{"id":627,"date":"2018-02-25T00:27:04","date_gmt":"2018-02-25T05:27:04","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/physicalgeologyh5p\/chapter\/13-5-measuring-geological-structures\/"},"modified":"2021-08-09T16:14:44","modified_gmt":"2021-08-09T20:14:44","slug":"13-5-measuring-geological-structures","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/physicalgeologyh5p\/chapter\/13-5-measuring-geological-structures\/","title":{"raw":"13.5 Measuring Geological Structures","rendered":"13.5 Measuring Geological Structures"},"content":{"raw":"Documenting the characteristics of geological structures is used to understand the geological history of a region.\u00a0One of the key features to measure is the orientation, or attitude<\/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 tectonic forces have folded or tilted them.\r\n\r\nThe orientation of a planar feature, such as a bed of sedimentary rock, can be described with two values. The strike <\/strong>of the bed is the compass orientation of a horizontal line on the surface of the bed. The dip <\/strong>is the angle at which the surface tilts down from the horizontal (Figure 13.35). The dip is measured perpendicular to strike, otherwise the dip angle that is measured will be smaller than the actual tilt of the bed.\r\n\r\n[caption id=\"attachment_623\" align=\"aligncenter\" width=\"550\"]\"\"<\/a> Figure 13.35<\/strong> Strike and dip for tilted sedimentary beds. Water provides a horizontal surface. The strike and dip symbol is a T with the long horizontal bar representing the strike direction, and the small tick mark indicating the dip direction. The dip angle is written next to the tick mark. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. View source.<\/a>[\/caption]\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 is 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 because you could draw a horizontal line in any of an infinite number of directions on the horizontal surface of the wall. Its dip would be 0\u02da.\r\n\r\nWhen reporting the dip, include the direction. For example, if the strike runs north-south and the dip is 30\u02da, it would be necessary to specify \u201cto the west\u201d or \u201cto the east.\u201d\u00a0 Similarly if the strike is northeast-southwest and the dip is 60\u02da, it would be necessary to say \u201cto the northwest\u201d or \u201cto the southeast.\u201d In the case of the vertical wall with a dip angle of 90\u02da, there is no dip direction. The dip points straight down, not toward any compass direction.\r\n\r\nMeasurement of geological features is done with a special compass that has a built-in clinometer<\/strong>, which is a device for measuring vertical angles. The strike is measured by aligning the compass along a horizontal line on the surface of the feature (Figure 13.36, left). The dip is measured by turning the compass on its side and aligning it along the dip direction (Figure 13.36, right).\r\n\r\n \r\n\r\n[caption id=\"attachment_624\" align=\"aligncenter\" width=\"648\"]\"\"<\/a> Figure 13.36<\/strong> Measurement of strike (left) and dip (right) using a geological compass with a clinometer.\u00a0 Source: Steven Earle (2015), CC BY 4.0. View source left<\/a>\/ right<\/a>[\/caption]\r\n\r\nStrike and dip are used to describe any other planar features, including joints, faults, dykes, sills, and even the foliation planes in metamorphic rocks.\u00a0Figure 13.37 shows an example of how we would depict the beds that make up an anticline on a map. The 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 beds in the middle are horizontal; this is denoted by a cross within a circle on the map.\u00a0The dyke is dipping at 80\u02da to the west.\u00a0The hinge line of the fold is denoted with a dashed line on the map, with two arrows pointing away from it, indicating the general dip directions of the limbs.\u00a0 If it were\u00a0a syncline, the arrows would point inward toward the line.\r\n\r\n \r\n\r\n[caption id=\"attachment_625\" align=\"aligncenter\" width=\"633\"]\"\"<\/a> Figure 13.37<\/strong> A depiction of an anticline and a dyke in cross-section (looking from the side) and in map view (or plan view) with the appropriate strike-dip and anticline symbols. Source: Steven Earle (2015), CC BY 4.0. View source.<\/a>[\/caption]\r\n\r\n \r\n
\r\n\r\nPutting Strike and Dip on a Map<\/strong>\r\n\r\nThis cross-section shows seven tilted sedimentary layers (a to g), a fault, and a steeply dipping dyke.\r\n\r\n[caption id=\"attachment_626\" align=\"aligncenter\" width=\"650\"]\"\"<\/a> Figure 13.38<\/strong> Practice with strike and dip symbols. Source: Steven Earle (2015), CC BY 4.0. View source.<\/a>[\/caption]\r\n
    \r\n \t
  1. Place strike and dip symbols on the map to indicate the orientations of the beds shown, the fault, and the dyke.<\/li>\r\n \t
  2. What type of fault is shown?<\/li>\r\n \t
  3. What kind of stress created the fault?<\/li>\r\n<\/ol>\r\n \r\n\r\n<\/div>\r\n \r\n\r\n ","rendered":"

