The Dehydration Effect: How Water Loss Drives Crystallization in Felsic Magmas

As felsic magma rises through the Earth’s crust, the reduction in pressure causes water to be released from the magma. This loss of water as the magma rises has a significant effect on its crystallization behavior. As the water content decreases, the melting point of the magma increases, leading to the onset of crystallization.

The effect of water loss on felsic magma crystallization

Water loss from a felsic magma promotes the crystallization of certain mineral phases. As the water content decreases, the magma becomes supersaturated with certain minerals, which then begin to crystallize out of the melt. This process is known as fractional crystallization and can lead to the formation of distinct mineral assemblages in the resulting igneous rocks.

One of the most important effects of water loss on felsic magma crystallization is the increased formation of quartz and alkali feldspar. These minerals are typically the first to crystallize from the magma because they are less soluble in the increasingly dehydrated melt. The crystallization of quartz and alkali feldspar can result in the formation of coarse-grained, intrusive igneous rocks, such as granite, or finer-grained, extrusive igneous rocks, such as rhyolite.

Implications for igneous rock formation and geologic processes

The loss of water from felsic magmas and the resulting changes in crystallization behavior have significant implications for igneous rock formation and the broader understanding of geologic processes. The composition and texture of the resulting igneous rocks can provide valuable insights into the evolution of the Earth’s crust and the processes that shape the planet’s surface.

In addition, the crystallization of felsic magmas can influence the distribution and concentration of certain elements, including economically important mineral resources. Understanding the role of water in felsic magma crystallization is critical to unraveling the complex geologic history of a region and to exploring the potential for economically viable mineral deposits.

In summary, the loss of water from a felsic magma is a critical factor in promoting the crystallization of specific mineral phases, leading to the formation of different igneous rock types and influencing broader geological processes. This understanding is essential for interpreting the Earth’s geologic record and informing our knowledge of the dynamic evolution of the planet.

FAQs

Here are 5-7 questions and answers about why the loss of water from a felsic magma encourages crystallization:

Why does the loss of water from a felsic magma encourage crystallisation?

The loss of water from a felsic magma encourages crystallization because water acts as a flux, lowering the melting point of the magma. As water is lost, the melting point rises, making it more likely for the minerals in the magma to crystallize out. The loss of water also increases the viscosity of the magma, further promoting crystallization.

How does the composition of a felsic magma impact its crystallization behavior?

Felsic magmas, which are high in silica and alkali content, tend to be more viscous and have a higher melting point compared to mafic magmas. This composition makes felsic magmas more prone to crystallization, especially as water is lost. The presence of minerals like quartz, feldspar, and mica in felsic magmas also encourages them to crystallize more readily than the less viscous, lower melting point mafic magmas.

What is the role of volatile elements in the crystallization of felsic magmas?

Volatile elements like water, carbon dioxide, and sulfur play a crucial role in the crystallization of felsic magmas. These volatile elements act as fluxes, lowering the melting point of the magma. As the volatile elements are lost, either through eruption or by diffusion, the melting point rises, and crystallization is encouraged. The loss of volatiles also increases the viscosity of the magma, further promoting crystal formation.

How does the crystallization of felsic magmas differ from mafic magmas?

Felsic magmas, which are higher in silica content, tend to crystallize more slowly and at lower temperatures compared to mafic magmas. This is due to the higher viscosity and melting point of felsic magmas. As a result, felsic magmas often exhibit more complex crystal assemblages and textures, with minerals like quartz, feldspar, and mica forming first, followed by other minerals. In contrast, mafic magmas typically crystallize more rapidly and at higher temperatures, with minerals like olivine and pyroxene forming first.

What are the implications of the loss of water on the eruption of felsic magmas?

The loss of water from felsic magmas can have significant implications for their eruption behavior. As water is lost, the magma becomes more viscous and crystalline, which can make it more likely to erupt explosively. The increased viscosity and crystal content can also lead to the development of highly viscous, slow-moving lava flows or domes. Additionally, the loss of water can cause the magma to become more buoyant, potentially leading to more violent eruptions as the magma rises more rapidly through the crust.