Which term describes a dark-colored igneous rock rich in magnesium and iron?

Which term describes a dark-colored igneous rock rich in magnesium and iron?

• A. Mafic
• B. Felsic
• C. Silicates
• D. Phaneritic

Answer: (A)

The main idea is how igneous rocks are described by their composition and color. A dark-colored rock that is rich in magnesium and iron is labeled mafic. The high content of ferromagnesian minerals like olivine and pyroxene gives the rock its dark color and dense feel, and it also means the rock has relatively lower silica compared to felsic rocks. Mafic rocks are commonly basalt and gabbro, which helps you connect the term to real examples. Felsic, by contrast, refers to light-colored rocks with high silica and light minerals like quartz and feldspar, so it doesn’t fit a dark, Mg-Fe rich rock. Silicates describe a broad mineral group that makes up most rocks but doesn’t specify this rock’s color or Mg-Fe richness. Phaneritic describes a texture (coarse-grained appearance from slow cooling) rather than composition or color, so it isn’t the right match here.

The main idea is how igneous rocks are described by their composition and color. A dark-colored rock that is rich in magnesium and iron is labeled mafic. The high content of ferromagnesian minerals like olivine and pyroxene gives the rock its dark color and dense feel, and it also means the rock has relatively lower silica compared to felsic rocks. Mafic rocks are commonly basalt and gabbro, which helps you connect the term to real examples.

Felsic, by contrast, refers to light-colored rocks with high silica and light minerals like quartz and feldspar, so it doesn’t fit a dark, Mg-Fe rich rock. Silicates describe a broad mineral group that makes up most rocks but doesn’t specify this rock’s color or Mg-Fe richness. Phaneritic describes a texture (coarse-grained appearance from slow cooling) rather than composition or color, so it isn’t the right match here.