Working through this chapter of the study guide will enable you to:

1. Name and describe some of the most abundant minerals found in Earth's crust.
2. State the basic physical properties that can be used to identify mineral samples.
3. List the three basic types of rocks that make up the crust of Earth and explain how they fit into the rock cycle.
4. Describe the various types of volcanic mountains that can form and tell how they differ from one another.
5. Explain how new sedimentary rocks can be formed from the remains of previous rocks.
6. Discuss the processes by which existing rocks can be changed by chemicals, heat, and pressure into different metamorphic forms.

Discussion

The composition and structure of the planet that we live on has long been of great interest to scientists and laymen alike. Much of the raw materials that we use to enhance our existence comes from the crust of Earth itself. Mineral formations are not only pretty to look at, as in gem quality stones for personal adornment, but also are the basis for many of the chemical and industrial products that we use in our daily lives. The finding and identification of these materials can be an interesting and rewarding experience, and many people today make a living doing just that.

Although many of the ideas that are covered in this chapter are somewhat complex, the overall discussion of Earth's crust and its physical composition is quite straightforward and can be easily grasped. Understanding this material is an important stepping stone for the next chapter in the textbook, which deals with the overall structure of Earth and the way that movements in its crust and mantle can produce significant, and sometimes catastrophic, events.


Section  21.1Minerals

Minerals are naturally occurring, inorganic, crystalline materials that have a well-defined chemical composition. They can be either elements or compounds, and each has a distinctive set of physical properties. Only about two dozen common minerals make up the majority of Earth's crust; however, in excess of 2000 have been found in small quantities. Over 98% of the crust is made up of only eight elements. In order of decreasing abundance by mass, they are oxygen, silicon, aluminum, iron, calcium, magnesium, sodium, and potassium. See Figure 11.9 in Chapter 11 of the textbook for a graphical representation of this distribution.

The term mineral is also used when referring to the elements in foods that are necessary, in small quantities, for the proper functioning of the human body. These are often discussed at the same time as other vital organic substances known collectively as vitamins, so it is quite common to be concerned, for example, about the vitamin and mineral content of your morning breakfast cereal. Some minerals fitting this classification are iron, sodium, iodine, manganese, magnesium, and copper.

Consolidated mixtures of crystalline minerals are called rocks. Most rocks are composed primarily of oxygen and silicon, much of which is in the form of silicon dioxide, commonly called silica. Combinations of these elements in which the oxygen-silicon ratios are greater than 2:1 are known as silicates. For example, quartz is composed of pure silica (SiO₂), whereas olivine is made up of the silicate tetrahedron (SiO₄)^4- in combination with several trace metals.

The most abundant mineral group in Earth's crust is the feldspar family. The members of this family are composed of oxygen, silicon, and aluminum plus at least one additional metallic element. The two main sub-classes of the feldspar family are plagioclase feldspar, which contains calcium or sodium, and orthoclase feldspar, which contains potassium.

Many non-silicate minerals are found in Earth's crust, among which are carbonates such as calcite, oxides such as hematite, and sulfides such as galena. There are also rare but important pure elements such as gold, silver, sulfur, copper, and diamond (carbon). Most of these minerals, however, appear as ores in which the important minerals make up only a small percentage. The discovery, mining, and refining of these ore deposits is the basis for a large portion of the valuable and useful minerals needed in our modern industrial society.

When discussing the physical properties of minerals, we use terms like crystal form, hardness (as measured on the Mohs' hardness scale), cleavage, fracture, color, and streak. Well-defined tests and observations can be made of these properties to help identify mineral samples. Applying these procedures to samples obtained from Earth's upper crust is quite easy and can even be fun. Many people have small personal collections of rocks and minerals that they have acquired through inexpensive purchase or simply by picking them up from the ground and bringing them home.


