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Boudinage
The Feature is found in Norway, Known as Boudinage structure commonly found in folded strata. With its rich geological history and prominent mountain belts like the Caledonian orogeny, Norway is an excellent place to observe such features. These structures provide valuable insights into ancient tectonic settings’ stress, strain, and metamorphic conditions.
This is a fascinating geological feature! Amphibolite boudins within quartz schists form when layers of amphibolite (metamorphosed basalt) are subjected to intense stretching during deformation. The stronger amphibolite layers resist deformation and break into discrete “boudins,” resembling sausage links, while the surrounding quartz schists flow and deform plastically due to their lower strength.
Description
The boudin layer consists of “strong” amphibolites surrounded by relatively weaker quartzites. Notice how the layering of the quartzites is deflected at the “pinches,” giving the impression that they flowed into the gaps between the boudins during extension. This phenomenon occurs during ductile deformation, where rocks gradually flow in a plastic manner over millions of years.
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Different rock types exhibit varying strengths during deformation, influenced significantly by factors such as temperature, pressure, and—importantly—the presence of water. Boudinage is a common geological structure that helps geologists determine which rocks are stronger relative to others and provides clues about the physical conditions under which deformation occurred.
What is Boudin
Boudins, derived from the French word for “sausage,” are fragments of original rock layers that have been stretched and segmented. They form in layers that are stronger and more resistant to deformation (i.e., more competent) compared to the surrounding rocks.
Boudinage is a structure where the competent rock layer is formed into sausage-shaped boudins or lenslike shapes compared to the incompetent surrounding rock bed. Boudin is a geological term for structures formed by extension, where a rigid tubular body, such as hornfels, is stretched and deformed amidst less competent surroundings.
Boudinage structures contain a rigid tubular body that has been stretched and deformed where embedded within more deformable (less competent) rocks. Pinch and swell structures, also called drawn or necked boudins, are a subset of boudins that retain the continuity of the drawn layers but show variable thinning of the original layer thickness. A structure is common in strongly deformed sedimentary and metamorphic rocks, in which an original continuous competent layer or bed between less competent layers has been stretched, thinned, and broken at regular intervals into bodies resembling boudins or sausages, elongated parallel to the fold axes.
The eye-shaped boudins mentioned in the article are called “pinch-and-swell” structures. In this case, the boudins are not entirely separated but remain connected by a thin amphibolite layer, resembling a “pinch” made by a finger. This structure indicates that both the boudinated layer and the surrounding rocks deformed in a ductile manner.
