Plate Tectonic Terms A-D

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Abyssal Sediments

Deep ocean-bottom fine grained sediments beyond the direct influence of the land. [Diag of diff depths + calcite sol]

Accretionary Wedge

Pile of sediments, characterized by repeated thrust faults, which accumulates on the oceanward side of a subduction zone. It grows due to sediment being scraped off the descending plate at a subduction zone.

Alkali basalt

Basalt containing normative nepheline and olivine.


Metamorphic rock consisting mainly of amphibole and plagioclase with little or no quartz.


A fine grained intermediate volcanic igneous rock characterised by the presence of oligoclase or andesine. Their chemistry and mineralogy are closely similar to those of the diorites. Porphyritic varieties are fairly common, both ferro-magnesian minerals and feldspars occurring as phenocrysts - the latter commonly showing zoning.


The earlier part of the Precambrian between 4000Ma and 2500Ma.


Partially molten layer below the lithosphere that is marked by low seismic wave velocities and high seimic attenuation.

Back Arc Basin

Basin located on the over-riding plate behind the volcanic island arc at a subduction zone.


Fine grained, dark, mafic igneous rock composed largely of plagioclase feldspar and pyroxene.

Benioff Zone

Inclined zone of earthquake foci that dips beneath an Island Arc or active continental margin at approximately 45'.

Blue Schist

A low temperature, high pressure regionally metamorphosed rock containing abundant glaucophane.


Group of high MgO (> 6 wt% MgO) andesites apparently restricted to fore arc regions, which suggests that rather special conditions are required for their generation. The most extensive occurences are in the Bonin Islands.


Central part of the Earth extending from the Gutenberg discontinuity with the mantle at 2900km to the centre of the Earth at 6370km. The composition is predominantly iron and nickel. The outer core, from about 2900-4980km, is fluid and the site of generation of the geomagnetic field, while the inner core, below 5120km, is solid. piccy from notes


That part of the Earth lying above the Mohorovicic Discontinuity. It is divided into two shells, a lower continuous one - the sima (acronym of <silica and magnesia) - and an upper, discontinuous layer - the sial (acronym of silica and aluminium


Flow banded, often dark coloured igneous rock that is the approximate extrusive equivalent of granodiorite or tonalite.
Its principal minerals are sodic plagioclase and quartz, which occur as phenocrysts in a glassy to microcrystalline groundmass. Mafic phenocrysts may also be present.


A coarse grained plutonic intermediate rock, consisting essentially of intermediate plagioclase feldspar (oligoclase to andesine), and one or more of the ferromagnesian minerals - biotite, hornblende, augite; quartz may be present in small amounts , up to 10%, and alkali feldspar may also occur, up to one third of the total feldspar. Sphene is a common accessory.

East Pacific Rise

A fast spreading ridge with geophysical studies indicating a possible continuous magma chamber.


High pressure rock containing garnet (pyrope) and pyroxene (omphacite).

Fore Arc

Region on the trench side of a volcanic arc.

Island Arc

Curved chain of volcanic Islands, many of which are located among the Circum-Pacific margins e.g. the Aleutian Islands and the Islands of Japan.
Diagram as an imagemap !?!

Island Arc Tholeiites

High silica basalts. Typical examples of tholeiitic arcs are the South Sandwich Islands, Tonga, the Izu Islands and the northern Lesser Antilles. To be completed


Outer rigid shell of Earth comprising the crust and upper mantle. Its base is at a depth of 2-3km under ocean ridges, increasing upto 180km beneath old oceanic crust. Beneath cratonic areas it is at least 250km thick and possibly as much as 500km.

Magnetic lineation

Linear magnetic anomalies of alternating polarity which run parallel to ocaenic ridges and are symmetrical about the crest of the ridge.

Mantle Convection

Plate Tectonic theory requires some system of horizontal forces that can cause plates to collide and combine or break up . The only possible mechanism seems to be convection. This is essentially motion induced by buoyancy, with lighter material rising and denser material sinking. In the mantle the convection is predominantly thermal convection in which the density variations are a result of temperature variations. There are two possible scales of convection in the mantle:
  1. Whole Mantle Convection
  2. Two Layer Convection

Marginal Basin

Basin located on the over-riding plate behind the volcanic Island arc at a subduction zone. The basin may be split into
  1. Active Zone - A zone of extension which splits the active arc and trench complex from the remnant arc and continent.
  2. Inactive Zone - Region containing submarine ridges (the remnant arcs).


