{"id":560,"date":"2019-06-11T14:51:29","date_gmt":"2019-06-11T14:51:29","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/12-4-measuring-geological-structures\/"},"modified":"2021-12-08T18:12:15","modified_gmt":"2021-12-08T18:12:15","slug":"12-4-measuring-geological-structures","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/12-4-measuring-geological-structures\/","title":{"raw":"12.4 Measuring Geological Structures","rendered":"12.4 Measuring Geological Structures"},"content":{"raw":"Geologists take great pains to measure and record geological structures because they are critically important to understanding the geological history of a region.\u00a0One of the key features to measure is the orientation, or [pb_glossary id=\"1715\"]attitude[\/pb_glossary]<\/strong>, of bedding.\u00a0We know that sedimentary beds are deposited in horizontal layers, so if the layers are no longer horizontal, then we can infer that they have been affected by tectonic forces and have become either tilted, or folded.\u00a0We can express the orientation of a bed (or any other planar feature) with two values: first, the compass orientation of a horizontal line on the surface\u2014the [pb_glossary id=\"1716\"]strike[\/pb_glossary]<\/strong>\u2014and second, the angle at which the surface dips below a horizontal plane, (perpendicular to the strike)\u2014the [pb_glossary id=\"1717\"]dip[\/pb_glossary]<\/strong> (Figure 12.4.1).\r\n\r\nIt may help to imagine a vertical surface, such as a wall in your house.\u00a0The strike is the compass orientation of the wall and the dip is 90\u02da from horizontal.\u00a0If you could push the wall so it\u2019s leaning over, but still attached to the floor, the strike direction would be the same, but the dip angle would be less than 90\u02da.\u00a0If you pushed the wall over completely so it was lying on the floor, it would no longer have a strike direction and its dip would be 0\u02da.\u00a0When describing the dip it is important to include the direction. In other words. if the strike is 0\u02da (i.e., north) and the dip is 30\u02da, it would be necessary to say \u201cto the west\u201d or \u201cto the east.\u201d\u00a0 Similarly if the strike is 45\u02da (i.e., northeast) and the dip is 60\u02da, it would be necessary to say \u201cto the northwest\u201d or \u201cto the southeast.\u201d\r\n\r\n[caption id=\"attachment_556\" align=\"aligncenter\" width=\"800\"]\"\"<\/a> Figure 12.4.1\u00a0 A depiction of the strike and dip of some tilted sedimentary beds.\u00a0 The dipping beds are shown partially covered with water so that you can visualize a horizontal line on the rock surface.\u00a0The notation for expressing strike and dip on a map is also shown.[\/caption]\r\n\r\nMeasurement of geological features is done with a special compass that has a built-in clinometer\u2014a device for measuring vertical angles.\u00a0An example of how this is done is shown on Figure 12.4.2.\r\n\r\n[caption id=\"attachment_557\" align=\"aligncenter\" width=\"900\"]\"\"<\/a> Figure 12.4.2 Measuring the compass direction of the strike (left) and the vertical angle of the dip (right) using a compass with a clinometer.[\/caption]\r\n\r\nStrike and dip are also used to describe any other planar features, including joints, faults, dykes, sills, and even the foliation planes in metamorphic rocks.\u00a0Figure 12.4.3 shows an example of how we would depict the beds that make up an anticline on a map.\r\n\r\n[caption id=\"attachment_558\" align=\"aligncenter\" width=\"600\"]\"\"<\/a> Figure 12.4.3 A depiction of an antiform and a dyke in cross-section (looking from the side) and in map view (a.k.a. plan view) with the appropriate strike-dip and antiform symbols.[\/caption]\r\n\r\nThe beds on the west (left) side of the map are dipping at various angles to the west.\u00a0The beds on the east side are dipping to the east.\u00a0The middle bed (light grey) is horizontal; this is denoted by a cross within a circle.\u00a0The dyke is dipping at 80\u02da to the west.\u00a0The hinge of the fold is denoted with a dashed line with two arrows that point away from it.\u00a0 If it was a synform, the arrows would point towards the line.\r\n
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Exercise 12.3 Putting strike and dip on a map<\/p>\r\n\r\n<\/header>\r\n

\r\n\r\n[caption id=\"attachment_559\" align=\"alignright\" width=\"450\"]\"\"<\/a> Figure 12.4.4[\/caption]\r\n\r\nThis cross-section shows seven tilted sedimentary layers (a to g), a fault, and a steeply dipping dyke.\u00a0Place strike and dip symbols on the map to indicate the orientations of the beds shown, the fault, and the dyke.\u00a0Then answer the questions.\r\n
    \r\n \t
  1. What type of fault is this, and is this an extensional or compressional situation?<\/li>\r\n \t
  2. What are the relative ages of the nine geological features shown here (seven beds, dyke, and fault)? Which are the youngest and oldest?<\/li>\r\n<\/ol>\r\nSee Appendix 3 for Exercise 12.3 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\n

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