Which of the following metamorphic rocks has the most weather resisting characteristics?
Understanding Weather Resistance in Metamorphic Rocks
The question asks us to identify which of the listed rocks, specifically focusing on weather resistance, is the most durable. Weathering is the process where rocks and minerals are broken down at the Earth's surface. Different rocks resist this breakdown differently based on their composition, structure, and how they were formed.
Let's look at the characteristics of each rock type provided in the options:
- Marble: This is a metamorphic rock formed from the recrystallization of limestone or dolostone. It is primarily composed of calcite (calcium carbonate, $\text{CaCO}_3$). Calcite is relatively soft and is susceptible to chemical weathering, particularly dissolution by acidic rainwater.
- Quartzite: This is a metamorphic rock formed from sandstone. It consists mainly of quartz grains ($\text{SiO}_2$) that have been recrystallized and fused together during metamorphism. Quartz is a very hard and chemically stable mineral. This makes quartzite highly resistant to both physical and chemical weathering processes.
- Slate: This is a fine-grained metamorphic rock formed from shale or mudstone. It is composed mainly of clay minerals and micas. Slate is known for its prominent foliation (slaty cleavage), which allows it to be split into thin sheets. While relatively durable for certain applications, the clay minerals can alter over time, and the cleavage planes can be weak points under physical stress or frost wedging, making it less resistant than very hard, non-foliated rocks like quartzite.
- Limestone: This is a sedimentary rock, not a metamorphic rock (although it is the parent rock of marble). Limestone is primarily composed of calcite ($\text{CaCO}_3$). Like marble, it is highly susceptible to chemical weathering, especially dissolution by acidic water.
Comparing the weather resistance of these rocks:
- Limestone and Marble are both composed mainly of calcite, which is easily dissolved by acid rain. They are the least resistant to chemical weathering among the options involving calcite.
- Slate, while more resistant than its parent rock shale, has a foliation that can be a point of weakness, and its component minerals (clays, micas) are less stable than quartz.
- Quartzite is composed almost entirely of quartz, one of the most stable and durable minerals found at the Earth's surface. The metamorphic process fuses the quartz grains tightly, creating a very hard and dense rock.
Therefore, based on their mineral composition and physical properties related to metamorphism, Quartzite exhibits the strongest resistance to weathering.
| Rock Type | Classification | Main Mineral(s) | Key Characteristic | Weather Resistance |
|---|---|---|---|---|
| Marble | Metamorphic | Calcite ($\text{CaCO}_3$) | Recrystallized texture | Low (Susceptible to acid rain) |
| Quartzite | Metamorphic | Quartz ($\text{SiO}_2$) | Fused quartz grains, very hard | High (Very resistant) |
| Slate | Metamorphic | Clay minerals, micas | Slaty cleavage (foliation) | Moderate (Weakness along cleavage) |
| Limestone | Sedimentary | Calcite ($\text{CaCO}_3$) | Fossil fragments common | Low (Highly susceptible to acid rain) |
Revision Table: Key Concepts in Rock Weathering
Understanding rock weathering involves knowing the types of processes and how different rock types respond.
| Concept | Description | Relevance |
|---|---|---|
| Physical Weathering | Breaking down rocks into smaller pieces without changing their chemical composition (e.g., frost wedging, abrasion). | Hard, dense rocks like quartzite resist abrasion well. Rocks with fractures or cleavage (like slate) can be vulnerable to frost wedging. |
| Chemical Weathering | Breaking down rocks through chemical reactions (e.g., dissolution, oxidation, hydrolysis). | Minerals like calcite ($\text{CaCO}_3$) are easily dissolved by weak acids (acid rain). Quartz ($\text{SiO}_2$) is highly resistant to most chemical weathering processes. |
| Mineral Stability | How easily a mineral breaks down under surface conditions. | Minerals like quartz are very stable at the Earth's surface, making rocks rich in quartz very resistant to weathering. Minerals like calcite are less stable. |
Additional Information: Metamorphic Rocks and Durability
Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or other metamorphic rocks) are subjected to high heat and pressure. This process can change their mineralogy, texture, and chemical composition, often resulting in rocks that are harder and more durable than their parent rocks.
- Parent Rock Influence: The original rock (protolith) plays a significant role in the resulting metamorphic rock's properties. Quartzite's exceptional durability comes directly from its protolith, sandstone, being primarily composed of stable quartz.
- Degree of Metamorphism: Higher grades of metamorphism often lead to larger crystal sizes and increased interlocking textures, which can enhance durability, though foliation (like in slate) can sometimes reduce overall strength in certain directions.
- Practical Applications: The weather resistance of metamorphic rocks makes them suitable for various construction and architectural purposes. Quartzite is often used for paving stones, building facades, and countertops due to its hardness and durability. Marble is prized for its beauty but requires more care in outdoor use due to its susceptibility to acid rain. Slate is commonly used for roofing and flooring due to its ability to split into thin sheets.
Understanding the composition and formation processes of metamorphic rocks helps explain why some, like quartzite, are exceptionally resistant to the relentless forces of weathering, making them very durable materials.
Important Questions from Miscellaneous Material
In roofing sheet terminology, CGI means -
Which of the following types of stones shows the least water absorption property?
Hydraulic lime is obtained by
Which one of the following is obtained by burning kankar or clayey lime stones ?
Consider the following statements related to the glass:
1. The addition of lime makes the glass fluid and suitable for blowing.
2. Excess of soda increases the safety.
3. Silica is used in the form of pure quartz.
Which of the above statements are correct?
