Question

Use your understanding of magmatic differentiation to explain how magmas of different composition can be generated in a cooling magma chamber.

Short answer

Different minerals crystallize and settle at different stages, changing the magma's composition and producing diverse igneous rocks.

Step-by-step solution

Understanding Magmatic Differentiation

Magmatic differentiation refers to the process by which a single magma can produce rocks of varying composition. This occurs as different minerals crystallize from the melt at different temperatures.

Fractional Crystallization

As a magma cools, minerals with high melting points crystallize first. These minerals typically include olivine and pyroxene, which are rich in iron and magnesium. As they form, these minerals are removed from the remaining melt, altering its composition.

Changing Composition of the Residual Melt

With the removal of early-forming minerals, the magma becomes progressively richer in silica and lighter elements such as sodium and potassium. This process leads to the formation of intermediate and then felsic magmas from an originally mafic source.

Role of Crystal Settling

As crystals form, they can settle to the bottom of the magma chamber due to gravity. This further removes them from the melt, enhancing the differentiation process. This separation allows the overlying melt to evolve and become more silica-rich.

End Result

The end result is a zoned magma chamber, where the composition varies from dense, mafic material at the bottom to lighter, felsic material at the top. This layered structure can lead to a diverse range of igneous rocks once the magma solidifies.

Key concepts

Crystalline Fractionation

Crystalline fractionation is a fascinating process in geological science, where a cooling magma differentiates to form distinct rock compositions. As magma cools, minerals do not solidify all at once. Instead, they crystallize at different temperatures depending on their chemical structure. This means that as the temperature drops, certain minerals solidify first and separate from the liquid magma.

For instance, minerals like olivine and pyroxene, which are rich in iron and magnesium, solidify at higher temperatures. This process is known as fractional crystallization. These early-formed minerals remove specific elements from the melt, altering its chemical composition.

Importantly, as these crystals form, they may settle to the bottom of the magma chamber. This gravitational settling enhances differentiation by removing these crystals from the remaining melt, thus preventing them from potentially re-dissolving. This separation is key to forming a diversity of rocks in a single magma chamber.

Silica-Enrichment

Silica-enrichment is a crucial outcome of magmatic differentiation, and it begins once early-forming minerals crystallize and are removed from the melt. As these minerals crystallize, elements that like to form silica-rich minerals, such as silicon, sodium, and potassium, remain in the liquid magma.

This gradual process transforms initially mafic magmas, which are rich in iron and magnesium, into felsic magmas rich in silica. The enrichment in silica and lighter elements drives the melt's evolution towards end-member compositions like rhyolite, which is markedly different from the original magma's composition.

Silica-enrichment is significant because it influences not only the chemical characteristics of the resulting igneous rocks but also their physical properties, including color, density, and melting points. The differentiation of magma through silica-enrichment can therefore lead to a wide array of geological formations with varied characteristics.

Mineral Crystallization

Mineral crystallization is the cornerstone of magmatic differentiation and significantly contributes to the evolution of magma compositions. As magma cools, its temperature decreases until it reaches a point where minerals begin to crystallize.

Each mineral has a specific temperature at which it forms from the liquid magma, dependent on its composition. In a typical cooling sequence, minerals like olivine and pyroxene crystallize first, followed by amphibole, biotite, and eventually quartz and feldspar as the temperature continues to drop.

These crystallization steps are characterized by the removal of elements from the melt, which changes the remaining magma's composition. This process can lead to the formation of various igneous rock types and the stratification of different mineral layers within the magma chamber. Mineral crystallization is essential for predicting the kinds of rocks that will form once the magma solidifies completely.

Magma Evolution

Magma evolution is an intricate process describing how the composition of magma changes over time due to various factors, including temperature variations, mineral crystallization, and crystal settling.

Initially, a magma chamber may have a uniform composition, but as minerals begin to crystallize out and are removed, the leftover melt becomes progressively more evolved. This is largely determined by the temperatures at which various minerals form and precipitate out of the liquid magma.

The removal of early-formed minerals, such as olivine, modifies the magma's original composition, leading to a transformation from a mafic to an intermediate and eventually felsic composition. Magma evolution is essential in understanding Earth's geological history because it accounts for the diversity of igneous rocks and their complex structures. Understanding this evolution helps geologists predict volcanic behavior and the nature of potential eruptions.