A - C

Abyssal Plain The flat area of the ocean basins between oceanic ridges and trenches or passive margins. Average depth of the abyssal plain is 4km below sea level. Typically the sedimentary rocks forming here are black shales and turbidites.

Accretionary Wedge (prism) A zone of deformed sediment made up of thrust slices scraped off a subducting oceanic plate and added onto the over-riding plate. These occur at oceanic-oceanic and oceanic-continental convergent (destructive) plate margins.

Alpine Orogenic Belt The result of the collision of the continents of Africa and Europe. The distant effects of this can be seen in the UK in the folded rocks of South East England: Wealden Anticline, Hampshire Basin and the tilting of the rocks along the Jurassic Coast.

Andesite A fine grained volcanic rock formed by a complex series of processes at convergent (destructive) plate margins. In the mantle, it is richer in silica and more viscous than the basalt from which it is derived. Andesitic magma typically produces explosive volcanic eruptions, such as those of Andean volcanoes and at Montserrat. Andesite is named after the Andes mountain chain.

Asthenosphere Layer of the Earth immediately below the lithosphere; this layer is hotter and weaker than the lithosphere above and is capable of plastic flow when stress is applied. The top of the asthenosphere is indicated when a temperature of 1300o centigrade is reached.

Basalt A fine grained volcanic rock formed by the partial melting of the mantle, typically at divergent (constructive) plate margins. Submarine eruptions of basalt produce pillow lavas which form the upper part of the oceanic crust.

Basin A depression in the surface of the Earth, in which sediments accumulate.

Benioff Zone An inclined zone of earthquake foci in the upper part of a subducting oceanic plate at a convergent (destructive) plate margin. These zones are sometimes named as Benioff-Wadati zones as Benioff and Wadati discovered them independently.

Caledonian Orogenic Belt The remains of an ancient fold mountain belt from approximately 490 - 390 Ma which trends NE - SW west through northern England and Scotland. It provides clear evidence of the closure of the Iapetus Ocean and plate collision in the UK during the Palaeozoic.

Contamination The addition of (any) rocks to a magma, creating inclusions that may or may not react with the magma.

Continental Crust A collective term for the crust that forming the continents; it has an average thickness of 35 km but can be up to 70 km under mountain ranges. Typically the composition is granitic with a density of 2.7 meaning that it is too buoyant to be subducted at convergent (destructive) plate margins. The oldest piece of continental crust is in Greenland and is 3,800 million years old.

Continental Drift Theory proposed by Alfred Wegener in 1915 to support the notion that the continents had changed position through geological time. Wegener was unable to propose a mechanism to explain how drift occurred and the idea was ignored for almost half a century.

Convection Currents Heat generated by the breakdown of radioactive minerals in the mantle is redistributed by currents that rise at the mid ocean ridges and descend at the ocean trenches. Convection currents were long thought to be responsible for driving plate motion but this is still the subject of intense debate (see slab pull).

Conservative Margin A plate margin where two plates slide past each other along a transform fault. The margin is characterised by shallow focus earthquakes but no volcanic activity. The San Andreas Fault Zone that separates the North American Plate from the Pacific Plate is an example; the Great Glen Fault in Scotland is a fossil transform fault. In this case both plates are moving in a north easterly direction but the Pacific Plate is moving faster than the North American Plate.

Convergent (Destructive) Oceanic-Oceanic Margin A plate margin where two oceanic plates collide and one is subducted to produce a volcanic island arc-oceanic trench system. The margin is characterised by andesitic volcanism and shallow, intermediate and deep focus earthquakes. An example is the Caribbean Islands (Montserrat) where the South American Plate is subducted westwards down the Puerto Rico Trench.

Convergent (Destructive) Oceanic – Continental Margin
A plate margin where an oceanic plate is subducted beneath a continental plate to produce an ocean-trench-mountain belt system. The continental margin is characterised by andesitic and rhyolitic volcanism and shallow, intermediate and deep focus earthquakes. As the oceanic plate is subducted, an accretionary wedge is formed. An example is where the Nazca Plate is subducted eastwards below the South American Plate along the Peru-Chile Trench. The Andes fold mountain chain is formed to the east of the trench.

Convergent (Destructive) Continental-Continental Margin A plate margin where an ocean closes and two continental plates collide. As continental plates are too buoyant to be subducted they deform and thicken on collision to form a mountain belt with the continental crust thickening to twice the normal average. These margins are characterised by shallow focus earthquakes, folding, faulting and regional metamorphism. In addition the base of the continental crust may melt to generate granitic magmas which rise and solidify as plutons. An example is The Himalayas, formed by the collision of India with South East Asia.

Crustal shortening The process that occurs in a fold mountain belt where the crust is laterally shortened by compression caused by plate collision. This reduction in length of the crust is accommodated by faulting, where one block is thrust over another, and/or by folding of the layers.