Page 416 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

380 part III The Earth–Atmosphere Interface
the uppermost mantle and the crust is the Mohorovicˇic´ discontinuity, or Moho. The outer layer is the crust.
The outer part of Earth’s crust is divided into two lay- ers. The uppermost mantle, along with the crust, makes up the lithosphere. Below the lithosphere is the astheno- sphere, or plastic layer. It contains pockets of increased heat from radioactive decay and is susceptible to slow convective currents in these hotter materials. The prin- ciples of buoyancy and balance produce the important principle of isostasy. Isostasy explains certain vertical movements of Earth’s crust, such as isostatic rebound when the weight of ice is removed.
Earth’s magnetic field is generated almost entirely within Earth’s outer core. Polarity reversals in Earth’s mag- netism are recorded in cooling magma that contains iron minerals. The patterns of geomagnetic reversal in rock help scientists piece together the history of Earth’s mobile crust.
seismic wave (p. 350)
core (p. 350)
mantle (p. 351)
Mohorovicˇic´ discontinuity (Moho) (p. 351) crust (p. 351)
lithosphere (p. 352) asthenosphere (p. 352) isostasy (p. 353)
geomagnetic reversal (p. 354)
4. MakeasimplesketchofEarth’sinterior,labeleach layer, and list the physical characteristics, tempera- ture, composition, and depth of each on your drawing.
5. How does Earth generate its magnetic field? Is the magnetic field constant or does it change? Explain the implications of your answer.
6. Describe the asthenosphere. Why is it also known as the plastic layer? What are the consequences of its convection currents?
7. What is a discontinuity? Describe the principal discontinuities within Earth.
8. Define isostasy and isostatic rebound, and explain the crustal equilibrium concept.
9. Diagram the uppermost mantle and crust. Label the density of the layers in g·cm−3. What two types of crust were described in the text in terms of rock composition?
■ Describe the three main groups of rock, and diagram the rock cycle.
A mineral is an inorganic natural compound having a specific chemical formula and possessing a crystalline structure. A rock is an assemblage of minerals bound to- gether (such as granite, a rock containing three minerals), or it may be a mass of a single mineral (such as rock salt).
Igneous rock forms from magma, which is molten rock beneath the surface. Lava is the name for magma once it has emerged onto the surface. Magma either in- trudes into crustal rocks, cools, and hardens, forming intrusive igneous rock; or extrudes onto the surface, forming extrusive igneous rock. The crystalline texture of igneous rock is related to the rate of cooling. Granite is a coarse-grained intrusive igneous rock; it is crystalline and high in silica, aluminum, potassium, calcium, and sodium. Basalt is a fine-grained extrusive igneous rock; it is granular and high in silica, magnesium, and iron. In- trusive igneous rock that cools slowly in the crust forms a pluton. The largest pluton form is a batholith.
Sedimentary rock is formed when loose clasts (grains or fragments) derived from several sources are compacted and cemented together in the process of lithification. Clastic sedimentary rocks are derived from the fragments of weath- ered and eroded rocks and the material that is transported and deposited as sediment. Chemical sedimentary rocks are formed either by biochemical processes or from the chemi- cal dissolution of minerals into solution; the most common is limestone, which is lithified calcium carbonate, CaCO3.
Any igneous or sedimentary rock may be transformed into metamorphic rock by going through profound physi- cal or chemical changes under pressure and increased temperature. The rock cycle describes the three principal rock-forming processes and the rocks they produce.
mineral (p. 355) rock (p. 355)
igneous rock (p. 355) magma (p. 355)
lava (p. 355)
intrusive igneous rock (p. 355) extrusive igneous rock (p. 355) granite (p. 356)
basalt (p. 356)
pluton (p. 357)
batholith (p. 357)
sedimentary rock (p. 357) sediment (p. 358)
lithification (p. 358)
limestone (p. 359) metamorphic rock (p. 360) rock cycle (p. 361)
10. What is a mineral? A mineral family? Name the most common minerals on Earth. What is a rock?
11. Describe igneous processes. What is the difference be- tween intrusive and extrusive types of igneous rocks?
12. Explain what coarse- and fine-grained textures say about the cooling history of a rock.
13. Briefly describe sedimentary processes and lithifica- tion. Describe the sources and particle sizes of sedi- mentary rocks.
14. What is metamorphism, and how are metamorphic rocks produced? Name some original parent rocks and their metamorphic equivalents.
■ Describe Pangaea and its breakup, and explain the physical evidence that crustal drifting is continuing today.
The present configuration of the ocean basins and conti- nents is the result of tectonic processes involving Earth’s interior dynamics and crust. Pangaea was the name Alfred Wegner gave to a single assemblage of continen- tal crust existing some 225 m.y.a. that subsequently broke apart. Wegener coined the phrase continental drift to describe his idea that the crust is moved by vast forces within the planet. The theory of plate tectonics is that Earth’s lithosphere is fractured into huge slabs or plates, each moving in response to gravitational pull and to flow- ing currents in the mantle that create frictional drag on the plate. Geomagnetic reversals along mid-ocean ridges on the ocean floor provide evidence of seafloor spreading which accompanies the movement of plates toward the continental margins of ocean basins. At some plate boundaries, denser oceanic crust dives beneath lighter continental crust along subduction zones.