    Documenting the characteristics of geological structures is used to understand the geological history of a region.\u00a0One of the key features to measure is the orientation, or attitude<\/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 tectonic forces have folded or tilted them.<\/p>\n

    The orientation of a planar feature, such as a bed of sedimentary rock, can be described with two values. The strike <\/strong>of the bed is the compass orientation of a horizontal line on the surface of the bed. The dip <\/strong>is the angle at which the surface tilts down from the horizontal (Figure 13.35). The dip is measured perpendicular to strike, otherwise the dip angle that is measured will be smaller than the actual tilt of the bed.<\/p>\n

    \"\"<\/a>
    Figure 13.35<\/strong> Strike and dip for tilted sedimentary beds. Water provides a horizontal surface. The strike and dip symbol is a T with the long horizontal bar representing the strike direction, and the small tick mark indicating the dip direction. The dip angle is written next to the tick mark. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. View source.<\/a><\/figcaption><\/figure>\n

    It 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 is 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 because you could draw a horizontal line in any of an infinite number of directions on the horizontal surface of the wall. Its dip would be 0\u02da.<\/p>\n

    When reporting the dip, include the direction. For example, if the strike runs north-south and the dip is 30\u02da, it would be necessary to specify \u201cto the west\u201d or \u201cto the east.\u201d\u00a0 Similarly if the strike is northeast-southwest and the dip is 60\u02da, it would be necessary to say \u201cto the northwest\u201d or \u201cto the southeast.\u201d In the case of the vertical wall with a dip angle of 90\u02da, there is no dip direction. The dip points straight down, not toward any compass direction.<\/p>\n

    Measurement of geological features is done with a special compass that has a built-in clinometer<\/strong>, which is a device for measuring vertical angles. The strike is measured by aligning the compass along a horizontal line on the surface of the feature (Figure 13.36, left). The dip is measured by turning the compass on its side and aligning it along the dip direction (Figure 13.36, right).<\/p>\n

     <\/p>\n

    \"\"<\/a>
    Figure 13.36<\/strong> Measurement of strike (left) and dip (right) using a geological compass with a clinometer.\u00a0 Source: Steven Earle (2015), CC BY 4.0. View source left<\/a>\/ right<\/a><\/figcaption><\/figure>\n

    Strike and dip are used to describe any other planar features, including joints, faults, dykes, sills, and even the foliation planes in metamorphic rocks.\u00a0Figure 13.37 shows an example of how we would depict the beds that make up an anticline on a map. The 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 beds in the middle are horizontal; this is denoted by a cross within a circle on the map.\u00a0The dyke is dipping at 80\u02da to the west.\u00a0The hinge line of the fold is denoted with a dashed line on the map, with two arrows pointing away from it, indicating the general dip directions of the limbs.\u00a0 If it were\u00a0a syncline, the arrows would point inward toward the line.<\/p>\n

     <\/p>\n

    \"\"<\/a>
    Figure 13.37<\/strong> A depiction of an anticline and a dyke in cross-section (looking from the side) and in map view (or plan view) with the appropriate strike-dip and anticline symbols. Source: Steven Earle (2015), CC BY 4.0. View source.<\/a><\/figcaption><\/figure>\n

     <\/p>\n

    \n

    Putting Strike and Dip on a Map<\/strong><\/p>\n

    This cross-section shows seven tilted sedimentary layers (a to g), a fault, and a steeply dipping dyke.<\/p>\n

    \"\"<\/a>
    Figure 13.38<\/strong> Practice with strike and dip symbols. Source: Steven Earle (2015), CC BY 4.0. View source.<\/a><\/figcaption><\/figure>\n
      \n
    1. Place strike and dip symbols on the map to indicate the orientations of the beds shown, the fault, and the dyke.<\/li>\n
    2. What type of fault is shown?<\/li>\n
    3. What kind of stress created the fault?<\/li>\n<\/ol>\n

       <\/p>\n<\/div>\n

       <\/p>\n

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