58 



GEOMORPHOLOGY: 



reefs as stages in a possible life history sequence. 

 For example, Darwin's hypothesis assumes: first, 

 corals growing along a shore to form a fringing reef; 

 second, building up of the original fringe associated 

 with progressive sinking of the land to form a barrier 

 reef; and finally, further building and complete land 

 sinking to construct an atoll. Darwin's hypothesis, 

 therefore, involves a shoreline of submergence and, 

 for full development of an atoll, a restricted land mass 

 such as an oceanic island. Another hypothesis is that 

 of glacial control. It presumes the same sequence of 

 reef formation as did Darwin; however, the rise of 

 waters over old reefs is specifically attributed to melt- 

 ing of ice since the last glacial age. Still another 

 hypothesis involves underwater platforms. This 

 premise does not propose a life cycle, but assumes 

 that a particular kind of reef forms in relation to a 

 platform and its wave action. 



DEPOSITION AND UNCONFORMITY 



Erosion produces fine materials that are deposited 

 in various ways. Under a given condition of topog- 

 raphy, the sediments accumulate in a thick layer of 

 fairly uniform particles. However, when conditions 

 are such that the essentially homogeneous material, 

 called a conformable bed, is eroded and then submerged, 

 new kinds of sediments are deposited on the older, 

 eroded bed, creating an unconformity. The fact that 

 there is a change from one bed to another creates the 

 unconformity, and the zone of contact between the 

 unlike layers is termed the plane of unconformity. Such 

 zones of contact are portrayed in many of the illustra- 

 tions in this chapter. 



CONSTRUCTIONAL LAND FORMS 



Discussions to this point have been mostly of de- 

 structive geological processes and destructional land 

 forms. Exceptions occur when land is elevated, be- 

 cause the elevation processes and their consequences 

 are constructive. In the remainder of this chapter, the 

 primary constructive processes, the constructional 

 land forms produced by the processes, and the de- 

 structive results of erosion on these land forms will 

 be considered. 



There are two groups of constructive processes. 

 The first, vulcamsm, produces land forms from sub- 

 stances originating beneath the earth's crust. Vul- 



canism is also called igneous activity. The second, 

 diastrophism, consists of various kinds of movements 

 that mold the earth's crust. 



VULCANISM 



Vulcanism, or igneous activity, encompasses all 

 aspects of the formation and movement of molten ma- 

 terial, magma, within and upon the earth's crust. 

 Magma moves toward the surface through zones of 

 crustal weakness, because this molten material is 

 under extreme pressure. Magma movement, depend- 

 ing upon whether or not it reaches the surface, is used 

 to categorize two classes of vulcanism, intrusive and 

 extrusive. Intrusive flows are those in which the 

 magma is trapped within the crust; extrusive flows, 

 those in which the magma reaches the surface. 



INTRUSIVE VULCANISM 



Intrusive flows also differ from extrusive in the gen- 

 eral nature of the rocks and land forms that are pro- 

 duced. In general, intrusive igneous rocks have larger 

 crystals than do extrusive, and intrusive land forms 

 are always constructed below the surface whereas ex- 

 trusive are upon the surface. Finally, the nature of 

 the land forms differs in the two kinds of flows. The 

 main land forms resulting from intrusive flows are 

 volcanic necks, dikes, sills, laccoliths, stocks, and 

 batholiths (Figure 4.21). Volcanic necks, also called 

 volcanic plugs, are igneous rock masses that fill the 

 opening, or vent, through which magma moves out of 

 a volcano. These plugs form from the last flow of 

 magma as a volcano becomes inactive. Dikes occur 

 wherever magma solidifies in cracks or fractures in 

 the crust; they may form in the cracks of volcano 

 walls or in any other fissure. Dikes range in size from 

 less than an inch wide and a few yards long to many 

 feet wide and many miles long. The largest, in Rho- 

 desia, is over 5 miles wide and 300 miles long. 



Sills resemble certain dikes, but sills form between 

 and are oriented parallel to the layers of rock strata. 

 This orientation occurs because the magma forces its 

 way between two layers (i.e., it spreads throughout a 

 zone of unconformity). Sills also resemble stocks and 

 batholiths, but are distinguished from them by being 

 formed near the surface, having their top and bottom 

 margins somewhat parallel to one another, and being 

 smaller. The average sill covers only a few acres or 

 square miles and is less than 100 feet thick. However, 



