What is a coarse-grained, dark-coloured, intrusive igneous rock that is widely used as a concrete aggregate, road base material, and crushed stone for railroad ballast?
Gabbro
Identifying Coarse-Grained Dark Intrusive Igneous Rocks
The question asks to identify a specific type of rock based on several key characteristics: it's coarse-grained, dark-coloured, an intrusive igneous rock, and has common uses in construction like concrete aggregate, road base, and railroad ballast.
Understanding the Characteristics
- Coarse-grained: This means the mineral crystals within the rock are large enough to be seen clearly with the naked eye. This texture develops when magma cools slowly, allowing crystals time to grow, typical of intrusive igneous rocks.
- Dark-coloured: This indicates the rock is rich in mafic minerals, which are typically dark silicates containing iron and magnesium (like pyroxene, amphibole, olivine).
- Intrusive igneous rock: This type of rock forms when magma cools and solidifies beneath the Earth's surface. The slow cooling leads to the coarse-grained texture.
- Uses: The specified uses (concrete aggregate, road base, railroad ballast) point towards a durable, strong rock that can be crushed and used for structural support and filler.
Evaluating the Options
Let's examine each option based on these characteristics:
- Quartzite: This is a metamorphic rock formed from sandstone. It is typically light-coloured and not an intrusive igneous rock. It does not fit the description.
- Conglomerate: This is a sedimentary rock made up of rounded pebbles and gravel cemented together. It is not an intrusive igneous rock and its texture is clastic, not crystalline coarse-grained in the igneous sense. It does not fit the description.
- Gabbro: This is an intrusive igneous rock that is typically coarse-grained. Its composition is mafic, meaning it is rich in dark-coloured minerals like pyroxene and plagioclase feldspar, making the rock dark. Gabbro is widely used in construction for the purposes mentioned due to its hardness and durability when crushed. This option fits all the characteristics.
- Sandstone: This is a sedimentary rock formed from sand-sized grains. It is not an intrusive igneous rock and its texture is granular, not coarse-grained in the intrusive igneous sense. It does not fit the description.
Based on the analysis, Gabbro is the rock that matches all the descriptions provided in the question.
| Rock Type | Type | Texture | Colour (Typical) | Fits Description? |
|---|---|---|---|---|
| Quartzite | Metamorphic | Granoblastic | Light | No (Not igneous, light) |
| Conglomerate | Sedimentary | Clastic (rounded gravel) | Varied | No (Not igneous) |
| Gabbro | Intrusive Igneous | Coarse-grained | Dark | Yes |
| Sandstone | Sedimentary | Clastic (sand grains) | Varied | No (Not igneous, texture) |
Conclusion: Identifying the Rock
The rock described as coarse-grained, dark-coloured, intrusive igneous, and used widely as concrete aggregate, road base material, and crushed stone for railroad ballast is Gabbro.
| Characteristic | Description | Relevance to Gabbro |
|---|---|---|
| Coarse-grained | Large visible crystals; slow cooling | Typical texture of Gabbro due to intrusive formation |
| Dark-coloured | Rich in mafic minerals (e.g., pyroxene, plagioclase) | Gabbro's composition is mafic, leading to dark colour |
| Intrusive igneous | Forms from magma cooling underground | Gabbro is a classic example of an intrusive igneous rock |
| Common Uses | Aggregate, road base, ballast | Gabbro's hardness and durability make it suitable for these applications |
Additional Information: Igneous Rocks and Uses
Igneous rocks are formed from the cooling and solidification of magma or lava. They are broadly classified into two main types:
- Intrusive (Plutonic) Igneous Rocks: Formed when magma cools slowly beneath the Earth's surface. The slow cooling allows large crystals to form, resulting in a coarse-grained texture. Examples include granite, diorite, and gabbro.
- Extrusive (Volcanic) Igneous Rocks: Formed when lava cools quickly on the Earth's surface. Rapid cooling results in small or microscopic crystals, leading to fine-grained textures (like basalt) or even glassy textures (like obsidian).
Many types of igneous rocks, both intrusive and extrusive, are used in construction and industry due to their strength and durability. Gabbro, being a hard, dense rock, is particularly well-suited for uses requiring crushed stone or aggregate, such as road construction, railway beds, and concrete production.
Important Questions from Geomorphology
Which one of the following is not an igneous rock?
Which of the following statements is/are correct?
1. Hypocenter is the point on the surface of the Earth, nearest to the focus.
2. Velocity of earthquake waves is higher in denser materials.
3. P waves move faster and are the first to arrive at the surface of the Earth.
Select the correct answer using the code given below:
Which one of the following is the lowermost/innermost intrusive igneous rock?
The formation of ‘tors’ on small rocky hills is associated with which among the following?
Which one of the following statements about metamorphic rocks is not correct?
