Science Fair Project Encyclopedia
See also folding
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of plastic (i.e. permanent) deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales. Folds form under very varied conditions of stress, hydrostatic pressure, pore pressure , and temperature, as evidenced by their presence in soft sediments, sedimentary rocks, the full spectrum of metamorphic rocks, and even as primary flow structures in some igneous rocks. A set of folds distributed on a regional scale constitutes a fold belt , a common feature of orogenic zones.
Types of folds
- Anticline: linear, strata dip away from axial center, oldest strata in center.
- Syncline: linear, strata dip toward axial center, youngest strata in center.
- Dome: nonlinear, strata dip away from center in all directions, oldest strata in center.
- Basin: nonlinear, strata dip toward center in all directions, youngest strata in center.
- Monocline: linear, strata dip in one direction between horizontal layers on each side.
- Recumbent: linear, fold axis oriented at low angle resulting in overturned strata below the fold axis.
- Slump: typically monoclinal, result of differential compaction or dissolution during sedimentation and lithification.
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