Batuan beku (Igneous Rocks)

Keywords

Magma                                   igneous rock                           extrusive igneous rock

volcanic rock                          intrusive igneous rock           plutonic rocks

texture                                    porphyry                                phenocryst

obsidian                                  felsic rock                               mafic rock

ultramafic rock                      intermediate rock                  basalt plateau 68

The Origins Of Magma

If you drilled a well deep into the crust, you would find that Earth temperature rises about 30ºC for every kilometer of depth. Below the crust, temperature continues to rise, but not as rapidly. In the asthenosphere (between depths of about 100 to 350 kilometers), the  temperature is so high that rocks melt in certain environments to form magma.

Figure 4–6 The lower box shows that increasing temperature, addition of water, and decreasing pressure all melt rock to form magma. The upper box shows that cooling, increasing pressure, and water loss all solidify magma to form an igneous rock.

CHARACTERISTICS OF MAGMA

Temperature

The temperature of magma varies from about 600º to 1400ºC, depending on its chemical composition and the depth at which it forms. Generally, basaltic magma forms at great depth and has a temperature near the high end of this scale. Granitic magmas, which form at shallower depths, tend to lie near the cooler end of the scale. As a comparison, an iron bar turns red hot at about 600ºC and melts at slightly over 1500ºC.

Chemical Composition

Because oxygen and silicon are the two most abundant elements in the crust and mantle, nearly all magmas are silicate magmas. In addition to oxygen and silicon, they also contain lesser amounts of the six other common elements of the Earth’s crust: aluminum, iron, magnesium, calcium, potassium, and sodium. The main variations among different types of magmas are differences in the relative proportions of these eight elements. For example, basaltic magma contains more iron and magnesium than granitic magma, but granitic magma is richer in silicon, potassium, and sodium. A few rare magmas are of carbonate composition. The rocks that form from these are called carbonatites and contain carbonate minerals such as calcite and dolomite.

Behavior

When a silicate rock melts, the resulting magma expands by about 10 percent. It is then of lower density than the rock around it, so magma rises as it forms—much as a hot air balloon ascends in the atmosphere. When magma rises, it enters the cooler, lower-pressure environment near the Earth’s surface. When temperature and pressure drop sufficiently, it solidifies to form solid igneous rock.

CLASSIFICATION OF IGNEOUS ROCKS

Magma can either rise all the way through the crust to erupt onto the Earth’s surface, or it can solidify within the crust. An extrusive igneous rock forms when magma erupts and solidifies on the Earth’s surface. Because extrusive rocks are so commonly associated with volcanoes, they are also called volcanic rocks after Vulcan, the Greek god of fire. An intrusive igneous rock forms when magma solidifies within the crust. Intrusive rocks are sometimes called plutonic rocks after Pluto, the Greek god of the underworld.

TEXTURES OF IGNEOUS ROCKS

The texture of a rock refers to the size, shape, and arrangement of its mineral grains, or crystals (Table 4–1). Some igneous rocks consist of mineral grains that are too small to be seen with the naked eye; others are made up of thumb-size or even larger crystals. Volcanic rocks are usually fine grained, whereas plutonic rocks are medium or coarse grained.

 

Figure 4–12 Basalt is a finegrained volcanic rock.

The holes are gas bubbles that were preserved

as the magma solidified in southeastern Idaho.

Extrusive (Volcanic) Rocks

After magma erupts onto the relatively cool Earth surface, it solidifies rapidly—perhaps over a few days or years. Crystals form but do not have much time to grow. The result is a very fine-grained rock with crystals too small to be seen with the naked eye. Basalt is a common very fine-grained volcanic rock (Fig. 4–12).

 

Figure 4–13 Porphyry is an igneous rock containing large

crystals embedded in a fine-grained matrix. This rock is rhyolite

porphyry with large pink feldspar phenocrysts.

If magma rises slowly through the crust before erupting, some crystals may grow while most of the magma remains molten. If this mixture of magma and crystals then erupts onto the surface, it solidifies quickly, forming porphyry, a rock with the large crystals, called phenocrysts, embedded in a fine-grained matrix (Fig. 4–13). In unusual circumstances, volcanic magma may solidify within a few hours of erupting. Because the magma hardens so quickly, the atoms have no time to align themselves to form crystals. The result is the volcanic glass called obsidian (Fig. 4–14)

Figure 4–14 Obsidian is natural volcanic glass. It contains no

crystals. (Geoffrey Sutton)

Figure 4–15 Although granite (a) and rhyolite (b) contain the same minerals, they have

very different textures because granite cools slowly and rhyolite cools rapidly.

Intrusive (Plutonic) Rocks

When magma solidifies within the crust, the overlying rock insulates the magma like a thick blanket. The magma then crystallizes slowly, and the crystals may have hundreds of thousands or even millions of years in which to grow. As a result, most plutonic rocks are medium to coarse grained. Granite, the most abundant rock in continental crust, is a medium- or coarse-grained plutonic rock

Figure 4–16 The names of common igneous rocks are based on the minerals and texture

of a rock. In this figure, a mineral’s abundance in a rock is proportional to the thickness

of its colored band beneath the rock name. If a rock has a fine grain texture, its name is

found in the top row of rock names; if it has a coarse grain texture, its name is in the second row.

NAMING IGNEOUS ROCKS

Geologists use both the minerals and texture to classify and name igneous rocks. For example, any medium- or coarse-grained igneous rock consisting mostly of feldspar and quartz is called granite. Rhyolite also consists mostly of feldspar and quartz but is very fine grained (Fig. 4–15). The same magma that erupts onto the Earth’s surface to form rhyolite can also solidify slowly within the crust to form granite. Like granite and rhyolite, most common igneous rocks are classified in pairs, each member of a pair containing the same minerals but having a different texture. The texture depends mainly on whether the rock is volcanic or plutonic. Figure 4–16 shows the minerals and textures of common igneous rocks. The chemical compositions of common igneous rocks are summarized in Figure 4–17. Granite and rhyolite contain large amounts of feldspar and silica, and so are called felsic rocks. Basalt and gabbro are called mafic rocks because of their high magnesium and iron contents (ferrum is the Latin word for iron). Rocks with especially high magnesium and iron concentrations are called ultramafic. Rocks with compositions between those of granite and basalt are called intermediate rocks. Once you learn to identify the rock-forming minerals, it is easy to name a plutonic rock using Figure 4–17 because the minerals are large enough to be seen. It is more difficult to name many volcanic rocks because the minerals are too small to identify. A field geologist often uses color to name a volcanic rock. Figure 4–17 shows that rhyolite is usually light in color: White, tan, red, and pink are common. Many andesites are gray or green, and basalt is commonly black. The minerals in many volcanic rocks cannot be identified even with a microscope because of their tiny crystal sizes. In this case, definitive identification is based on chemical and X-ray analyses carried out in the laboratory.