Definition - What does Axial Surface mean?
An Axial Surface is a surface which is defined by joining all the hinge lines of folding surfaces that are compacted together. When found planar in strike and dip it is called an axial plane. When the Axial Surface gets intersected by another surface, it is referred to as an axial trace. There are different types of folds beneath the earth's surface and the Axial Surface is of two types – planar and non-planar. The folds in a rock can be formed because of pore pressure, hydrostatic pressure, temperature gradient etc.
Petropedia explains Axial Surface
Different forms of fold in the earth's bed are formed over years when the bedrock is exposed to a number of conditions such as hydrostatic pressure, stress, and temperature to name a few. When hinge lines connect with each other within a fold and get stacked together, they form an Axial Surface. The Axial Surface is of two types – planar and non-planar. A planar Axial Surface is denoted by a strike and dip of the surface plane; this is called an axial plane. When non-planar, the Axial Surface gets intersected by another surface and is referred to as an axial trace.