ROCKS AND SOUS 



75 



TABLC 5.3 CLASSIFICATION Of METAMORPHIC ROCKS 



Original Composition Metomorphic Process Melamorphic Rock 



Variable 



Variable 



Conglomerate 



Sandstone 



Shale 



Fine-grained 

 igneous rocks 



Coarse-grained 

 igneous rocks 



Limestone 



Dolomitic lime- 

 stone 



Diatomite 



Hydrothermal 



Hydrothermal 

 Pressure and heat 

 Hydrothermal 



and/or pressure 



and heat 

 Pressure and heat 



Pressure and heat 

 Pressure and heat Gneiss 



Steatite (soap- 

 stone) 

 Serpentine 

 Gneiss or schist 

 Quartzite 



Slate to phyllite to 



schist 

 Schist 



Pressure and heat 

 Pressure and heat 



Marble 

 Dolomitic marble 



Pressure and heat Chert 



rocks .are associated with certain land forms. In some 

 respects the following treatment of forms and struc- 

 tures of rocks is review. 



IGNEOUS ROCKS 



The physical features of igneous rocks are related 

 to whether the rocks are intrusive or extrusive. 



Forms. Extrusive igneous rocks are shaped into 

 volcanoes and allied surface forms. Intrusive 

 igneous rocks are wedged into dikes, sills, laccoliths, 

 stocks, and batholiths. 



Structures. Igneous rocks are irregular in struc- 

 ture and have great resistance to erosion and pres- 

 sure. The outcome of these outside forces acting upon 

 igneous rocks is usually some kind of jointing or 

 faulting along lines or planes of weakness. Joints 

 assume a rectangular, columnar, circular, or angular 

 pattern; faults occur along an essentially flat plane of 

 weakness. 



SEDIMENTARY ROCKS 



Forms. Sedimentary rocks include both unconsoli- 

 dated and consolidated remains of past life or of any 

 kind of rock. In general, sedimentary rocks are prod- 

 ucts of erosion of other rocks and of deposition of the 

 erosion fragments in horizontal layers. The layers 

 may or may not be compacted or cemented. 



Structures. Although sedimentary rocks joint and 

 fault as do igneous rocks, the lesser resistance of 



sedimentary rocks to pressure allows them to fold. 

 However, jointing, faulting, and folding are all com- 

 mon. Typically, joints open at right angles to the 

 plane of deposition and in a manner to separate rec- 

 tangular blocks from the main mass. Extreme pres- 

 sures tend to cause faulting and block mountains. 

 Even greater pressures may develop grabens, horsts, 

 or complex faults. If jointing and faulting do not 

 occur, folding is common. In fact, some type of fold- 

 ing is generally present — horizontal strata are rare. 

 The folding leads to both simple and complex folds, 

 some of which as a result of intense pressure may be 

 overturned, asymetrical, or faulted. In many cases, 

 sediments are affected by more than one of these 

 forces and complex mountains result. 



METAMORPHIC ROCKS 



Metamorphic rock structure resembles that of its 

 parent material. However, some differences between 

 the parent material and the metamorphic product 

 generally occur; the metamorphic rock tends to be 

 harder, more crystalline, and deeper in color. 



SOILS 



Soil is a complex mixture of minerals and organic 

 compounds. Because it is a decomposition product 

 of rocks and organisms, soil is a sedimentary ma- 

 terial and sometimes is considered a sedimentary 

 rock. 



FACTORS OF SOIL FORMATION 



Five factors contribute to the formation of soil. 

 The first two, type of parent and organic materials, 

 provide the substance that eventually develops into 

 soil. Parent materials are the rocks that erode to 

 fine inorganic particles and organic materials are the 

 plant and animal remains that decompose to soil 

 humus. These sources of soil may display a seem- 

 ingly conflicting relationship with mature soils. For 

 example, the sources may differ in two localities but 

 the same mature soil may form in both areas. Also, 

 identical parent and organic sources can become dif- 

 ferent mature soils in remote locations. Although 

 this might be perplexing, it does not belie the im- 

 portance of the first two factors. Rather, it implies 

 the possible importance of the other three factors. 



