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Structural Geology

Columnar jointing

Vertical columnar jointing forms as a result of cooling, particularly of lava. The joint systems associated with cooling are typically polygonal in nature due to the cooling introducing stresses that are isotropic in the plane of the layer. A good example of columnar jointing can be see at Giant's Causeway in Ireland.

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Fault

A discontinuity over which there has been displacement of rock strata. See Reverse Fault, Normal Fault and Strike-Slip Fault. There are many terms associated with faults.

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Graben

A trench formed by subsidence between two parallel faults. The word is derived from grabe, the German word for "grave".

 

Melange

A melange is formed in the accretionary wedge as sediment and oceanic crust is scraped off the descending plate in a subduction zone. The melange comprises a strongly brecciated unit with large blocks of pre-existing rocks in a deformed fine grained matrix. Olistostromes are similar units but have a gravity sliding origin rather than a tectonic one. For a melange to be described as such it must be of mapable size, contain 'exotic' clasts (i.e. not derived from immediately adjacent units) and be matrix supported.

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Nonconformity

An unconformity that exists between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock was deposited on the pre-existing, eroded metamorphic or igneous rock.

Normal Fault

A fault caused by extension.

Schematic normal fault

Reverse Fault

A fault caused by compaction. A low angle reverse fault is often called a "Thrust Fault".

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Slickenside

A polished or smooth fault surface. The direction of the last fault movement can be determined by feeling which direction is smoothest with your hand.

Strike-Slip Fault

A fault caused by horizontal shear.

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Stylolite

A pressure-dissolution structure commonly found in chalks and other carbonates. The stylolite appears as a jagged line on the rock surface. Increasing pressure causes the carbonate ions to dossolve and migrate. This does not occur along a straigh line, bit along crystal boundaries, which in turn gives rise to the jagged appearence of the stylolite. See http://www.geo.lsa.umich.edu/~vdpluijm/animations/stylolite.mov for a video of formation.

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