Geological Structures Definitions

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Undergraduate Geology - Part 1 (Geological Structures) Flashcards on Geological Structures Definitions, created by siobhan.quirk on 09/05/2013.
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Flashcards by siobhan.quirk, updated more than 1 year ago
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Created by siobhan.quirk almost 11 years ago
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Question Answer
Define bed a unit of sedimentation, which can vary considerably in thickness
Define bedding plane marks the break between beds. It represents a break in sedimentation, a change in composition or grain size, or a change in colour of sediments
Define dip the maximum inclination of a bed measured from the horizontal using a clinometer
Define strike the direction at right angles to the dip. There is zero dip in this direction. It is a bearing measured from north using a compass
Define apparent dip is a dip that is measured to be less than the maximum inclination
Define stress force applied to rocks
Define stress (change of length of line)/(the original length of line)
Define tension the force trying to pull rocks apart - the Earth's crust will be lengthened
Define compression the force trying to push rocks together - the Earth's crust will be shortened
Define shear forces forces which act along a plane in the rock and promote sliding along that plane
Define competent rocks are strong and brittles and tend to joint and fault
Define incompetent rocks are weak and plastic and tend to fold and develop cleavage
Define joint a fracture in competent rocks along which no observable movement has occurred
Define tectonic joints form as a result of folding and cause tension joints parallel to the axial plabne and cross joints on the limbs
Define cooling joints forms as a result of contraction on cooling of igneous rock
Define unloading joints often horizontal as well as vertical and form as a result of lower pressure near to the surface
Define fault a fracture in a rock along which there has been an observable amount of displacement
Define fault plane the surface along which movement takes place
Define throw the amount of vertical displacement measured vertically between the top of the same bed seen on both sides of the fault
Define fault dip the angle between the fault plane and the horizontal
Define hanging wall the side that lies above the fault plane
Define footwall the side that lies below the fault plane
Define downthrow the side of the fault that has moved down
Define upthrow the side of the fault that has moved up
Define slickensides the polishing and striations found on a fault plane indicating the direction of relative movement
Define fault breccia composed of fragments produced by rocks fracturing during faulting
Define fold limb the section of a fold between one hinge and the next, forming at the side of a fold
Define hinge the line along which there is a change in amount and/or direction of dip, forming the most sharply curved part of the fold
Define crest the highest point of a folded bed
Define trough the lowest point of a folded bed
Define axial plane a plane that joins the hinges of all the beds. It bisects the fold
Define axial plane trace the outcrop of the axial plane at the Earth's surface
Define plunge the angle of dip of the axial plane from the horizontal
Define antiform an upward closing fold
Define synform a downward closing fold
Define cleavage planes of weakness in a rock along which the rock will split
Which rocks does cleavage form in? incompetent sedimentary rocks such as shale or clay made from clay minerals
Where does cleavage form? perpendicular to the maximum pressure that forms a fold, so it at an angle to the bedding planes
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