Mafic vs. Felsic: A Comprehensive Comparison

Definition

Mafic Rocks: The term “mafic” is derived from “magnesium” and “ferric” (iron), reflecting the high concentrations of these elements in mafic rocks. Mafic rocks are rich in ferromagnesian minerals such as olivine, pyroxene, amphibole, and biotite. They contain a lower percentage of silica (45-55%), which contributes to their characteristic dark color. Examples of mafic rocks include basalt, gabbro, and diabase.

Felsic Rocks: The term “felsic” combines “feldspar” and “silica,” indicating the high silica content in these rocks. Felsic rocks are rich in silica (over 65%) and contain minerals such as quartz, potassium feldspar, and plagioclase feldspar. These rocks also have higher concentrations of lighter elements like aluminum, potassium, and sodium, which gives them a lighter color. Granite, rhyolite, and dacite are common examples of felsic rocks.

The classification of igneous rocks into mafic and felsic categories is foundational in geology, helping to explain the processes that shape Earth’s crust. These two types of rocks are distinguished by their mineral composition, origin, physical properties, and the tectonic environments in which they form. Here’s a closer look at each aspect of mafic and felsic rocks.

1. Mineral Composition

• Mafic Rocks
  • Key Minerals: Mafic rocks are primarily composed of dark-colored, iron- and magnesium-rich minerals. These include:
  • Olivine: A greenish mineral common in basalt and gabbro, typically formed at high temperatures.
  • Pyroxene: A group of silicate minerals that are dark in color and rich in iron and magnesium.
  • Amphibole: Another dark mineral found in many mafic rocks, contributing to their overall dark color.
  • Biotite: A dark mica mineral present in some mafic rocks.
  • Silica Content: Mafic rocks have a lower silica content (45-55%), which makes them denser and darker.
• Felsic Rocks
  • Key Minerals: Felsic rocks are dominated by light-colored minerals rich in silica and aluminum. These include:
  • Quartz: A hard, crystalline mineral that is highly abundant in felsic rocks like granite.
  • Feldspar: The most common mineral group in Earth’s crust, including both potassium feldspar (orthoclase) and plagioclase feldspar, which are key components of felsic rocks.
  • Muscovite: A light-colored mica found in many felsic rocks.
  • Minor Minerals: Felsic rocks may also contain minerals like zircon, apatite, and sphene, which are often present in small amounts.
  • Silica Content: Felsic rocks have a high silica content (over 65%), contributing to their lighter color and lower density.

2. Formation Processes

• Mafic Rocks
  • Mantle-Derived Magma: Mafic rocks typically form from magma that originates in the Earth’s mantle. This magma is generated through partial melting of mantle peridotite, which is rich in magnesium and iron.
  • Tectonic Settings:
  • Mid-Ocean Ridges: At divergent plate boundaries, such as mid-ocean ridges, mafic magma rises to form new oceanic crust. Basalt, a common mafic rock, is the primary rock type found at these ridges.
  • Hotspots: Intraplate hotspots, like those that created the Hawaiian Islands, also produce mafic magma, which forms extensive basaltic lava flows.
  • Subduction Zones: While less common, mafic rocks can also form in subduction zones where mantle material is partially melted.
• Felsic Rocks
  • Crustal Melting: Felsic rocks form from magma that originates primarily from the partial melting of continental crust, which is rich in silica and aluminum.
  • Tectonic Settings:
  • Convergent Boundaries: Felsic rocks are often associated with convergent plate boundaries, where continental crust is thickened and partially melted. This process can create large plutonic bodies of granite, as well as explosive volcanic eruptions that produce rhyolite.
  • Continental Rifts: In some cases, felsic magmas can form in continental rift settings where the crust is extended and melted.
  • Post-Collisional Settings: After major continental collisions, the thickened crust can partially melt to produce felsic magmas that intrude as granite batholiths.

3. Physical Properties

• Mafic Rocks
  • Color: The dark color of mafic rocks (black, green, or dark gray) is due to the high content of ferromagnesian minerals like olivine and pyroxene.
  • Density: With densities typically between 3.0 and 3.3 g/cm³, mafic rocks are heavier than their felsic counterparts. This higher density is a result of the high iron and magnesium content.
  • Viscosity: Mafic magma has low viscosity, meaning it flows easily. This results in less explosive volcanic eruptions and the formation of broad shield volcanoes, like those found in Hawaii.
• Felsic Rocks
  • Color: Felsic rocks are generally light-colored (white, pink, light gray) due to their high quartz and feldspar content.
  • Density: The density of felsic rocks is lower, typically between 2.6 and 2.9 g/cm³, because they contain more silica and lighter elements like potassium and sodium.
  • Viscosity: Felsic magma is highly viscous, meaning it does not flow easily. This can lead to the buildup of pressure and explosive eruptions, as seen in stratovolcanoes like Mount St. Helens.

4. Geological Structures and Landforms

• Mafic Rocks
  • Oceanic Crust: The Earth’s oceanic crust is predominantly composed of mafic rocks, especially basalt. The continuous formation of basalt at mid-ocean ridges is a key process in the creation of new ocean floor.
  • Volcanic Islands: Many volcanic islands, such as those in the Hawaiian and Icelandic archipelagos, are primarily composed of mafic basaltic rock.
  • Lava Plateaus: Large mafic lava flows can create extensive plateaus, such as the Deccan Traps in India, which are composed of basalt.
• Felsic Rocks
  • Continental Crust: The Earth’s continental crust is largely composed of felsic rocks, particularly granite. These rocks form the cores of many mountain ranges and continental shields.
  • Mountain Ranges: Major mountain ranges, like the Himalayas and the Sierra Nevada, contain large granite intrusions formed from felsic magmas.
  • Calderas: Felsic volcanic activity can create large calderas, such as Yellowstone, which result from the collapse of land following a massive eruption.

5. Examples of Famous Locations

• Mafic Rocks
  • Hawaiian Volcanoes: The Hawaiian Islands are an iconic example of mafic volcanism, with their vast basaltic shield volcanoes.
  • Iceland: Iceland, located on the Mid-Atlantic Ridge, is another prominent location where mafic rocks dominate, particularly in its numerous volcanic formations.
• Felsic Rocks
  • Yellowstone National Park: Known for its explosive volcanic history, Yellowstone is underlain by rhyolitic magma, a typical felsic rock.
  • Sierra Nevada Batholith: This large batholith, composed mainly of granite, is an excellent example of felsic intrusions in a mountain range.

Conclusion

Mafic and felsic rocks represent the extremes of igneous rock compositions, each playing a crucial role in the Earth’s geology. Mafic rocks, with their dark color, higher density, and formation in oceanic settings, contrast with the lighter, less dense felsic rocks that form much of the continental crust. Understanding these differences allows geologists to interpret the processes that have shaped our planet’s surface over billions of years, from the creation of new oceanic crust to the building of towering mountain ranges. Whether you’re exploring the basaltic landscapes of a volcanic island or the granite peaks of a mountain range, the concepts of mafic and felsic rocks are fundamental to understanding the Earth.