Page 424 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

388 part III The Earth–Atmosphere Interface
sedimentary rock—called continental platforms—surround these shields and appear quite stable over time. Examples of such stable platforms include the region from east of the Rockies to the Appalachians and northward into central and eastern Canada, a large portion of China, eastern Eu- rope to the Ural Mountains, and portions of Siberia.
On the MasteringGeography website, you will find a map of seven world structural regions, including shields and their surrounding sedimentary deposits. Various mountain chains, rifted regions, and isolated volcanic areas are also noted on the map, which you can refer back to as you read through this chapter.
Building Continental Crust
and Accretion of Terranes
The formation of continental crust is complex and takes hundreds of millions of years. It involves the entire se- quence of seafloor spreading and formation of oceanic crust, eventual subduction and remelting of that oce- anic crust, and the subsequent rise of remelted material as new magma, all summarized in Figure 13.5. In this
process of crustal formation, you can literally follow the cycling of materials through the tectonic cycle.
To understand this process, study Figure 13.5 and the inset photos. Begin with the magma that originates in the asthenosphere and wells up along the mid-ocean ridges. Basaltic magma is formed from minerals in the upper mantle that are rich in iron and magnesium. Such magma contains less than 50% silica and has a low- viscosity (thin) texture—it tends to flow. This mafic ma- terial rises to erupt at spreading centres and cools to form new basaltic seafloor, which spreads outward to collide with continental crust along its far edges. The oceanic crust, being denser, plunges beneath the lighter conti- nental crust, into the mantle, where it remelts. The new magma then rises and cools, forming more continental crust in the form of intrusive granitic igneous rock.
As the subducting oceanic plate works its way under a continental plate, it takes with it trapped seawater and sediment from eroded continental crust. The remelting incorporates the seawater, sediments, and surrounding crust into the mixture. As a result, the magma, generally called a melt, migrating upward from a subducted plate,
Dacite, from Mount St. Helens, is higher in silica
Magma with an andesitic-to-granitic composition derived from partial melting of subducted oceanic plate and remelting of continental crust
Intrusive body
Continental crust
Spreading centre
Basalt, from Hawai‘i, is lower in silica
Oceanic ridge
Trench
Basaltic oceanic crust
Basaltic magma derived from partial melting of asthenosphere,
or deeper plume
Asthenosphere
1. Material from the asthenosphere upwells along seafloor spreading centres.
▲Figure 13.5 Crustal formation. Material from the asthenosphere upwells along seafloor spreading centres. Basaltic ocean floor is subducted beneath lighter continental crust, where it melts, along with its cargo of sediments, water, and minerals. This melting generates magma, which makes its way up through the continental crust to form igneous intrusions and extrusive eruptions. [Photos by Bobbé Christopherson.]
3. Melting generates magma, which makes its way up through the continental crust to form igneous intrusions and extrusive eruptions.
2. Basaltic ocean floor is subducted beneath lighter continental crust, where it melts, along with its cargo of sediments, water, and minerals.
Subduction