For the sake of simplicity, the process of
continental evolution is considered in three successive stages.
STAGE I: THE EARLY VOLCANIC STAGE
The beginning of crustal evolution is marked by the formation of broad swells on the basaltic ocean floor. These represent the early stages in the development of volcanic islands. Development of these swells leads to block faulting, extrusion of lavas, and their protrusion above the sea level to form volcanic islands (Eg Solomon Is in SW Pacific). The islands are composed of pillow lavas ( basalts poor in olivine with spilites and keratophyres). Pyroclastic material is absent, but products of erosion have started forming.
By this time the island is quite large ( 150 Km
in length) and new swells are developing either in a linear or arcuate
arrangement with respect to the earlier swells. The older swells are undergoing
extensive erosion and much sedimentation. The sedimeents become folded, faulted
and deformed due to near vertical block faulting and gravity collapse.
STAGE II: THE GEOSYNCLINAL STAGE
The volcanic islands are by now well established and are enlarged by bodily uplift, volcanic accretion and sedimentation. Volcanic products are becoming more felsic and pyroclastic material is becoming prominent. Earliest plutonic rocks of granitoid texture and dioritic- granodioritic composition appear (in pipe or stock form). These are of shallow subvolcanic nature. These may be products of magmatic differentiation or transformation of the deeply buried pyroclastic rocks.
The oldest swells have become quite large ( Java
& Sumatra) with smaller intervening swells beginning to coalesce
(Aleutian-Malaysian). Volcanic festoons isolated from continents, or extending
outwards from these are characterized by igneous sedimentation (with formation
of reefs in appropriate climates). There is a mixing of volcanic material with
products of continental erosion around volcanic festoons bordering continents.
There is, therefore, a hybrid sedimentation on the continental side of the arc
(the miogeosyncline) while volcanic sedimentation progresses in the outward
seaward trough (the eugeosyncline). Vulcanism at this stage becomes more felsic.
STAGE III: SHELF AND SHIELD STAGE
The volcanic islands are by now welded to each other and also to the continental margins. Intense fault movement, compressional folding, more felsic plutonic intrusions, waning of vulcanism and rapid erosion are characteristic of this stage of crustal evolution. Plutonic rocks are granitic and pegmatitic in composition.
Continuing folding, faulting and varying degrees of metamorphism leads to the formation of a "Continental Shield". This stage is marked by outpouring of flood basalts and stabilization of the crustal segment. Erosion and peneplanation lead to the development of broad flat areas susceptible to inundation during sea-level rises resulting in extensive deposits of detrital sediments, reefs (carbonate), evaporites and chemical sediments. Movement of the shoreline in response to sea-level fluctuations leads to the interfingering of shallow water marine and fluviatile sediments, particularly in the lower reaches of braided streams, deltas and outwash fans. Mineral deposits forming at this stage include a variety of igneous and sedimentary ores, and deposits formed during the earlier stages undergo substantial metamorphism.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)