Section  21.2Rocks

Most minerals are not found in a pure state either on or beneath the surface of Earth. Large quantities of these materials are combined into aggregates called rocks. Rocks are classified by the three basic ways in which they are formed. Aggregates that have cooled from molten magma, whether on the surface or deep in Earth's interior, are known as igneous rocks. Those formed from the fragments of old rocks that have been deposited after erosion by air, water, or other chemical or mechanical means are called sedimentary rocks. Finally, any rocks that have changed their structures due to heat and/or pressure after the time they were initially formed are referred to as metamorphic rocks. These three types of rock are closely interrelated, as can be seen by studying the rock cycle, which is depicted in Fig. 21.10 of the textbook.


Section  21.3Igneous Rocks

It is believed that Earth began as a molten sphere of material, and thus the first rocks were formed from the cooling of hot magma into solid rock. This means that igneous rocks can be considered the basis of all other types of rocks.

Today, igneous rock forms either deep beneath Earth's surface or at Earth's surface as the result of volcanic activity. Molten rock called magma is often forced to the surface by volcanic action. Once it reaches the surface, molten rock is referred to as lava. Cooled lava retains this name even after it has solidified into igneous rock.

Igneous rock makes up about 80% of Earth's crust. Magma that cools and solidifies as lava at or near the surface of Earth is called extrusive rock, and magma that solidifies deep within Earth's interior is referred to as intrusive rock.

The outermost layer of Earth is a thin, rigid outer shell called the lithosphere, which is split up into giant surface plates. The lithosphere rides on top of a semimolten region known as the asthenosphere. The rigid surface plates are driven across the asthenosphere by the slow convective circulation of this easily deformable plastic material as heat works its way outward from Earth's central core region. The motion of these large lithospheric plates of solid rock across Earth's surface is explained by a theory called plate tectonics.

As plate tectonics occurs, the plates may bump into each other at convergent boundaries, pull away from each other at divergent boundaries, or slip past each other at transform boundaries. It is at these boundaries that we find most volcanic activity, and it is also where the majority of earthquakes occur.

The actual interaction of plates in quite complex, as will be seen in even more detail in Chapter 22, but the basic ideas needed in this chapter are that new rock can form in regions where plates are diverging, because of the upwelling of magma from the asthenosphere, and that one plate often is driven under another plate when they converge, producing a subduction zone in which the descending plate melts and its lighter, volatile material works its way back to the surface as volcanic gases and lava.

The texture of igneous rock is mainly determined by the type and size of mineral grains that form as it cools. Different minerals crystallize out of magma at different temperatures as the molten material cools. The order in which the most common minerals crystallize is related to the magma's temperature, and the size of the crystals that form from the various minerals present is dependent on the rate of cooling. When magma reaches the surface, a volcano is formed and lava can flow across Earth's surface. The texture, or grain size, in igneous rock created from this lava also depends on the rate of cooling, with the smallest grain size occurring in the fastest-cooling materials. On the other hand, if magma never reaches the surface, it tends to cool at a much slower rate, and the crystals formed in these subterranean regions will be much larger in size.

The color of igneous rock is determined primarily by the silica content of the rock. Rocks that are rich in silica often contain sodium and potassium. Granite and similar silica-rich rock is generally light in color and low in density. Rock that has lower silica content usually contains iron, magnesium, and calcium, which make it darker in color and also denser than silica-rich rock. Rocks, such as basalt, make up the material found in low-lying crustal regions such as ocean basins. Because of its lower density and high-silica content, granite is the most common rock found at higher elevations in volcanic islands or in continental mountain ranges.


Section  21.4Igneous Activity

Igneous rock that has cooled slowly below Earth's surface is found in formations called plutons. Such igneous bodies can be either concordant, if they form parallel to the grain in the surrounding rock, or discordant if they cut across this grain. Examples of concordant formations are sills and laccoliths, whereas examples of discordant bodies are batholiths and dikes.

Volcanoes appear where magma is squeezed all the way to Earth's surface. In these cases, steam and other gases are often expelled together, with molten rock in the form of lava. Solid material ranging in size from dust particles to boulders is also a common product of volcanic eruptions. Violent eruptions occur in areas where great pressure is built up by gases that are trapped because the emerging lava is thick and does not flow easily. If the lava is more fluid (has a lower viscosity), volatile gases can more easily escape and the eruptions are usually less violent. This results in lava rivers or fountains that can often be safely observed close at hand, such as the brilliant pyrotechnic displays that occur frequently in the Hawaiian Islands. The viscosity of lava depends on its temperature and on its silica content, with the lower temperature and silica-rich lava being the thickest and most viscous.