Emplacement of part of the oceanic crust onto the continental crust at a destructive plate margin.

Ocean Basin

Part of the Earths crust that lies beyond the continental margin, including abyssal plains and hills , oceanic ridges, volcanic islands and trenches.

Oceanic Crust

Thin (~7km), young (<200Ma) crust of three layers
  1. Layer 1 Topmost layer consists of sediments.
  2. Layer 2 Pillow lavas underlain by sheeted dykes.
  3. Layer 3 Gabbro and underlying ultrabasic rocks.


A suite of mafic and ultra-mafic igneous rocks consisting of basaltic pillow lavas, dolerite dykes, gabbros and peridotites , associated with pelagic sediments, which represent segments of obducted oceanic crust.
Diagram Troodos, J.T. notes

Pillow Basalts

Spherical or ellipsoidal sturctures usually composed of basaltic lava. These formations are the result of the rapid cooling of hot, fluid magma that comes in contact with water, such as occurs when lava flows into the sea or into water saturated sediments e.g. beneath a glacier. Pillow lavas at Curacao

Plate Tectonics

Synthesis of geological and geophysical observations in which the Earths lithosphere is thought to be divided into seven large rigid plates, and several smaller ones, that are moving relative to each other.
Diagram wilson pg 4
The plates move over the weak asthenosphere and interact with each other along relatively narrow zones of volcanic and seismic activity. Internally, the plates are aseismic and may contain both oceanic and continental crust. The theory of plate tectonics formulated during the late 1960's, unified and expanded the earlier hypothesis of continental drift and sea floor spreading.
Seismic activity is concentrated along ocean ridges, ocean trenches and transform faults.
Three types of plate boundaries are recognised.
  1. Constructive
  2. Destructive
  3. Conservative
  1. Constructive Margin
  2. A constructive margin is a region of the Earth, such as a mid ocean ridge, where plates separate and magma uprises to form new oceanic crust.
    Diagram of mid Atlantic ridge
  3. Destructive Margin
  4. Oceanic trenches are the sites of destructive plate boundaries, where plates converge and oceanic lithosphere is consumed in subduction zones. There are three types of destructive margins:
    1. Ocean - Ocean boundary
    2. Where the oceanic parts of two plates converge, an island arc is formed on the overriding plate.
      Diag wilson pg 156
    3. Ocean - Crust boundary
    4. Where oceanic plate converges with a continental part of another, the oceanic plate is subducted beneath the continental plate. The edge of the continental plate is compressed and folded to form a mountain range.
      Diag. wilson pg 200
    5. Continental Collision
    6. Continents may collide if the oceanic lithosphere between them is subducted. The collision of two continents results in underthrusting and crustal thickening , leading to the formation of mountain ranges such as the Alpine - Himalayan chain.
    diag. J.T notes.
  5. Conservative margin
  6. The third type of plate boundary is the conservative boundary, where material is neither created or destroyed. These boundaries are transform faults where plates slide past each other.


A synthetic model mantle composition devised by A.E.Ringwood. It is proposed as representing the upper mantle composition from which basaltic magma could be derived on partial melting.

Remnant Arc

Inactive arc located behind the active volcanic arc and separated from it by an inter-arc basin.

Salt Dome

A dome like structure formed by the buoyant ascent of relatively low density evaporites through a sediment overburden by halokinesis. Often constitutes an effective hydrocarbon trap in combination with the deformed sediments. diag sedimentary rocks Pettijohn

Slab Pull Force

The mechanism whereby the gravitational pull of a downgoing slab in a subduction zone exerts a lateral force on the plate attached to the slab.

Seafloor Spreading

A hypothesis, proposed by the American geophysicist Harry Hess at Princeton University in 1960, that oceanic crust forms along the mid-oceanic ridge system and spreads out laterally away from it. Thermal anomalies and low seismic wave velocities over the mid-ocean ridges support the hypothesis. Linear, normal and reversed magnetic anomalies found in basalts generated at the ridge axis, where they acquited a thermoremnant magnetization as they cooled also lend support to the seafloor spreading hypothesis.

Subduction Flip

(Type in Def)

Volcaniclastic Material

Rocks formed by extrusive volcanic activity and containing fragments or a clastic fabric. ??