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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Gurmeet Singh posted an Question
July 28, 2020 • 05:21 am
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30 points
30 points
- IIT JAM
- Geology (GG)
Causes of plate motion (iv) gravity : ridges are close to isostatic equilibrium but sinks are characterised by topogiap trenches which shows the largest negativ
causes of plate motion (iv) Gravity : Ridges are close to isostatic equilibrium but sinks are characterised by topogiap trenches which shows the largest negative gravity anomalies. The gravitational difference may au building that ca to the plate motion.
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Sajan sarthak
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1.As fresh magma wells up at mid-ocean ridges to form new young, oceanic lithosphere, a higher elevation is formed at spreading ridges. 2.The new oceanic crust gradually cools and thickens with age and is pushed ‘downhill’ as new magma emerges from the active zone of divergence behind it (and thus adds distance from the ridge). This force is regarded as a secondary force and is referred to by some as ‘ridge push’. 3.Slab pull is thought to be a more significant gravitational force acting on plates. 4.In the current understanding of plate motion the movement is driven by the weight of cold, older, dense plate material sinking into the mantle at deep ocean trenches and pulling the rest of the plate slab with them as gravity causes them to slide downwards. 5.At constructive margin. Oceanic crust is thin, so the high-density mantle rocks occur at shallower depths than under the continents. This causes a general increase of the Earth’s gravity field over the oceans, giving positive gravity anomalies. However, over the ridge systems gravity decreases toward the axis so that a “negative” anomaly is superposed on the normally positive oceanic gravity anomaly. The effect is due to the local density structure under the ridge. It has been interpreted in terms of anomalous mantle material with density slightly less than normal. The density is low because of the different mantle composition under the ridges and its high temperature. 6.At destructive margin...Gravity anomalies across subduction zones have several distinctive features. Seaward of the trench the lithosphere flexes upward slightly before it begins its descent, causing a weak positive anomaly; the presence of water or low-density sediments in a deep-sea trench gives rise to a strong negative gravity anomaly; and over the descending slab a positive anomaly is observed, due in part to the mineralogical conversion of subducted oceanic crust to higher-density eclogite
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Rahul kumar Best Answer
Ridges are considered as +be gravity anomaly places because of higher lithospheric thickness than usual whereas trenches are considered as -ve gravity anomaly places because of lesser lithospheric thickness than usual. Now, since both (rigdes and trenches) are two major factors that drive plate tectonics, that's why any imbalance (+ve and -ve gravity anomaly) will cause plate movement. By movement of plates, the lithospheric plate would try to attain isostatic equilibrium.