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Igneous Rock: Felsic and Mafic Help

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By — McGraw-Hill Professional
Updated on Aug 31, 2011

Introduction to Felsic and Mafic Rock

Igneous rocks are divided into two types depending on composition: felsic and mafic . Felsic rock is affected by heat, either from magma coming to the surface from extreme depths in the Earth or by the friction between continental plates. Although igneous rock contains some combined rock like the deep continental plate rock melted by moving magma, overall igneous rocks are either felsic or mafic.

Felsic

The first igneous rock type, felsic , is made up of light-colored igneous rocks that have high levels of silica-containing minerals like quartz and feldspar . Plagioclase feldspar that is higher in calcium crystallizes at higher temperatures than plagioclase having higher levels of sodium. When a rock is formed by different minerals, it tends to melt at a temperature below that of any one contributing mineral. This happens because different elements have different chemical properties that have an effect on their melting points.

Granite and granodiorites are the best known and most frequently seen intrusive igneous rocks containing about 70% silica. These mostly light-colored quartz and orthoclase feldspar minerals are found only in the continental crust. When different minerals like quartz and feldspar mix with granite, it takes on its well-known gray or pink color.

Felsic rock has high levels of silica in its composition. The name felsic comes from a combination of the words feldspar and silica.

Rhyolite has the same composition as granite, but it is an extrusive igneous rock. It has a pale brown to gray color and is finely grained. The majority of rhyolites are made up of volcanic glass with no obvious crystals. They are much less common and found in much smaller pockets of extrusive igneous rock than their intrusive cousins.

Intermediate Igneous Rock

The volcanic igneous rocks in the intermediate class, that are a lot like granodiorite, are dacite and andesite .

Granodiorite has a lot of quartz like granite, but not a lot of silica. Diorite , the phaneritic cousin of andesite, contains feldspar and a lot of other mafic minerals like biotite and pyroxene that give it a darker color. Diorite is in between the granites and gabbro in composition and properties.

A transition rock type between the rhyolitic and basalitic magmas is an intermediate igneous rock like andesite . This type doesn’t contain particularly high or low levels of silica, but is pretty average. Andesite is named after the volcanic Andes Mountains of South America and is made up of plagioclase and amphibole. Andesites and diorites are both equally common.

Mafic Rock

Mafic rock contains high levels of magnesium and ferric (iron-containing) minerals. The word mafic comes from a combination of these two mineral names.

Rocks which are low in silica, but high in magnesium and iron, form dark-colored mafic rocks like pyroxenes and olivines. Their main minerals are calcium-rich feldspar and calcium- and magnesium-rich pyroxenes.

The Earth’s ocean floor is made up of basalt. Mafic rock contains only about 50% silica by weight. It is commonly dark gray, but can be green, brown, or black.

Figure 6-3 lists the main minerals found in felsic and mafic rock according to their composition.

Igneous Rock

 

Fig. 6-3. Felsic and mafic rock have different amounts of silica and magnesium.

When geologists looked closer at these two rock types, they found rocks that looked very different, but had the same composition. Some geologists thought that texture differences must be related to the way magma crystallized and then reached the surface, either by slow boat or by rocket ship.

Practice problems of this concept can be found at: Igneous Rock Practice Test

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