What is the Difference Between Rhyolite and Granite

Rhyolite and granite are two felsic igneous rocks. They contain many of the same minerals, with quartz, alkali feldspar (like sanidine), and plagioclase feldspar being the most common. However, there is a distinct difference between rhyolite and granite.

What is the difference between rhyolite and granite? Rhyolite has much finer crystals than granite, which has a more coarse texture.

Key Areas Covered

1. What is Rhyolite 
      – Definition, Features
2. What is Granite
      – Definition, Features
3. Similarities Between Rhyolite and Granite
      – Outline of Common Features
4. Difference Between Rhyolite and Granite
      – Comparison of Key Differences
5. FAQ: Rhyolite and Granite
      – Answers to Frequently Asked Questions

Key Terms

Rhyolite, Granite, Igneous Rock

What is Rhyolite

Rhyolite is a volcanic rock that has a high silica content. It is an extrusive igneous rock. It is usually pink or grey in color and has small grains that are difficult to observe without a hand lens. The composition of rhyolite includes quartz, plagioclase, and sanidine, with minor amounts of hornblende and biotite. There may be trapped gases inside the rock. Granitic magma that has partially cooled in the subsurface gives rise to rhyolite. A rock with two grain sizes is formed when this magma erupts. Groundmass consists of the smaller crystals formed at the surface, whereas phenocrysts are the larger crystals that form beneath the surface. Formation of rhyolite is seen in continental or continental margins and rarely in oceanic eruptions.

Rhyolite’s higher silica content makes it unsuitable for use in the construction industry. However, due to its pleasing appearance, it is often used for decorative creations.

What is Granite

Granite is a coarse-grained igneous rock that is formed deep underground. It originates from molten rock (magma) that cools and crystallises slowly. This slow cooling allows for the formation of visible mineral grains, primarily quartz, and feldspar, giving granite its characteristic speckled appearance.

Granite gets its color from other minerals like mica and amphiboles. The color of granite may vary from pink and grey to even black. Granite is strong and durable. These properties make it very useful in the construction field. It is used to create countertops, floor tiles, and monuments due to its resistance to weathering and wear and tear.

Apart from its uses in the construction industry, granite is also used to study Earth’s history. Because it forms deep within the Earth’s crust, granite’s presence often signifies the core of continents. Studying these ancient rocks helps geologists unravel the story of our planet’s formation and evolution.

Similarities Between Rhyolite and Granite

1. Rhyolite and granite are classified as felsic igneous rocks.
2. Both rhyolite and granite are formed from the cooling and solidification of magma or lava.
3. They contain many of the same minerals, with quartz, alkali feldspar (like sanidine), and plagioclase feldspar being the most common.

Difference Between Rhyolite and Granite

Definition

• Rhyolite is an extrusive igneous rock that has a high content of silica, whereas granite is a coarse-grained igneous rock that is formed deep underground.

Type of Rock

• Rhyolite is an extrusive igneous rock, whereas granite is an intrusive igneous rock.

Formation

• Rhyolite cools rapidly on the surface, resulting in a fine-grained texture, while granite cools slowly underground, resulting in a coarse-grained texture.

Weathering

• Due to its finer grain size and sometimes glassy nature, rhyolite can be more susceptible to weathering than granite.

Conclusion

In conclusion, there is a distinct difference between rhyolite and granite in terms of their texture, cooling conditions, and resistance to weathering, although both share similarities in mineral composition and formation from magma or lava. Rhyolite, with its fine-grained texture and rapid surface cooling, stands apart as a product of explosive volcanic activity, while granite’s coarse-grained structure, formed from slow underground cooling, renders it durable and resistant to weathering.

FAQ: Rhyolite and Granite 

1. What cools faster, rhyolite or granite?

Rhyolite cools rapidly on the surface, whereas granite cools slowly underground. Therefore, rhyolite cools faster than granite. This cooling gives rhyolite a fine-grained texture.

2. What is similar between granite and rhyolite?

Both rhyolite and granite are classified as igneous rocks and both rhyolite and granite are formed from the cooling and solidification of magma or lava. Moreover, they contain many of the same minerals, including quartz and feldspar.

3. Do rhyolite and granite have the same composition?

Yes, both rhyolite and granite have the same mineral composition, with minerals like quartz, alkali feldspar (like sanidine), and plagioclase feldspar. But their grain size decides its distinct appearance.

4. Why is rhyolite unique?

Rhyolite’s high silica content makes rhyolite magma extremely viscous, which favours explosive eruptions. This higher silica content in rhyolite makes it unsuitable for use in the construction industry.

5. What is more resistant to weathering, granite or rhyolite?

Granite is generally more resistant to weathering than rhyolite. This is due to granite being a coarse-grained, hard, and dense igneous rock.

Reference:

1. “Granite.” Encyclopedia Britannica.
2. “Rhyolite.” Wikipedia. Wikipedia Foundation.

Image Courtesy:

1. “Granite (Cape Ann Complex, Late Ordovician to Late Silurian, 426-450 Ma; Rockport, Massachusetts, USA) 2” By James St. John –  (CC BY 2.0) via Commons Wikimedia
2. “PinkRhyolite” By Michael C. Rygel – Own work (CC BY-SA 3.0) via Commons Wikimedia