Page 393 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 12 The Dynamic Planet 357
Table 12.2
Classification of Igneous Rocks
Intrusive igneous rock that cools slowly in the crust forms a pluton, the general term for any intru- sive igneous body, regard- less of size or shape. (The Roman god of the under- world, Pluto, is the name- sake.) The largest plutonic form is a batholith, defined as an irregular-shaped mass with an exposed sur- face greater than 100 km2. Batholiths form the mass of many large mountain ranges—for example, the Coast Range batholith of British Columbia and Wash- ington State, the Sierra Nevada batholith in Cali- fornia, and the Patagonian batholith in South America.
Smaller plutons in- clude the magma conduits of ancient volcanoes that have cooled and hardened. Those that form parallel to layers of sedimentary rock are sills; those that cross layers of the rock they in- vade are dikes (Figure 12.7). Magma also can bulge be- tween rock strata and pro- duce a lens-shaped body called a laccolith, a type of sill. In addition, magma
Silica content of magma
Chemical composition
Mineral content
Colouration
Melting temperature
Resistance to weathering
Intrusive rock type
(coarse-grained)
Extrusive rock type
(fine-grained)
Felsic High
High in potassium and sodium
Quartz Feldspars
Lighter
Lower Higher
Granite
Rhyolite
Medium
Mica Amphibole
Diorite
Andesite
Mafic (Ultramafic) Low
High in magnesium and iron
Pyroxine (Olivine) Darker
Higher Lower
Gabbro
Basalt
[granite: givaga/Shutterstock. Diorite and basalt: Tyler Boyes/Shutterstock. gabbro: Siim Sepp/Shutterstock. rhyolite: richard M. Busch. Andesite: John Cancalosi/Alamy.]
its physical characteristics. Felsic igneous rocks, such as granite, are high in silicate minerals, such as feldspar and quartz (pure silica), and have low melting points. The category name is derived from feldspar and silica. Rocks formed from felsic minerals generally are lighter in colour and less dense than those from mafic miner- als. Mafic igneous rocks, such as basalt, are derived from magnesium and iron (the Latin word for iron is ferrum). Mafic rocks are lower in silica and higher in magnesium and iron and have high melting points. Rocks formed from mafic minerals are darker in colour and of greater density than those from felsic minerals. Ultramafic rocks have the lowest silica content; an example is peridotite (less than 45% silica).
Igneous Landforms If igneous rocks are uplifted by exogenic processes, the work of air, water, and ice then sculpts them into unique landforms. Figure 12.7 illus- trates the formation environments of several intrusive and extrusive igneous rock types and landforms.
conduits may solidify in roughly cylindrical forms that stand starkly above the landscape when finally exposed by weathering and erosion. Shiprock volcanic neck in New Mexico is such a feature, rising 518 m above the surrounding plain; note the radiating dikes in the aer- ial photo in Figure 12.7. Striking examples of smaller plutons can be seen in the Monteregian Hills east of Montreal.
Sedimentary Processes
Solar energy and gravity drive the processes that form sedimentary rock, in which loose clasts (grains or frag- ments) are cemented together (Figure 12.8). The clasts that become solid rock are derived from several sources: the weathering and erosion of existing rock (origin of the sand that forms sandstone), the accumulation of shells on the ocean floor (which make up one form of lime- stone), the accumulation of organic matter from ancient plants (which form coal), and the precipitation of miner- als from water solution (origin of the calcium carbonate, CaCO3, that forms chemical limestone). Sedimentary