Igneous Rock Types by Silica Content

Igneous rocks are formed from the cooling and solidification of molten rock (magma or lava). They are broadly classified based on their chemical composition, particularly their silica ($\text{SiO}_2$) content. This classification helps geologists understand the source and formation conditions of the rocks.

The main classifications based on silica content are:

• Acidic (Felsic) Rocks: High silica content (> 63% $\text{SiO}_2$). Rich in minerals like quartz, feldspar, and mica. Light in color. Examples: Granite (intrusive), Rhyolite (extrusive).
• Intermediate Rocks: Silica content between 52% and 63% $\text{SiO}_2$. Contain a mix of felsic and mafic minerals. Examples: Diorite (intrusive), Andesite (extrusive).
• Basic (Mafic) Rocks: Silica content between 45% and 52% $\text{SiO}_2$. Rich in minerals like pyroxene, olivine, and plagioclase feldspar. Dark in color. Examples: Gabbro (intrusive), Basalt, Dolerite (extrusive/hypabyssal).
• Ultrabasic (Ultramafic) Rocks: Very low silica content (< 45% $\text{SiO}_2$). Dominated by minerals like olivine and pyroxene. Very dark color. Examples: Peridotite (intrusive).

Identifying Acid Igneous Rock Options

Let's look at the provided options and classify them based on their typical silica content:

• Dolerite: Dolerite is a fine-grained, dark-colored intrusive or hypabyssal igneous rock. It is chemically equivalent to basalt or gabbro. Dolerite is classified as a basic igneous rock, with silica content typically between 45% and 52%.
• Rhyolite: Rhyolite is a fine-grained, extrusive volcanic rock with a composition similar to granite. It is typically light in color. Rhyolite is classified as an acidic (felsic) igneous rock, with silica content typically greater than 69% $\text{SiO}_2$.
• Peridotite: Peridotite is a coarse-grained intrusive rock that is the main constituent of the Earth's upper mantle. It is dominated by olivine and pyroxene. Peridotite is classified as an ultrabasic (ultramafic) igneous rock, with very low silica content, typically less than 45%.
• Gabbro: Gabbro is a coarse-grained intrusive igneous rock with a composition similar to basalt. It is typically dark-colored. Gabbro is classified as a basic (mafic) igneous rock, with silica content typically between 45% and 52%.

Based on this classification, Rhyolite is the only acid type igneous rock among the given options.

Summary of Rock Types:

Therefore, Rhyolite is the acid igneous rock from the provided list.