A geological joint is a generally planar fracture formed in a rock from extensional stress.

Joints are typically formed due to unroofing of a rock which is currently exposed at the surface by erosion of the overlying strata. Joints are distinct from faults because joints do not have any significant offset of strata either vertically or horizontally. This article is about the geologic use of the term, for other uses see Stratum (disambiguation) Interstate road cut through limestone and shale strata in eastern Tennessee In geology and related fields, a stratum (plural: strata) is a layer of rock or soil with internally consistent characteristics that distinguishes it... Old fault exposed by roadcut near Harrisburg, Pennsylvania. ...

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. Look up lava, Aa, and pahoehoe in Wiktionary, the free dictionary. ... Basalt Basalt is a common gray to black volcanic rock. ...

Columnar jointing in the basalt of the Giant's Causeway in Ireland

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. 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. It has been suggested that Giants Causeway Legend be merged into this article or section. ... ...

Columnar jointed basalt in Turkey

Joint propagation directions can 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.

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