Correct Option
The correct option isRapid cooling on the earth's surface
Explanation
Igneous rocks are formed by the cooling and solidification of molten rock (magma or lava). The texture of these rocks, specifically the size of the mineral crystals (grain size), is primarily determined by the rate of cooling.
Option Analysis
- Slow cooling inside the earth is incorrect: Slow cooling occurs deep inside the Earth's crust, where heat dissipates slowly. This allows mineral crystals ample time to grow large, resulting in a coarse-grained texture. This characterizes intrusive (plutonic) igneous rocks, such as granite.
- Rapid cooling on the earth's surface is correct: Extrusive igneous rocks form when magma erupts onto the Earth's surface as lava. Upon exposure to the atmosphere or water, the lava undergoes rapid cooling. This rapid solidification denies ions the time to organize into large crystal lattices, resulting in a fine-grained (aphanitic) structure. Examples include basalt and rhyolite.
- Mixing with sediments is incorrect: Mixing with sediments is a process related to the formation of sedimentary rocks or the contamination of magma, but it is not the primary cause of the fine-grained texture in igneous rocks.
- Metamorphism due to heat is incorrect: Metamorphism refers to the alteration of pre-existing rocks under high heat and pressure without melting. It results in metamorphic rocks, not extrusive igneous rocks.
Key Takeaway: The texture of igneous rocks is defined by cooling history: rapid cooling on the surface produces fine-grained extrusive rocks, while slow cooling at depth produces coarse-grained intrusive rocks.