the continental crust will occur 2) An isostasy problem in three parts.. In esse
ID: 117353 • Letter: T
Question
the continental crust will occur 2) An isostasy problem in three parts.. In essence, the goal here is to acquire some insight into the variations in sub-surface structure across a west-east transect of central California. We'll loosely divide this transect into three regions: the coastal zone, the central valley, and the Sierra Nevada. We'll ignore the coastal ranges (it's all seashore for this problem). The three densities you'll need to know are: density of maantle = 3.3 gem": density of sediments-2.1 g/cm) (likely to be a modest underestimate); and density of "normal" continental crust (loosely granitic composition) = 2.75 g My advice: As you're doing the problems, draw a picture first A) Assume that the coastal zone is composed of "normal" density continental crust, and that you can disregard sediments. If this coastal zone has an elevation of 4 km above the surrounding ocean basin (e.g. sea level)- and yes, we'll ignore oceanic crust and water for this problem- what is the thickness of continental crust in this region? B) Now, assume that the central valley lies at the same elevation as the coastal zone (e.g. sea level), and includes a pile of sediments that is 8 km thick on top of it. If these sediments are underlain by continental crust with "normal" density, what is the total depth to the mantle beneath the Central Valley? Let's say the Sierras have an average elevation of 2 km, and occupy about the same areal extent as the Central Valley. If 2 km of material is eroded off of these mountains and converted to sediments of the same thickness, and then stacked on top of the Valley sequence from (B) above, how much will the topography of the Valley change due to the added sediment load (ie. how much does the elevation of the Valley change in response to these added sediments- of course, once the system reaches isostatic equilibrium)? Note that the Sierras will maintain considerable topography...think about it. Don't worry about the mass loss in this conversion-just figure some mass is lost out the Sacramento River. C)Explanation / Answer
A) elevation is given which is 4 km
According to isostasy it should be balance below mean sea level
i.e, 4+4 = 8 km
B)
By using Airy model
Which says
Density of sediment x thickness of sediment = density of continental crust x thickness of continental crust
2100 x 8 = 2750 x thickness of continental crust
Thickness of continental crust = 6 km
Depth = thickness of sediment + thickness of continental crust
= 8 + 6
= 14 km
C) the mass is added in above one
That is 14 + 2 = 16 km
Sediment of 2 km is added hence we did addition
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