The rocks which are formed due to cooling of magma at a considerable depth from earth's surface are called
🌋 Understanding Igneous Rocks
This question is about the classification of Igneous Rocks. All igneous rocks are formed from the cooling and solidification of molten rock material (magma or lava). However, they are classified into different types based on where this cooling takes place.
Magma: Molten rock that exists beneath the Earth's surface.
Lava: Molten rock that has erupted onto the Earth's surface.
🔬 Detailed Analysis of Formation Types
A. Plutonic Rocks (Intrusive)
This is the correct answer. Plutonic rocks are formed when magma gets trapped deep within the Earth's crust and cools down very slowly over thousands or millions of years. This slow cooling allows large mineral crystals to grow.
- Formation Location: At a considerable depth inside the Earth.
- Cooling Rate: Very Slow.
- Resulting Texture: Coarse-grained (large, visible crystals), known as a phaneritic texture.
- Example: Granite.
C. Volcanic Rocks (Extrusive)
Volcanic rocks are formed when lava erupts onto the Earth's surface and cools very quickly in contact with the air or water. This rapid cooling does not allow time for large crystals to form.
- Formation Location: On the Earth's surface.
- Cooling Rate: Very Fast.
- Resulting Texture: Fine-grained (microscopic crystals), known as an aphanitic texture, or even glassy (no crystals) like obsidian.
- Example: Basalt, Rhyolite.
B. Hypabyssal Rocks (Intermediate)
Hypabyssal rocks are an intermediate type. They are formed when magma cools at a relatively shallow depth below the surface, for example, in sills or dikes. The cooling rate is faster than for plutonic rocks but slower than for volcanic rocks.
- Formation Location: Shallow depth below the surface.
- Cooling Rate: Medium / Intermediate.
- Resulting Texture: Medium-grained or often porphyritic (containing a mix of large and small crystals).
- Example: Dolerite.
D. Igneous Rocks
This is the broad parent category that includes all three types mentioned above (Plutonic, Hypabyssal, and Volcanic). While it's technically true that the rock in the question is an igneous rock, it's not the most specific or best answer. The question asks for the specific name for rocks formed at great depth.
📊 Quick Comparison of Igneous Rock Types
| Type | Formation Depth | Cooling Rate | Crystal Size (Texture) | Example |
|---|---|---|---|---|
| Plutonic (Intrusive) | Great Depth | Slow | Large (Coarse-grained) | Granite |
| Hypabyssal | Shallow Depth | Medium | Medium / Mixed | 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.
- Volcanic = Volcano: This is the easiest one to remember. Volcanic rocks come from volcanoes, which are on the surface.
- Slow Cooling = Large Crystals: Imagine making rock candy. If you let the sugar water cool slowly, you get large sugar crystals. If you cool it fast, you get tiny ones. The same principle applies to magma.
- 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.