Divergent (Constructive) Margin A plate margin where two oceanic plates are moving away from each other. Divergent margins are responsible for the formation of ocean basins, which start out as rift valleys e.g. the African Rift. As the plates move apart, pressure on the underlying mantle is reduced and it partially melts to form basalt magma. New oceanic crust is formed at the ridge then moves away in both directions, cools and subsides. These margins are characterised by shallow focus earthquakes and basaltic volcanism. An example is the Mid Atlantic Ridge.
Dolerite A medium grained intrusive igneous rock, similar in composition to basalt, but with a slower rate of cooling. It is typically is associated with intrusive igneous bodies such as dykes and sills.
Flood basalts Flood basalts form as a result of large-scale volcanic eruptions of fluid basalt in the ocean or on the continent. They form extensive plateaus in many continents composed of multiple layers of basalt lava erupted over a relatively short time (1 million years or even less) to form a “trap” (step-like) landscape. They are associated with a rising mantle plume and continental rifting which leads to decompression melting. Examples include the Deccan Traps of India, Siberian Traps of Russia and flood basalts of Iceland. They are often suggested as the cause of mass extinction events in the fossil record.
Focus (earthquake) The point of fracture of rocks at depth within the Earth, giving rise to earthquakes. These are classified as shallow (<70 km), intermediate (70 – 300 km) and deep (>300 km).
Folding The process where rocks are deformed by compressional forces and associated with a shortening of the crust. The rocks show plastic deformation and flow to form anticlines (arch-shaped) and synclines (trough shaped, like kitchen sinks). Associated with convergent (destructive) plate margins.
Gabbro A coarse grained intrusive igneous rock, similar in composition to basalt, but which cool very slowly to produce large crystals. It is typically is associated with large scale intrusive igneous bodies (plutons and batholiths).
Gondwana Translated as ‘the forest of the Gonds’ (an Indian tribe) this is the name given to the large land mass of the southern hemisphere that consisted of South America, Africa, Arabia, Madagascar, India, Sri Lanka, Australia, New Zealand and Antarctica all joined as a single continent.
Granite A coarse grained igneous rock comprising the minerals quartz, feldspar, and mica. It corresponds to the average composition of the continental crust. It is formed at convergent (destructive) plate margins, typically in fold mountain (orogenic) belts.
Heat Flow A measure of the heat being conducted through the surface rocks of the Earth. Measurements are taken down boreholes and the highest values correspond to mid-ocean ridges (divergent plate margins), whilst the lowest values occur at ocean trenches (convergent plate margins).
Hess Harry Hess proposed the idea of Sea Floor Spreading, following echo sounding work to reveal the topography of the ocean basins.
Hot Spot An area of abnormally intense active volcanism thought to be underlain by a mantle plume. Many hot spots, for example Hawaii are located in the middle of a lithospheric plate whilst others such as Iceland are located on divergent (constructive) plate margins.
Iapetus Ocean The Palaeozoic ocean that existed in the southern hemisphere between the continents of Laurentia, Baltica and the micro-continent of Avalonia. Closure of this ocean resulted in the Caledonian Orogeny.
Intrusion Igneous rocks that crystallise beneath the Earth’s surface. Smaller scale intrusions may be called dykes and sills; the very largest are plutons and batholiths.
Laser Measurements A method that uses lasers to measure the distance between two or more points. Successive measurements are sufficiently accurate to show how fast plates are moving
Laurasia The name given to the large land mass of the northern hemisphere that consisted of North America, Greenland, Europe and Asia all joined as a single continent.
Laurentia An ancient continent, comprising what is now Scotland, Greenland and North America, on the western margin of the Iapetus Ocean during the Caledonian Orogeny.
Lithosphere The outer cool, rigid and brittle layer of the Earth. It comprises the crust (oceanic or continental) and part of the upper mantle. The base of the lithosphere is marked by a temperature of 1300o C. It has an average of thickness of 100 km but varies from just 1 to 2 km thick at mid-ocean ridges up to 300 km thick beneath mountain ranges. It is dived up into a number of lithospheric plates.
Lithospheric Plate A segment of the lithosphere which has earthquake activity along its margins and in certain situations, eg when associated with subduction zones. Lithospheric plates can consist of just oceanic areas (Nazca Plate) or a combination of oceanic and continental areas (Eurasian Plate). Seven major, eight minor and numerous micro plates have been identified.