Understanding Orogeny: Formation of Mountain Ranges
Orogeny: The Birth of Mountains
Orogeny: The set of geological processes that give rise to large mountain ranges or orogens. They form on the borders of converging tectonic plates and, depending on the type of convergence, may be of the Andean and island arc collision type.
The Tectonic Orogeny Before Plate Theory: Fixism
Early orogenic theories did not consider the mobility of the masses. According to the fixist hypothesis, the distribution of continents and oceans has always been the same. The main fixist hypotheses were the fixist theory and the contractionary geosynclinal theory.
Theories of Mobility
The theory of continental drift emerged from the work of Alfred Wegener. Wegener’s theory was not well received at a time when scientists did not accept continental mobility, so the idea of mobile continents was banished in favor of fixism.
Orogenesis According to Wegener
Mountains originated from the refolding of continents and of the sediments deposited in marine basins caused by the movement of continents over the oceanic crust.
The Andean Orogenic Type
These are marginal mountain ranges, such as the Andes, which form due to the subduction of an oceanic plate beneath the continental edge of a mixed plate. They are located at the edges of continents and are characterized seismically by great activity and numerous active volcanoes.
Formation of an Andean-Type Orogenic Belt
Sedimentation
Sediment from the erosion of surface rocks is deposited in the continental margin, coinciding with the mature stage of the Wilson cycle in an ocean basin.
Folding
Subduction causes the folding of these sediments. In deep areas, they are under high pressures and temperatures, leading to metamorphism. High temperatures melt the rocks, causing associated volcanism. This is the last stage of the Wilson cycle.
Uplift
Compressive movement ceases when the range is pushed slowly up by the partially molten upper mantle rocks due to isostasy.
Collision Orogeny
These originate from continent-continent type convergence. For that reason, they are also called intracontinental. Ridge collision involves the disappearance of the ocean that lay between them.
The Formation of a Collision Orogen
Sedimentation
Sediment is deposited in the ocean basin lying between the two continents as they approach due to subduction of the oceanic edge of the mixed plate. This coincides with the oceanic basin reducing stage of the Wilson cycle.
Folding
The approach of the continental blocks causes the sediments deposited in the ocean basin to fold intensively. This is the close oceanic basin stage of the Wilson cycle.
Obduction
The oceanic crust disappears by subduction. The collision of continental blocks is resolved by obduction. Temperatures in metamorphic rocks are higher at depth. This is the final stage of the Wilson cycle.
Island Arcs
These are islands that originate from ocean-ocean type convergence. As a result of the collision, a mountain system is developed in the passive plate, and a trench forms.
Island Arcs: Location and Structure
They develop over the ocean due to the accumulation of volcanic products. This volcanic activity is related to the melting of rocks deep in some areas of the passive plate.
The Origin of Relief
Orogenesis
Creates relief through endogenic geological processes.
Gliptogenesis
All exogenic geological processes alter rocks.