Geologic joint
From Wikipedia, the free encyclopedia
A joint is a generally planar fracture formed in a rock as a result of extensional stress. Joints are distinct from faults because joints do not have any significant offset of strata either vertically or horizontally.
Joints form in solid, hard rock when a rock is stretched past its elastic modulus. When this happens the rock fractures, in a plane perpendicular to the extensional stress (the direction of finite stretch) and parallel with the minimum compressive stress, which is usually vertical (eg. the atmosphere above a column of rock, with a horizontal ground surface).
Joints are typically formed due to erosion of the overlying strata exposed at the surface. This removal of overlying rock results in extensional stresses and the fracturing of underlying rock. Joints can also form via cooling of hot rock masses, particularly lava, which form cooling joints, the underlying structure of columnar jointing or columnar basalts.
Measurement of joint patterns can be useful in characterising the tectonic history of extension through an area because of this relationship between the direction of finite stretch and the minimum compressive stress. Joint propagation directions can also be analyzed by characterising plumose structures on the faces of joints, which manifest as fan-shaped irregularities which originate at the point of rupture and propagate toward the site of minimum compressive stress.
