Because of the Andean Uplift (see the ast two posts), many changes took place in the narrow western area of Andean South America. One of those changes would have been rivers which would have flowed from the original mountains down to the sea but with the flattening of those mountains and the rising of new mountains “whose height is great,” would have changed the origin points of most rivers, or at least changed the direction of flow toward the sea.
Recently, Sandra Villacorta (left), a Research Fellow, College of Engineering, Charles Darwin University with a Ph.D in Modelling and Environmental Risk Analysis, and an MSc degree in Geological Engineering, has completed extensive field work for the Polytechnic University of Madrid (Universidad Politécnica de Madrid). She and her colleagues, who studied the dynamic geologic processes of the Apurímac Region, including the crustal tectonic rock cycle, completed extensive mapping and georeferencing of the processes involved with the Apurímac River—referred to by locals as the Divine River, or the River of Divinity.
This work referenced the vulnerable areas for mass movements, especially in the Abancay, Andahuaylas or Chalhuanca districts in the area west of Cuzco, and extending south between that city and Ayacucho—an area of steep canyons, a strong river, and mountains. In other words, a narrow strip of wilderness.
Red Circle: The Abancay, Andahuaylas and Chalhuanca Districts where the georeferencing processes are located, making this entire area a critical geologic zone, where major slides, flows and mountain movements have occurred
In this Apurímac region, thirty-two critical zones of direct geologic hazards of severe earth and mountain movement have been identified by Ingemmet, using geologic studies, satellite photography and extensive field work. While current work in the study is meant to relocate and prevent future loss of life from severe geologic damage, it is important to consider the overall area of geologic weakness as a historic factor in understanding the region roundabout the Apurímac River. This area of ancient removal deposits and unstable rock masses that underlie the current concerns for future damage to the area rests in the unstable area dating back into pre-historic times, no doubt clear to the period of the Andean Uplift.
The geologic structural properties that make up this region may well have been permanently eroded due to the changes that took place during the Uplift some two thousand years ago, leading to the continuing and worsening instability of the region today. It should also be noted that the north and central parts of the Apurimac Region (shown in the red circle on the map above), are more likely for the occurrence of landslides and earth movement today as a result of till movement and colluvial and till materials, mostly with steep slopes and substrates formed by slates, phyllites, limestones, sandstones with interbedded shales, siltstones and mudstones, and conglomerates, tuffs and pyroclastic rocks.
Change in some rivers could result in a change in the Sidon River as a result of mountains changing
It should be noted that, according to geologists, subduction orogeny (subduction zones developing belts of deformation in the overriding plate’s crust, which in part leads to mountain building, has been occurring since the establishment of the continental area of South America). These subduction zones, the area where an oceanic plate is being forced down into the mantle by plate tectonic forces, causing enormous friction between the plates, has resulted in a continuous movement of the continental area, both those parts above the surface and those beneath, throughout pre-historic times, as well as the more modern historic period.
When the tectonic plates collide the underlying plate is consumed into the Earth’s mantle, creating a hot magma that erupts from volcanoes on the surface of the overlying plate. These volcanoes form a volcanic arc—a chain of volcanoes, at a depth of about 60 miles and hundreds to thousands of miles long, that form above a subduction zone, that is, an island volcanic arc forms in an ocean basin via ocean to ocean subduction
When two oceanic plates collide against each other, the older and therefore heavier of the two subducts beneath the other, initiating volcanic activity in a manner similar to that which occurs at an oceanic-continental convergent plate boundary and forming a volcanic island arc.
Showing that tectonic plate subduction is an ongoing process of learning, Dr. Sev Kender (left), a Research Fellow from the School of Geography at The University of Nottinghamin in Britain, stated: ““We found the crust to be much younger than expected, a stunning discovery indicating that we needed to readjust our ideas of how the subduction zone formed. The crust has chemical characteristics indicating it was formed at the time the subduction zone started, rather than much earlier. The crust may have formed in an extensional setting through seafloor spreading, in some ways similar to that formed at mid-ocean ridges today, although in this case near the newly-formed subduction zone.”
With this understanding, Kender went on to say: “We found that in plate tectonics, as opposed to the previously believed theory of long-time (millions of years) development of subducting crusts being induced, found that the crust can form spontaneously along a previous line of weakness without previously being uplifted, thus suggesting an immediate development of subduction not previously understood (Richard J. Arculus, et al, “A Record of Spontaneous Subduction Initiation,” Nature Geoscience Vol.8, 2015, pp728-733).
Mid-ocean ridges, which are found in all ocean basins, are where fresh new oceanic crust is formed and are the opposite of subduction zones. There are numerous ‘transform faults’ near ridges today, enormous fractures through the crust that form due to the spreading plates interaction with the curvature of the earth.
Such understanding shows a remarkable change in thinking by scientists about the time involved in the subduction process. The Mantle Plume forms and as it rises, and drives the mantle upward, lifting the surface of the land upward—with the spontaneous movement, this can result in sudden rising of the surface, quickly forming mountains. The larger the plume that falls away, the higher the mountain-building.
While it is understandable that the uninformed observer of geography considers landforms to have been fairly constant throughout history, these subductions occur because Earth has limited surface areas, convection and conduction of heat from the core as the Earth cools off means that denser basaltic ocean plates have to subduct until the oceanic crust is under a high enough pressure and temperature that minerals start to melt.
This, of course, has caused the altering of plate tectonics, the continents, mountains, and topography of the Earth in general. Some areas, especially along the Pacific Rim where subduction occurs more readily, resulting in earthquakes and volcanic action, are far more active, and thus, far more altering than areas, such as Europe, Asia and Africa. Obviously the subduction Zone Physics, that is the sinking of the oceanic lithosphere (sediments, crust, mantle), by contrast of density between the cold and old lithosphere and the hot asthenospheric mantle wedge, while not the only one, is the strongest force to drive plate motion and is the dominant mode of mantle convection.
(See the next post regarding the Andean Uplift and the Apurímac River)
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