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Plate Tectonics - common sites of vulcanicity accretionary prism and the high pressures and low
temperatures causes metamorphism which can create
The main source of the Earth's energy is the sun, and - cause up warping or doming of the crust greenschists and blueschists.
this radiant energy drives the cycles which redistributes The youngest island is volcanically active since they
sediments in the surface processes part of the rock are over the hot spot while the older islands have Volcanic Island Arcs:
cycle. moved away and have therefore become extinct. When one oceanic plate is forced under another
The thermal energy, which is part of the Earth internal oceanic plate, than an arcuate subduction zone is
heat, maintains the active earth beneath the surface. Destructive Margins (Convergent plates): created, this can occur if two oceanic plates converge,
The thermal heat occurs from the decay of radioactive This is where two lithospheric plates mover towards or if an edge of a plate meets an immoveable objects
metals, such as uranium and potassium in the each other, and there are three main types. and the plate snaps. The plate than starts to descend
subsurface rocks, together with a little heat from the Oceanic crust has a thin sedimentary layer of muds into the mantle.
primeval earth. passing down into pillow basalts, dolerite sheet The descending plate has a considerable influence on
Volcanic hot springs, such as Old Faithful, and the high intrusions and gabbros. the overriding plate, and the most notable one is the
temperatures of deep mines are evidence of a high Continental crust has a wider variety, but its overall creation of the arc of volcanoes et back by 10km.
temperature within the earth despite the cold and brittle composition is that of granite. from where the rim of the trench was created.
rocks upon the surface. The oceanic crust is denser because there is a greater The lavas that erupt are always intermediate andesite
Heat flows vary from place to place, mainly where thickness of the upper mantle beneath the oceanic lava.
magma is rising such as in mid-Atlantic ridges, island crust of the oceanic lithospheric plate. The descending slab of the subducted plate is much
arcs and hot spots and rarely occur in places such as *oceanic crust is doomed to destruction. Therefore, no cooler than the medium into which it is plunging. This
ocean trenches. oceanic crust is older than 200m.y old* creates conditions of higher pressure and lower
Below the Asthenosphere, the rise in geotherm is less After the oceanic crust is created at a constructive temperatures than the surrounding geological
rapid due to the molten rock that is below the core- margin it is carried along until it is destroyed by the environment, which causes metamorphism of the
mantle boundary. destructive margin. local rocks to occur.
By process, hot rock rises to replace cooler rock which Two kinds of metamorphism occur, greenschist
sinks, warms up, rises again, transferring the internal Andean Margins: metamorphism and blueschist metamorphism, which
heat of the Earth to the surface. When oceanic and continental crust collides, it is the can be useful in identifying ancient or old subduction
denser oceanic crust which will be subducted into the zones since the blueschist metamorphism occurs in the
Constructive Margins (Divergent plates): mantle. This causes melting and the newly created subduction zone, and the greenschist metamorphism
The magma which rises to the surface is deflected magma will begin to rise. This magma starts of as occurs from further in from the rim of the plate.
laterally due to the cold and brittle rocks and it causes basaltic but as it rises up through the granitic
sufficient drag on the underside of the lithosphere to continental crust it becomes contaminated by melting Himalayan Margins (continental collision):
tear it apart. some of the rocks as it passes. This is known as Continental plates can't be subducted due to their
This is called a constructive margin, and the main contamination by assimilation, and causes the lava composition; therefore, when two continental plates
forces are tensional forces which causes normal faults that erupts to be andesite since the silicon level has collide they will lock together and form larger and
which are associated with rift valleys such as the East risen. newer continents.
African Rift Valley. The subduction zone where the subduction of the New young fold mountains or cordilleras will form
These margins often form long, linear ridges of the oceanic crust takes place is a site of earthquake foci as along these margins, with a structural trend approx. at
floors of ocean basins which can be thousands of km in the plates grind past each other. right angles to the direction of the movement of the
length, and contains much volcanic activity. The The main stresses that occur are compressional so the plates.
volcanic activity is usually submarine in origin since man geological structures formed are folding and The Himalayan Mountain chain formed this way when
most constructive margins are found below sea level. reverse faults. India crashed into Asia.
The magma is basic, and therefore produces rocks The sediments between the two leading edges of the Igneous activity is at a minimum here, since there is no
such as basalt, dolerite, Gabbro and some peridotite. plates are piled high to for cordillera or young fold subduction as it is seen in the Andean margins and
Mid-ocean ridges are rifted in their axial planes with mountains such as the Andes and Cascade Mountains Volcanic Island arcs.
different spreading rates and plate rotation about a in America, which are often composed of granite Oceanic sediments are trapped by the collision and
pole. This causes the ridges to be offset or displaced batholiths. Volcanoes can be present, and can be of deformation may be extreme. Compressional forces
along large faults similar to wrench faults. These are acid r intermediate type. create structures such as folds, nappes and thrust
known as transform faults. An oceanic trench can be found above the surface faults. Rocks can also be affected by metamorphism
Hot spots occur where a plume of magma rises to the position of the subduction zone. so that both their mineralogy and texture will have
surface, and are best seen in the Hawaiian Islands The ophiolite suite is where irregular masses of the been altered.
They are described as;- oceanic crust, such as pillow lavas, dolerites and
deeper material being plastered or obducted against
- regions of high heat flow the inner walls of the trench. Is also known as the