Short wavelength folds formed within a larger wavelength fold structure — normally associated with differences in bed thickness. A good analogy is bending a phone book, where volume preservation is accommodated by slip between the pages of the book. Typically, folding is thought to occur by simple buckling of a planar surface and its confining volume. The volume change is accommodated by layer parallel shortening the volume, which grows in thickness.
Folding under this mechanism is typically of the similar fold style, as thinned limbs are shortened horizontally and thickened hinges do so vertically. If the folding deformation cannot be accommodated by flexural slip or volume-change shortening buckling , the rocks are generally removed from the path of the stress. This is achieved by pressure dissolution, a form of metamorphic process, in which rocks shorten by dissolving constituents in areas of high strain and redepositing them in areas of lower strain.
Folds created in this way include examples in migmatites, and areas with a strong axial planar cleavage. Thursday, November 11, Sign in. Forgot your password? Get help. Password recovery. Geology Page. Home Latest News Video. Debris Flow Dynamics. Sampling Hot Molten Lava. Incredible moment Anak Krakatau erupts, Oct Download Google Earth For Free.
Remote Sensing Downloader. Thunder Egg. Home Geology Geological Folds. Geology Top Posts. Share on Facebook. Credit: W. Freeman and Company A wave-like geologic structure that forms when rocks deform by bending instead of breaking under compressional stress. Tightness of folding The tighness of folds can be described as open limbs dip gently , tight limbs dip steeply or isoclinal limbs are parallel.
Orientation of axial plane The orientation of the axial plane relative to the horizontal together with the orientation of fold limbs allow subdivision into upright axial plane vertical, limbs symmetric , overturned axial plane moderately inclined, one limb overturned , or recumbent axial plane near horizontal, one limb inverted. Recommended For You Research provides understanding about expansion and contraction of the tropical rain belt.
The axis is an imaginary line that marks the center of the fold on the map. In map view, a syncline appears as a set of parallel beds that dip toward the center. In a syncline the youngest beds, the ones that were originally on top of the rest of the beds, are at the center, along the axis of the fold. Anticlines and synclines form in sections of the crust that are undergoing compression, places where the crust is being pushed together.
A plunging anticline or a plunging syncline is one that has its axis tilted from the horizontal so that the fold is plunging into the earth along its length. Plunge direction is the direction in which the axis of the fold tilts down into the earth. In map view, a plunging anticline makes a U-shaped or V-shaped pattern that points, or closes, in the direction of plunge. A cross-section at a right angle to the axis of a plunging anticline looks the same as an anticline. In map view, a plunging syncline makes a U-shaped or V-shaped pattern that opens in the direction of plunge.
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform , which is a purely descriptive term for any fold that is convex up. Therefore if age relationships between various strata are unknown, the term antiform should be used. On a geologic map, anticlines are usually recognized by a sequence of rock layers that are progressively older toward the center of the fold because the uplifted core of the fold is preferentially eroded to a deeper stratigraphic level relative to the topographically lower flanks.
The strata dip away from the center, or crest , of the fold. Figure 2. Anticline with syncline visible at far right. Note the man standing before the formation, for scale. If an anticline plunges i. Anticlines are often flanked by synclines although faulting can complicate and obscure the relationship between the two.
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. Folded 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 This photograph shows folding in the same area of the Rocky Mountains as Figure Parks and Plates.
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