Page 391 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 12 The Dynamic Planet 355
(a) Granite shaped by weathering processes near Joshua Tree National Park, California, the southern region of the extensive California batholith (we examine weathering in Chapter 14.)
(b) Basaltic lava flows on Hawai`i. The glowing opening is a skylight into an active tube where molten lava flows; the shiny surface is where lava recently flowed out of the skylight.
▲Figure 12.5 examples of intrusive granite and extrusive basalt. [(a) Egon Bömsch/Imagebroker/SuperStock. (b) Bobbé Christopherson.]
in newly formed rock. These stripes illustrate global pat- terns of changing magnetism. Later in this chapter, we see the importance of these magnetic reversals.
earth Materials and the rock Cycle
We have already mentioned several types of rocks, such as granite and basalt, and described processes involving rocks and their formation. To understand and classify rocks from a scientific viewpoint, we must begin with minerals, which are the building blocks of rocks. A min- eral is an inorganic, or nonliving, natural solid compound having a specific chemical formula and usually possess- ing a crystalline structure. Each mineral has its own characteristic colour, texture, crystal shape, and density, among other unique properties. For example, the common mineral quartz is silicon dioxide, SiO2, and has a distinc- tive six-sided crystal. Ice fits the definition of a mineral, although water does not.
Mineralogy is the study of the composition, proper- ties, and classification of minerals (see www.mindat.org/). Of the more than 4200 minerals known, about 30 are the most common components of rocks. Roughly 95% of Earth’s crust is made up of silicates, one of the most widespread mineral families—not surprising consider- ing the percentages of silicon and oxygen on Earth and their readiness to combine with each other and with other elements. This mineral family includes quartz, feldspar, clay minerals, and numerous gemstones. Sev- eral other groups of minerals are also important: Oxides are minerals in which oxygen combines with metallic elements; sulfides and sulfates are minerals in which sulfur compounds combine with metallic elements; and carbonates feature carbon in combination with oxygen
and other elements such as calcium, magnesium, and potassium.
A rock is an assemblage of minerals bound together (such as granite, a rock containing three minerals); or a mass composed of a single mineral (such as rock salt); or of undifferentiated material (such as the noncrystalline glassy obsidian); or even solid organic material (such as coal). Scientists have identified thousands of differ- ent rocks, all of which can be sorted according to three types that depend on the processes that formed them: igneous (formed from molten material, as in Figure 12.5), sedimentary (formed from compaction or chemical processes), and metamorphic (altered by heat and pres- sure). The movement of material through these pro- cesses is known as the rock cycle and is summarized at the end of this section. Figure 12.6 maps the distribu- tion of rock types across Canada.
Igneous Processes
An igneous rock is one that solidifies and crystallizes from a molten state. Igneous rocks form from magma, which is molten rock beneath Earth’s surface (hence the name igneous, which means “fire-formed” in Latin). When magma emerges at the surface, it is lava, although it retains its molten characteristics. Overall, igneous rocks make up approximately 90% of Earth’s crust, al- though sedimentary rocks, soil, or oceans frequently cover them.
Igneous Environments Magma is fluid, highly gaseous, and under tremendous pressure. The result is that it ei- ther intrudes into crustal rocks, cooling and hardening below the surface to form intrusive igneous rock, or it extrudes onto the surface as lava and cools to form extru- sive igneous rock. Extrusive igneous rocks result from
Skylight