Question

If no crystallization takes place, then the rock is

• A. Holocrystalline
• B. Plutonic
• C. Glassy
• D. Hypabyssal

Hint:

• Igneous rock texture depends on the cooling rate of magma/lava.
• Rapid cooling (e.g., lava erupting onto the surface) prevents crystal growth, resulting in a glassy texture (e.g., obsidian). No crystals are formed.
• Slow cooling (e.g., magma deep underground) allows large crystals to form, resulting in a phaneritic or holocrystalline texture.
• Holocrystalline: Rock is entirely crystalline.
• Plutonic: Igneous rocks formed at depth (slow cooling).
• Hypabyssal: Igneous rocks formed at shallow depths.

Answer 1

The Correct Option is C

The texture of an igneous rock refers to the size, shape, and arrangement of its mineral crystals. Crystallization occurs when molten rock (magma or lava) cools and solidifies. The rate of cooling significantly influences the extent of crystallization and the resulting texture.

• If no crystallization takes place, it means the molten rock cooled so rapidly that ions did not have time to arrange themselves into an orderly crystalline structure. The atoms are "frozen" in a disordered state, similar to the structure of glass. Such a rock is said to have a glassy texture (option c). Obsidian is a common example of a volcanic rock with a glassy texture.
• Holocrystalline (option a): This term describes an igneous rock that is entirely composed of crystals, meaning complete crystallization occurred. This typically happens with slow cooling.
• Plutonic (option b): Also known as intrusive igneous rocks, these rocks form from magma that cools slowly deep beneath the Earth's surface. Slow cooling allows for the growth of large, visible crystals (phaneritic texture), and these rocks are typically holocrystalline.
• Hypabyssal (option d): These are intrusive igneous rocks that form at relatively shallow depths below the surface, cooling more rapidly than plutonic rocks but more slowly than volcanic (extrusive) rocks. They often have textures intermediate between plutonic and volcanic rocks, such as porphyritic texture.

Therefore, if no crystallization occurs, the resulting rock has a glassy texture. \[ \boxed{\text{Glassy}} \]