Volcanic eruptions can occur from long fractures in Earth's surface in the form of fissure eruptions, or in a more localized manner from the top or sides of large volcanic mountains. Fissure eruptions happen in continental regions where extensive surrounding areas can be covered by flood basalts, a very low-viscosity form of lava that can form flows several hundred meters thick, or they can occur in ocean basins at diverging plate boundaries where even more extensive flows can produce layers of lava several thousand meters thick.

Volcanic mountains can take on several forms. Shield volcanoes are low-profile mountains with gently sloping sides and extremely wide bases formed from the flow of non-viscous basaltic lava. Another common type is the stratovolcano, which forms from high-viscosity lava eruptions in which the violent activity produces composite layers of lava and tephra in a characteristic high, steep-sided formation. Yet another type of volcano can be formed almost completely out of tephra in a low, steeply sloped structure usually only a few hundred meters high, called a cinder cone.

The eruption of any specific volcano is, for the most part, quite unpredictable. New volcanoes can form suddenly, and old dormant ones may spring to life with little warning. However, in some areas of Earth's surface such activity is quite commonplace and can be expected to occur quite regularly. The most famous of these areas is the outer rim of the Pacific Ocean basin, which forms nearly a complete circle of currently active or recently active volcanoes. This volcanic region is commonly referred to as the Ring of Fire. Other more localized regions of continual volcanic activity are the famous hot spots, over one of which the Hawaiian Islands are still forming. Another very active region is the mid-oceanic trench on the floor of the Atlantic Ocean about halfway between the Americas and the mainlands of Europe and Africa, where seafloor spreading was first discovered and where it is still taking place today.


Section  21.5Sedimentary Rocks

Loose sediment is created by various forms of decomposition and erosion that will be discussed in detail in Chapter 23. The consolidation of this sediment forms sedimentary rock. This type of rock makes up only about 5% of Earth's entire crust, but it covers the surfaces of more than 75% of the continents and ocean basins. This means that sedimentary rock forms in relatively thin layers. Sedimentary deposits are also important because they contain abundant supplies of oil and coal, as well as metal deposits and other materials used primarily in the construction industry.

Sedimentary rock is made up of detrital sediments (rock and mineral fragments), organic sediments, and chemical sediments. These materials are transported by streams and rivers and eventually deposited in layers at the bottom of lakes and oceans. The consolidation of these layers leads to the formation of sedimentary rock. The conditions under which sedimentary rock was formed can be determined by studying characteristics such as color, sorting, rounding, bedding, fossil content, ripples, and mud cracks. Tables 21.5 and 21.6 in the textbook show some of the most common classifications of sedimentary rock and describe their properties.


Section  21.6Metamorphic Rocks

Metamorphic rocks have been changed under the influence of high temperature and/or extreme pressure deep beneath the Earth's surface. Sedimentary, igneous, and metamorphic rock can all undergo metamorphic changes, but sedimentary rock is especially susceptible to such processes. Metamorphic changes can be either mechanical or chemical, and they may involve only the materials present in the original rock, or, in some cases, may have additional minerals introduced into the formation by the metamorphic process itself.

There are three basic types of metamorphism: (1) contact metamorphism, caused by thermal processes, (2) shear metamorphism, induced by high-pressure conditions, and (3) regional metamorphism, involving a combination of both thermal and high-pressure influences over large areas deep underground. Once the changes have occurred, metamorphic rock is classified according to its texture, mineral composition, and ability to split along smooth planes. In advanced metamorphism this splitting is referred to as foliation. Marked foliation is most often associated with extensive regional metamorphic activity. Table 21.7 in the textbook shows the common types of metamorphic rock, together with some of the identifying characteristics associated with each of these types.

Return to Previous Page