Pick up the plutonic rock from the following:
🌋 Understanding Igneous Rock Classification by Depth
All igneous rocks form from cooling magma, but their final characteristics are determined by the depth at which they cool. This depth controls the cooling rate, which in turn dictates the size of the mineral crystals in the rock.
🔬 Detailed Analysis of the Options
A. Granite
This is the correct answer. Granite is the quintessential example of a Plutonic (or Intrusive) rock. It forms when large bodies of magma get trapped and solidify at great depths within the Earth's crust. The insulating effect of the surrounding rock allows the magma to cool extremely slowly over millions of years, giving ample time for large, interlocking crystals of quartz, feldspar, and mica to grow.
B. Dolerite
Dolerite is a Hypabyssal rock. This means it cools at a medium depth, such as in dikes or sills. Its cooling rate is faster than granite's, so its crystals are smaller and medium-grained.
C. Basalt
Basalt is a Volcanic (or Extrusive) rock. It forms from lava that cools very quickly on the Earth's surface. This rapid cooling prevents the growth of large crystals, resulting in a fine-grained texture.
D. All the above
This is incorrect because the three rocks listed represent the three different depth classifications (Plutonic, Hypabyssal, and Volcanic), they are not all Plutonic.
📊 Quick Comparison of Igneous Rock Types
| Classification | Formation Depth | Cooling Rate | Crystal Size (Texture) | Example |
|---|---|---|---|---|
| Plutonic (Intrusive) | Great Depth | Slow | Large (Coarse-grained) | Granite |
| Hypabyssal | Shallow / Medium Depth | Intermediate | Medium-grained | Dolerite |
| Volcanic (Extrusive) | Surface | Fast | Small (Fine-grained) | Basalt |
💡 Study Tips
- Plutonic = Pluto: Think of Pluto, the Roman god of the underworld. His domain was deep underground, just like where plutonic rocks form.
- Slow Cooling = Large Crystals: Remember that the slower the cooling, the more time the crystals have to grow, resulting in a coarse-grained texture like you see in granite countertops.
- Intrusive vs. Extrusive: Plutonic rocks are "intrusive" because the magma intrudes into existing rock layers deep underground. Volcanic rocks are "extrusive" because the lava is extruded (pushed out) onto the surface.