Diastrophism

Diastrophism or tectonism is the group of mechanisms by which large-scale portions of the Earth's crust are deformed by natural processes. Such diastrophic processes lead to the formation of continents and ocean basins, mountain systems and rift valleys, and other related features by the chief mechanisms associated with the plate tectonic movements of the lithospheric plates of the planet. The study of diastrophism, or of tectonic processes, is the central unifying principle in much of modern-day geology and geophysics.

Five main kinds of diastrophic processes occur:

• 1.
  Orogeny, in which narrow mountain belts are formed
• 2.
  Epeirogeny, which is broad regional uplift or down warping of the crust
• 3.
  Isostasy, which is broad down warping or uplift related to loading and unloading of the crust
• 4.
  Eustasy, which is worldwide sea-level changes from addition or removal of water or through tectonic change in ocean basin volume
• 5.
  Igneous processes, which include intrusive ones, or plutonism, which are emplacements of molten magma at depth in the Earth, and extrusive or volcanic emanations of lava and pyroclastics on the surface.

The main driver of all diastrophic processes is the movement of the great tectonic plates, whereby new rock is created at the spreading centers and old rock subsides down subduction zones to be melted and partially recycled.

Mountain building orogenesis is the group of processes whereby plate tectonics drives up the great ranges as the moving lithospheric plates rift apart in some places and collide in other places. Thus there are five main kinds of orogenies:

• 1.
  Uplift and rifting of a continent above a spreading center, which produces moderatesized mountain uplifts and volcanoes, much like those in East Africa
• 2.
  Island arc growth into a cordillera above a subduction zone at a continental margin, as happened with the ancient Appalachian mountains
• 3.
  Collision of a volcanic island arc with a continent, as happened in the Andes and elsewhere
• 4.
  Collision of two continents, as happened with India as it moved into Asia and pushed up the Himalayas
• 5.
  The tectonic aneurysm, as happened at both ends of the Himalaya, where great river systems such as the Indus and the Tsangpo-Brahmaputra were captured and diverted so that their ferocious denudation unloaded the crust so quickly that its pressure release at depth resulted in igneous and metamorphic effects and a mountain pop-up on the surface

Epeirogeny in which the crust is broadly and slowly up-warped or down-warped without significant faulting, folding, metamorphism, or igneous activity and other tectonism is not well understood. It may reflect dynamic effects of motions in the mantle below the crust, phase transitions in the mantle that the crust accommodates, or be the result of variations in the white-hot and plastic aesthenosphere below the lithosphere as the surficial plates move laterally over it. In any case, areas such as the Colorado Plateau and much of Central North America that were once below sea level are now well above it, which reflects their epeirogenesis.

Isostasy, the balance in Earth's crust, which is likened to icebergs floating in water, is the situation wherein any great load such as water or ice added or removed from the crust will result in up-warping or down-warping as a result. The great glacier masses on North America and the Scandinavian countries during the Pleistocene, for example, greatly depressed the crust, with the result that Hudson Bay and the Baltic Sea still represent remnants of that down-warping once the ice melted away. Compensatory isostatic uplift is slowly under way in both places to restore the crust to the higher level it was at once prior to the advent of the glaciation. Similarly, in the state of Utah, Great Salt Lake is the small remnant of the once-huge, Lake Superior–sized, Lake Bonneville of the Pleistocene. Prominent wave-cut beach lines that are now high and dry were established at the high stand of this lake. Those old beach lines around islands in the deeper center of Lake Bonneville are now much higher in elevation than [Page 737]those on the lake's periphery, which is a reflection of the greater isostatic rebound where the water was once much deeper.

...