. Accumulation of organic remains over millions of years.
• C. Cooling and solidification of magma.
• D. Chemical precipitation in shallow water.

Answer 1

Answer:

C. Cooling and solidification of magma.

Igneous Rock Formation

Origin from Magma and Lava

• Igneous rocks are formed from the cooling and solidification of molten rock material.
• This molten rock is known as magma when it is beneath the Earth's surface.
• When magma erupts onto the Earth's surface, it is called lava.

Two Main Types of Igneous Rocks

1. Intrusive (Plutonic) Igneous Rocks:
   • Formed when magma cools and solidifies slowly beneath the Earth's surface.
   • The slow cooling allows large crystals to form, giving these rocks a coarse-grained texture.
   • Examples: Granite, Gabbro, Diorite.
2. Extrusive (Volcanic) Igneous Rocks:
   • Formed when lava cools and solidifies rapidly on the Earth's surface (or under shallow water).
   • The rapid cooling results in small crystals or a glassy texture (no crystals).
   • Examples: Basalt, Obsidian, Pumice, Rhyolite.

Key Processes

• Melting: The process where solid rock melts to form magma. This occurs due to high temperatures within the Earth.
• Cooling and Crystallization: As magma or lava loses heat, minerals begin to crystallize and interlock to form solid rock. The rate of cooling is a crucial factor determining crystal size.
• Solidification: The final stage where the molten material becomes a solid rock.

Geological Significance

• Igneous rocks form the Earth's crust, particularly the oceanic crust (basalt) and large parts of the continental crust (granite).
• They are often the primary rocks from which other rock types (sedimentary and metamorphic) are derived through weathering, erosion, and metamorphism.