Question

Why is compositional layering a common feature of mafic plutons but not of felsic plutons?

Short answer

Compositional layering is more common in mafic plutons than in felsic plutons due to the differences in magma viscosity and mineral density. Mafic magma's lower viscosity allows for efficient mineral movement and mixing, with dense minerals settling to the bottom of the chamber, leading to distinct compositional layers. In contrast, felsic magma's higher viscosity results in a more homogeneous distribution of minerals, making compositional layering less likely to occur.

Step-by-step solution

Understand the composition of mafic and felsic plutons

Mafic plutons are formed from relatively low-viscosity magma rich in magnesium and iron, which typically crystallize into minerals like pyroxene, olivine, and plagioclase feldspar. Felsic plutons, on the other hand, are formed from magma with a high silica content and are rich in minerals like quartz, alkali feldspar, and biotite or muscovite mica.

Basics of Compositional Layering

Compositional layering in plutons occurs when different mineral compositions precipitate out of magma, forming distinct layers. This can be a result of processes like gravity settling, where denser minerals sink to the bottom of the magma chamber or convection currents within the cooling magma.

Mafic Plutons and Compositional Layering

In mafic plutons, the minerals that form are generally denser and have higher melting points than those in a felsic pluton. Since mafic magma has lower viscosity, it allows for efficient movement and mixing of minerals, such that denser minerals can easily settle to the bottom of the chamber. This leads to the formation of distinct compositional layers due to gravity settling and convection.

Felsic Plutons and Lack of Compositional Layering

In contrast, felsic magma has a higher viscosity, making it more difficult for minerals to move and mix within the magma chamber. This results in a more homogeneous distribution of minerals throughout the cooling magma, rather than forming distinct layers. Additionally, since felsic minerals are generally less dense and have lower melting points, there may be less differentiation in density and melting temperature between minerals, making gravity settling less likely to produce compositional layering. In conclusion, compositional layering is more common in mafic plutons due to the low magma viscosity allowing for efficient mineral movement and mixing, as well as their dense minerals settling to the bottom of the magma chamber. In felsic plutons, the high magma viscosity and less density differentiation between minerals make compositional layering less likely to occur.

Key concepts

Mafic Plutons

Mafic plutons are fascinating geological features formed from magma that is rich in magnesium and iron. This magma crystallizes into minerals such as pyroxene, olivine, and plagioclase feldspar.

These minerals are generally denser and have higher melting points compared to those in felsic counterparts. The formation of mafic plutons is interesting because of their tendency to exhibit compositional layering.

This occurs when minerals settle into distinct layers as the magma cools and solidifies. The low viscosity of mafic magma plays a pivotal role here. It allows minerals to move more freely. As a result, denser minerals can settle at the bottom, forming layers.

Felsic Plutons

Felsic plutons differ from mafic ones due to their high silicon content. This leads to the formation of minerals like quartz, alkali feldspar, and micas such as biotite.

With higher viscosity, felsic magma is more resistant to flow compared to its mafic counterpart. This has a significant impact on the internal structure of felsic plutons. High viscosity prevents minerals from moving freely, resulting in a more uniform distribution of minerals. Thus, felsic plutons rarely exhibit compositional layering. The minerals crystallize more uniformly throughout the magma as it cools. Additionally, the generally lower density and melting point differences among felsic minerals further discourage the formation of distinct mineral layers.

Mineral Composition

Mineral composition is key to understanding the differences between mafic and felsic plutons. Each has a unique set of minerals that form during the cooling process.

- **Mafic Plutons**: Predominantly feature minerals such as pyroxene, olivine, and plagioclase feldspar; which are dense and formed at higher temperatures. - **Felsic Plutons**: Composed of minerals like quartz, alkali feldspar, and mica such as biotite or muscovite. These differences in mineral composition influence not just the physical properties, but also the way these plutons develop internal layering. In mafic plutons, the density and melting point of these minerals aid in creating compositional layers as they settle.

In contrast, the minerals in felsic plutons do not exhibit such significant density and melting point variations, hence layer formation is minimal.

Magma Viscosity

The viscosity of magma is a major factor in determining the structure and layering within plutons. Viscosity is essentially the resistance of the magma to flow. - **Mafic Magma**: Has low viscosity, mainly due to lower silica content. This allows minerals to flow relatively freely and settle into distinct layers in the magma chamber. - **Felsic Magma**: Exhibits high viscosity due to its high silica content. This results in restricted movement of minerals, leading to a more uniform mineral distribution. Viscosity influences not only the movement of individual mineral grains but also the overall behavior of the cooling magma. In mafic plutons, the low viscosity supports dynamic processes like convection currents and gravity settling of minerals.

This contributes to the rich, layered textures often observed. Meanwhile, the high viscosity in felsic magma inhibits these processes, leading to a more homogeneous structure.