{"id":549,"date":"2017-01-30T18:29:36","date_gmt":"2017-01-30T18:29:36","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/physicalgeology\/?post_type=chapter&p=549"},"modified":"2017-01-30T18:57:35","modified_gmt":"2017-01-30T18:57:35","slug":"12-2-folding","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-purchase-physicalgeology\/chapter\/12-2-folding\/","title":{"raw":"12.2 Folding","rendered":"12.2 Folding"},"content":{"raw":"

When a body of rock, especially sedimentary rock, is squeezed from the sides by tectonic forces, it is likely to fracture and\/or become faulted if it is cold and brittle, or become folded if it is warm enough to behave in a plastic manner.\n\nThe nomenclature and geometry of folds are summarized on Figure 12.5.\u00a0 An upward fold is called an anticline<\/strong>, while a downward fold is called a syncline<\/strong>.\u00a0In many areas it\u2019s common to find a series of anticlines and synclines (as in Figure 12.5), although some sequences of rocks are folded into a single anticline or syncline.\u00a0A plane drawn through the crest of a fold in a series of beds is called the axial plane<\/strong> of the fold.\u00a0The sloping beds on either side of an axial plane are limbs<\/strong>.\u00a0An anticline or syncline is described as symmetrical<\/strong> if the angles between each of limb and the axial plane are generally similar, and asymmetrical<\/strong> if they are not.\u00a0If the axial plane is sufficiently tilted that the beds on one side have been tilted past vertical, the fold is known as an overturned<\/strong> anticline or syncline.\n\n[caption id=\"attachment_1017\" align=\"aligncenter\" width=\"400\"]\"Figure<\/a> Figure 12.5 Examples of different types of folds and fold nomenclature.\u00a0Axial planes are only shown for the anticlines, but synclines also have axial planes. [SE][\/caption]\u00a0\n\nA very tight fold, in which the limbs are parallel or nearly parallel to one another is called an isoclinal<\/strong> fold<\/strong> (Figure 12.6).\u00a0Isoclinal folds that have been overturned to the extent that their limbs are nearly horizontal are called recumbent folds<\/strong>.\n\n[caption id=\"attachment_1018\" align=\"aligncenter\" width=\"400\"]\"Figure<\/a> Figure 12.6 An isoclinal recumbent fold [SE][\/caption]\nFolds can be of any size, and it\u2019s very common to have smaller folds within larger folds (Figure 12.7).\u00a0 Large folds can have wavelengths of tens of kilometres, and very small ones might be visible only under a microscope. Anticlines are not necessarily, or even typically, expressed as ridges in the terrain, nor synclines as valleys.\u00a0Folded rocks get eroded just like all other rocks and the topography that results is typically controlled mostly by the resistance of different layers to erosion (Figure 12.8).\n\n[caption id=\"attachment_1019\" align=\"aligncenter\" width=\"400\"]\"Figure<\/a> Figure 12.7 Folded limestone (grey) and chert (rust-colored) in Triassic Quatsino Formation rocks on Quadra Island, B.C.\u00a0 The image is about 1 m across. [SE][\/caption]\u00a0\n\n[caption id=\"attachment_1020\" align=\"aligncenter\" width=\"400\"]\"Figure<\/a> Figure 12.8 Example of the topography in an area of folded rocks that has been eroded.\u00a0In this case the green and grey rocks are most resistant to erosion, and are represented by hills. [SE][\/caption]\n<\/p>

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Exercises<\/h3>\nExercise 12.1\u00a0 Folding Style<\/strong>\n\nThis photograph shows folding in the same area of the Rocky Mountains as Figure 12.1.\u00a0 Describe the types of folds using the appropriate terms from above (symmetrical, asymmetrical, isoclinal, overturned, recumbent etc.).\u00a0 You might find it useful to first sketch in the axial planes.\n\n\"Folding<\/a>\n\n<\/div>\n\u00a0<\/strong>","rendered":"

When a body of rock, especially sedimentary rock, is squeezed from the sides by tectonic forces, it is likely to fracture and\/or become faulted if it is cold and brittle, or become folded if it is warm enough to behave in a plastic manner.<\/p>\n

The nomenclature and geometry of folds are summarized on Figure 12.5.\u00a0 An upward fold is called an anticline<\/strong>, while a downward fold is called a syncline<\/strong>.\u00a0In many areas it\u2019s common to find a series of anticlines and synclines (as in Figure 12.5), although some sequences of rocks are folded into a single anticline or syncline.\u00a0A plane drawn through the crest of a fold in a series of beds is called the axial plane<\/strong> of the fold.\u00a0The sloping beds on either side of an axial plane are limbs<\/strong>.\u00a0An anticline or syncline is described as symmetrical<\/strong> if the angles between each of limb and the axial plane are generally similar, and asymmetrical<\/strong> if they are not.\u00a0If the axial plane is sufficiently tilted that the beds on one side have been tilted past vertical, the fold is known as an overturned<\/strong> anticline or syncline.<\/p>\n

\"Figure<\/a><\/p>\n

Figure 12.5 Examples of different types of folds and fold nomenclature.\u00a0Axial planes are only shown for the anticlines, but synclines also have axial planes. [SE]<\/p>\n<\/div>\n

\u00a0<\/p>\n

A very tight fold, in which the limbs are parallel or nearly parallel to one another is called an isoclinal<\/strong> fold<\/strong> (Figure 12.6).\u00a0Isoclinal folds that have been overturned to the extent that their limbs are nearly horizontal are called recumbent folds<\/strong>.<\/p>\n

\"Figure<\/a><\/p>\n

Figure 12.6 An isoclinal recumbent fold [SE]<\/p>\n<\/div>\n

Folds can be of any size, and it\u2019s very common to have smaller folds within larger folds (Figure 12.7).\u00a0 Large folds can have wavelengths of tens of kilometres, and very small ones might be visible only under a microscope. Anticlines are not necessarily, or even typically, expressed as ridges in the terrain, nor synclines as valleys.\u00a0Folded rocks get eroded just like all other rocks and the topography that results is typically controlled mostly by the resistance of different layers to erosion (Figure 12.8).<\/p>\n

\"Figure<\/a><\/p>\n

Figure 12.7 Folded limestone (grey) and chert (rust-colored) in Triassic Quatsino Formation rocks on Quadra Island, B.C.\u00a0 The image is about 1 m across. [SE]<\/p>\n<\/div>\n

\u00a0<\/p>\n

\"Figure<\/a><\/p>\n

Figure 12.8 Example of the topography in an area of folded rocks that has been eroded.\u00a0In this case the green and grey rocks are most resistant to erosion, and are represented by hills. [SE]<\/p>\n<\/div>\n

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Exercises<\/h3>\n

Exercise 12.1\u00a0 Folding Style<\/strong><\/p>\n

This photograph shows folding in the same area of the Rocky Mountains as Figure 12.1.\u00a0 Describe the types of folds using the appropriate terms from above (symmetrical, asymmetrical, isoclinal, overturned, recumbent etc.).\u00a0 You might find it useful to first sketch in the axial planes.<\/p>\n

\"Folding<\/a><\/p>\n<\/div>\n

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Candela Citations<\/h3>\n\t\t\t\t\t
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