Understanding Igneous Rocks and Silica Content

Igneous rocks are formed through the cooling and solidification of magma or lava. These rocks are classified based on their chemical composition, texture, and mode of formation. One common way to classify igneous rocks is based on their silica (SiO₂) content.

Classifying Igneous Rocks by Silica Content

Geologists categorize igneous rocks into different groups based on the percentage of silica they contain. The main categories are:

• Felsic: Rocks with high silica content, typically greater than 65%. These rocks are usually light in color.
• Intermediate: Rocks with silica content between 52% and 65%.
• Mafic: Rocks with low silica content, typically between 45% and 52%. These rocks are usually dark in color.
• Ultramafic: Rocks with very low silica content, typically less than 45%. These are also very dark and dense.

Analyzing the Given Igneous Rocks

Let's look at the typical silica content of the igneous rocks listed in the options:

• Basalt: Basalt is a common extrusive (volcanic) mafic rock. Its silica content is generally in the range of 45% to 52%.
• Granite: Granite is a common intrusive (plutonic) felsic rock. It is characterized by a high silica content, typically exceeding 65% and often ranging from 68% to 77%.
• Periodotite: Periodotite is an ultramafic intrusive rock. It has very low silica content, usually less than 45%, and is rich in minerals like olivine and pyroxene.
• Gabbro: Gabbro is a mafic intrusive rock, which is the intrusive equivalent of basalt. Its silica content is similar to basalt, ranging from about 45% to 52%.

Comparing the silica content ranges, we need to find the rock that contains silica in excess of 65%.

• Basalt: $\text{<} 65\%$
• Granite: $\text{>} 65\%$
• Periodotite: $\text{<} 65\%$
• Gabbro: $\text{<} 65\%$

Based on this comparison, Granite is the igneous rock that typically contains silica in excess of 65%.

Conclusion

Igneous rocks are classified by their silica content, among other factors. Rocks with over 65% silica are termed felsic. Out of the given options, Granite is a felsic rock known for its high silica content, which is typically well above 65%.