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An in-depth exploration of the processes involved in the formation of igneous rocks, including magma creation, crystallization, and classification based on texture and composition. the different types of igneous rocks, such as volcanic and plutonic, and their respective subcategories, as well as the economic uses and landscapes where they occur.
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Typology: Exercises
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Igneous rocks are initially classified based on where they were formed (See Figure 4.2 on page 69).
Volcanic Rocks – Rocks originating from the eruption of molten material at Earth’s surface. These rocks are also referred to as extrusive rocks.
Pyroclastic materials are materials that are explosively ejected from an erupting volcano.
Plutonic Rocks – Igneous rocks that solidify below the surface of the Earth. These rocks are also referred to as intrusive rocks.
Oxides of aluminum (Al), magnesium (Mg), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and other elements in trace concentrations may also be present in magma.
Based on the decreasing abundance of silica in the magma or in the rock-forming minerals, both magma and igneous rock compositions are further categorized as:
Ultramafic rocks are composed almost entirely of olivine and pyroxene. Mafic rocks also contain Ca-rich plagioclase feldspar.
These rocks tend to be dark gray to black in color, because the dark Mg- and Fe-rich minerals are more prevalent than the light-colored feldspar.
Crystal size is an important component of rock texture. The crystal size in igneous rocks is dependent on many factors; however, the cooling rate of the magma is generally the most important factor.
When magma cools quickly, the resulting rock consists of very small crystals which generally cannot be seen by the naked eye (Figure 4.2b, page 69). This fine-grained texture is called aphanitic (Figure 4.3, page 70).
In some cases, magma begins to crystallize slowly at depth, producing large crystals. Subsequent movement into colder environments at or near the surface results in rapid crystallization of the remaining melt.
The resulting igneous rock consists of large crystals surrounded by smaller crystals (see Figure 4.2d, page 69). The texture of such a rock is called porphyritic.
A special approach is necessary to classify the pyroclastic materials erupted from volcanoes, since most of them have glassy textures. Pyroclastic materials include pumice, ash, and any other erupted materials.
As a result, we distinguish these by the size of the fragments (see Figure 4.4 and Table 4.1 on page 72).
The classification scheme for igneous rocks combines both composition and structure (see Figure 4.3, page 70).
The oceanic crust is mostly composed of basalt and gabbro.
Pyroclastic deposits are named on the basis of fragment size and whether the fragments are loose or consolidated into rock (Table 4.1, page 72). Compositional terms may be used as modifiers to complete the descriptive name of pyroclastic igneous rocks.
Igneous rocks have a wide variety of uses in construction and industry. These uses include:
Extrusive and intrusive processes are revealed at the Earth’s surface by:
Most volcanoes are found at or near divergent and convergent plate boundaries; only a few occur within plates.
Igneous activity today occurs in active tectonic settings; therefore, geologists infer that ancient volcanic rocks and their intrusive equivalents formed in active tectonic regions in the past.
Volcanic necks also form close to the surface. Unlike dikes, which form when volcanic eruptions occur along elongate fissures, volcanoes may also be fed by nearly cylindrical pipes. When erosion eventually exposes the rocks filling the pipes, a volcanic neck is formed (see Figure 4.8, page 75).
