S - Z

Sea Floor Spreading The process by which oceans are formed at divergent (constructive) plate margins. New oceanic crust is formed as two oceanic plates move apart. Radiometric dating and fossil evidence shows that the sea floor becomes progressively older in both directions away from mid ocean ridges. First proposed by Harry Hess following echo sounding work to reveal the topography of the ocean basins.

Slab Pull As oceanic lithosphere cools, it becomes denser and thicker. At a convergent plate boundary the oceanic lithosphere sinks beneath the adjacent plate forming an ocean trench and subduction zone. As a result of its own weight, the descending plate is pulled by gravity through the mantle asthenosphere, which is hotter and less rigid. This force is known as slab pull. It is believed to be the major force driving plate motions.

Stoping Stoping is a mining term used to describe the removal of overlying roof rock so that it falls to the floor of a mine. As magmas rise up, they mimic the same process, exploiting weaknesses such as joints, bedding planes and faults, breaking off blocks of the country rock that then fall through the rising magma onto the floor of the magma chamber.

Subduction Zone A zone where oceanic lithosphere is recycled back into the mantle. At the surface the subduction zone generally coincides with the bottom of oceanic trenches. At depth, the subduction zone is marked by earthquakes, some of which are deeper than 400 km.

Tectonic plate see lithospheric plate

Tertiary Volcanic Province An area of extensive basaltic volcanism in the north west of the UK which marked the opening of the North Atlantic Ocean approximately 57 million years ago. The Giant’s Causeway in Northern Ireland, and the geology of eastern Greenland, are clear evidence for this event.

Tethys Ocean The ocean that opened between the continents of Gondwana and Laurasia during the breakup of Pangea.

Thrust Fault A reverse fault at an angle of less than 45 (but generally at a much shallower angle). Thrusts indicate horizontal shortening by compression. They are often found near convergent (destructive) plate margins, but may occur some distance from them. Examples include the Lizard Thrust in Cornwall and the Moine Thrust in Scotland.

Transform Fault A transform fault marks a conservative plate margin, i.e. one in which two plates slide past one another and crust is neither created nor destroyed, but is conserved. The San Andreas Fault is a transform fault that separates the Pacific Plate from the North American Plate. Transform faults cut the entire lithosphere and are most common in the oceans, where the segments of a mid-ocean ridge are connected together by active transform faults. The Great Glen Fault in Scotland is a fossil transform fault lying within a continent.

Triple Junction The point where three lithospheric plates meet. It has been suggested that mantle plumes are responsible for the upward doming of the crust at these locations, producing a three-way radial fracture. An example is the Red Sea, Gulf Aden and East African Rift Valley.

Tuzo Wilson John Tuzo Wilson was a Canadian geologist who synthesised all the plate tectonic components into a logical cycle. The Wilson Cycle starts with rifting followed by sea floor spreading, then subduction and continental collision and accounts for the formation and break up of supercontinents. He was also the first to recognise the significance of hot spots and transform faults.

Variscan orogenic belt The remains of an ancient orogenic event in Devon and Cornwall. This involved the collision of Gondwana and Laurasia to form Pangea. Evidence for this collision is provided by the Lizard Ophiolite, the Cornubian granite batholith and the intensely folded sedimentary rocks at Bude. The deformation continues under the Mesozoic and Cainozoic rocks of southern England and re-emerges in France and Germany. (Also called the Hercynian and Amorican event).

Vine Fred Vine was the PhD student of Drummond Matthews at Cambridge University. Together, they were the first to interpret correctly the pattern of normal and reversed magnetic anomalies in ocean floor basalts (eg in the Pacific Ocean floor next to California) as the result of sea floor (ocean-floor) spreading.

Volcanic Island Arc A chain of volcanoes generally with an arc shape, which runs parallel to an oceanic trench at a convergent (destructive) oceanic-oceanic plate margin. The islands of the Caribbean are a good example where the Atlantic ocean-floor is being subducted westwards below the Caribbean plate.

Wegener Alfred Wegener (1880 - 1930) was a German meteorologist, polar researcher and geophysicist, who proposed the theory of Continental Drift in his 1915 work, 'The origins of continents and oceans'. He died during his fourth expedition to Greenland.