Question

Identify the group that contains only examples of Igneous rocks :

• A. Granite, Gabbro, Limestone
• B. Basalt, Tuff, Sandstone
• C. Pegmatite, Breccia, Slate
• D. Granite, Basalt, Pegmatite

Answer 1

Answer:

D. Granite, Basalt, Pegmatite

Igneous Rocks: Formation and Classification

Definition and Formation:

• Igneous rocks are formed from the cooling and solidification of molten rock material, known as magma (beneath the Earth's surface) or lava (on the Earth's surface).
• The word 'igneous' is derived from the Latin word 'ignis', meaning 'fire'.
• The texture of igneous rocks is largely determined by the rate of cooling:
• Slow cooling leads to the formation of larger crystals (coarse-grained texture, e.g., Granite).
• Rapid cooling results in smaller crystals (fine-grained texture, e.g., Basalt).
• Very rapid cooling (quenching) can result in glassy textures with no visible crystals (e.g., Obsidian).

Types of Igneous Rocks:

• Intrusive (Plutonic) Igneous Rocks: Formed when magma cools and solidifies slowly beneath the Earth's surface. They are typically coarse-grained.
• Granite: A common intrusive igneous rock composed primarily of quartz, feldspar, and mica. It is usually light-colored and granular. Often used as a building and decorative stone.
• Pegmatite: An exceptionally coarse-grained intrusive igneous rock, often with very large crystals of minerals like feldspar, quartz, and mica. Formed from highly evolved, water-rich magmas. Gemstones like tourmaline and topaz are often found in pegmatites.
• Extrusive (Volcanic) Igneous Rocks: Formed when lava cools and solidifies rapidly on the Earth's surface. They are typically fine-grained or glassy.
• Basalt: A common extrusive igneous rock, typically dark-colored and fine-grained. It is rich in magnesium and iron. Basalt forms the oceanic crust and large volcanic plains. Columnar jointing is a characteristic feature of some basalt flows.

Examples of Igneous Rocks:

• The provided correct answer correctly identifies Granite (intrusive), Basalt (extrusive), and Pegmatite (intrusive) as examples of igneous rocks.