Some basic ideas.

New shapes

Shape image.Many rocks show more than just location fabric. Individual grains, rather than having near spherical shapes, can be strongly flattened ellipsoids. Commonly the orientations of the strongly ellipsoidal grains line up creating a new rock structure. In this case the fabric is defined by the shapes of grains and so this type of organisation has the general name: SHAPE FABRIC.

An aside: lines and planes; lineation and foliation.

GabbroIn general the intensity of the shape fabric increases as the rock deforms. In some rocks this can happen to such an extent that the original location fabric is impossible to see. Shape fabrics are important to structural geologists because they allow some qualitative measure on the amount of deformation. But they can also be used to understand geometrically how the rock has been distorted.

Granite in the Alps.Consider a ball of something soft - like plasticene. It has an original spherical shape. But this can be distorted into ellipsoids of different shapes. The shapes of ellipsoids can be described in terms of the relative lengths of the long axis, short axis and an intermediate axis (creating three dimensions with the axes at 90 to each other). If you push straight down on it the plasticene becomes pancake-shaped. The technical adjective for this is oblate. Here the short axis is reduced (of course) while the intermediate and long axes are about the same (and increased). To a squash in one direction results in a stretch in two directions. If a rock is made up of grains of this shape that are all aligned it will have a strong PLANAR fabric.

Gabbro.Now let's imagine another experiment with our plasticene ball. Rather than squashing it we can draw it out into a cigar shape. The technical adjective for this is prolate. Now the lump has a single long axis and two short axes that are more or less equal. If a rock is made up of grains that are all aligned it will have a strong LINEAR fabric. The rock itself would have the structure of a bunch of pencils (or drinking straws).

Granite.Finally we can perform another experiment with our plasticene ball smearing it by moving our hand across it on a table top. The ball will deform into an ellipsoid that is both flattened (increasingly parallel to the table-top) and elongate in the direction we smear in. The ellipsoid now has three axes of different length - rather like a pitta bread! In an ideal example the squashing in the direction of the short axis is matched by an elongation in the direction of the long axis, with the intermediate axis remaining unchanged. The technical term for this type of distortion is PLANE STRAIN.

An aside: tectonites

The various types of shape fabric in rocks represent important information for understanding deformed rocks. These uses are described elsewhere.

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