How does mafic magma become felsic

Chemical composition by mass for four rock samples are shown in the following table. Compare these with those in the table above to determine whether each of these samples is felsic, intermediate, or mafic.

In partial melting, some components of a mixture melt before others do. In the case of mafic magma, it is produced when ultramafic rocks undergo partial melting.

In general, silicate minerals with more silica will melt before those with less silica. This means the partial melt will have more silica than the rock as a whole. A number of processes that take place within a magma chamber can affect the types of rocks that form once magma cools and crystallizes.

If the magma has a low viscosity— which is likely if the magma is mafic—the crystals that form early, such as olivine Figure 7. This process is called fractional crystallization. The formation of olivine removes iron- and magnesium-rich components, leaving the overall composition of the magma near the top of the magma chamber more felsic. The crystals that settle might either form an olivine-rich layer near the bottom of the magma chamber.

Or, because the lower part of the magma chamber is likely to be hotter than the upper part, the crystals might remelt. If remelting happens, crystal settling will make the magma at the bottom of the chamber more mafic than it was to begin with Figure 7. If the rock in which the magma chamber is located called the country rock is more felsic than the magma, the country rock may also melt, adding to the magma already in the magma chamber Figure 7.

Sometimes magma carries fragments of unmelted rock, called xenoliths , within it. Melting of xenoliths can also alter the composition of magma, as can re-melting of crystals that have settled out of the magma. Skip to content The minerals that make up igneous rocks crystallize solidify, freeze at a range of different temperatures. Figure 7. Understanding how the reaction series was derived is key to understanding what it means.

Click the image for source and terms of use. Continuous Series At about the point where pyroxene begins to crystallize, plagioclase feldspar also begins to crystallize. This explains why a cooling magma can have some crystals within it and yet remain predominantly liquid.

The sequence in which minerals crystallize from a magma is known as the Bowen reaction series Figure 3. As the temperature drops, and assuming that some silica remains in the magma, the olivine crystals react combine with some of the silica in the magma see Box 3.

As long as there is silica remaining and the rate of cooling is slow, this process continues down the discontinuous branch: olivine to pyroxene, pyroxene to amphibole, and under the right conditions amphibole to biotite.

At about the point where pyroxene begins to crystallize, plagioclase feldspar also begins to crystallize. At that temperature, the plagioclase is calcium-rich anorthite see Figure 2. As the temperature drops, and providing that there is sodium left in the magma, the plagioclase that forms is a more sodium-rich variety.

In , he joined the Carnegie Institution in Washington, D. Working mostly with basaltic magmas, he determined the order of crystallization of minerals as the temperature drops. The method, in brief, was to melt the rock to a magma in a specially made kiln, allow it to cool slowly to a specific temperature allowing some minerals to form , and then quench it cool it quickly so that no new minerals form only glass.

The results were studied under the microscope and by chemical analysis. This was done over and over, each time allowing the magma to cool to a lower temperature before quenching. The Bowen reaction series is one of the results of his work, and even a century later, it is an important basis for our understanding of igneous rocks.

The word reaction is critical. As the temperature continues to drop, olivine becomes unstable while pyroxene becomes stable. The early-forming olivine crystals react with silica in the remaining liquid magma and are converted into pyroxene, something like this:. This continues down the chain, as long as there is still silica left in the liquid.

In cases where cooling happens relatively quickly, individual plagioclase crystals can be zoned from calcium-rich in the centre to more sodium-rich around the outside. This occurs when calcium-rich early-forming plagioclase crystals become coated with progressively more sodium-rich plagioclase as the magma cools.

Common rock-forming mafic minerals include olivine, pyroxene, amphibole, biotite mica, and the plagioclase feldspars. Mafic magmas are usually produced at spreading centers, and represent material which is newly differentiated from the upper mantle.

Common mafic rocks include basalt and gabbro. Please note that some geologists with questionable motives switch the order of the magnesium and iron and come up with the term "femag. Felsic, on the other hand, is used for silicate minerals, magmas, and rocks which have a lower percentage of the heavier elements, and are correspondingly enriched in the lighter elements, such as silicon and oxygen , aluminum, and potassium.