Causes of uplift of Earth's Surface- making mountains 8. 09.12.18…
Causes of uplift of Earth's Surface- making mountains 8. 09.12.18
Deflection of the vertical
George Biddle Airy calculated this during his geodetic survey of India in 1853, which showed that there was a smaller deviation in in gravity than expected (based on the mass of Tibet topography alone) was because of a mass deficit at a depth. He hypothesised that this was because of thicker than normal crust (which has a low density) beneath the mountain.
Measure of how far the gravity has been shifted by local anomalies such as nearby mountains.
This concept best describes the correlation between high topography and thick crust was first described quantitatively by George Airy. Now refereed to as the Airy istostasy.
Why does crustal thickening cause uplift of the surface and hence mountains? - Archimede's Principle.
Isostasy is essentially the same as Archimede's principle, with vertical "columns" of lower density crust floating on higher density mantle. Mantle is not magma, it is a solid that deforms in a ductile manner at high temperature and low strain rates, so behaves like a fluid over geological time scales.
Definition- a narrow region of mantle material that is much hotter (and therefore less dense) than the surrounding mantle, so it will flow upwards to the surface.
Note plumes are not magma plumes, the are once again the solid mantle flowing as a ductile material at high temperatures and pressures.
The class example of a plume is the one under the Hawaiin islands.
Some melting does occur when the mantle material impinges on the base of the lithosphere, this is because;
Decompression melting - As the mantle moves upwards to areas of low pressure, the reduction in overlying pressure enables the rock to melt, leading to magma formation.
The additional heat of the plumes can lead to partial melting of the lithosphere and intra-plate magmatism (volcanoes).
Intra-plate magmatism- occurs within tectonic plates and is generally not related to plate boundaries and movements.
Topography of Africa
The high topography of Southern and East Africa is partly caused by this dynamic mechanism of Mantle Plumes.
Largest non-orogenic (process leading to mountain regions etc) region of elevated topography.
"anomalously" high compared to other continents.
Surrounded on 3 sides by mid-ocean ridges.
The lithosphere (shallow mantle) is cold and so seismically fast, so the topography of Africa cannot be caused by shallow thermal anomalies.
More detailed seismic studies show the lithosphere is cold down to 200-300km beneath the high plateau.
The surface heat flow is also low in Southern Africa. the Lithosphere is cold so it should be more dense and hence be anomalously low, but it isn't. why?
Because, if we look deeper into the mantle there is evidence of a huge mantle seismic anomaly that is interperated as a mantle "super" plume. There is a seismically slow region that rises upwards from the core mantle boundary beneath southern Africa towards Ethiopia.
This super plume, drives active upward mantle flow in the deep mantle that causes uplift on a continental scale
Example of Airy's model of isostasy- Tibetan Plateau
Explains why crustal thickness causes mountains.
Large plateau with a mean elevation of c. 5km.
The Bouguer gravity anomaly shows the actual measures of gravity at the surface, whether it is higher or lower than our geoid model.
There are negative values beneath Tibet, indicating that there is less mass than expected. So less gravity and mass than expected supports Airy's idea.
The crustal thickness of the moho under Tibet was shown to be 65-75km, much thicker than the "normal" crust of 35-40km. These estimates are based on analysing reflection of seismic waves off the base of the crust.
The same anomaly is shown for the Andes. So this relationship is not unusual and applies to al major mountain ranges.
2 major causes of uplift and subsidence.
Istostatic (bouyance force)
Dynamic (active vertical flow within the mantle).
uplift= work done against gravity.
Subsidence= work done by gravity.
Mantle plumes under Iceland- how do we know?
The seismic waves are very slow in a cylindrical mass below Iceland (i.e. the mass is less dense, therefore hotter, than the surrounding mantle)
The plumes extend to at least 400km depth and 300km wide.
Remember- hot mantle is seismically slow.