Question

question 35 2 pts assuming a typical density of mafic crust and a mantle density of 3.3 g/cm³, what percentage of a block of oceanic crust resting on the mantle would extend above the surface? 18% 43% 9% 90% question 36 2 pts true or false: the mantle is composed of molten material that undergoes plastic deformation over geologic timescales. true false

Explanation:

Step1: Recall isostasy principle

The isostasy principle states that the pressure at a given depth in the mantle is the same beneath a column of crust and the surrounding mantle. Let the density of the crust be $
ho_{c}$ (a typical density of mafic crust is around $2.9\ g/cm^{3}$), the density of the mantle be $
ho_{m}=3.3\ g/cm^{3}$, the total height of the crustal block be $h$ and the height of the part of the crust above the mantle - crust interface be $h_{1}$ and the height of the part of the crust below the interface be $h_{2}$. So, $
ho_{c}h=
ho_{m}h_{2}$.

Step2: Express $h_{2}$ in terms of $h$

$h_{2}=\frac{
ho_{c}}{
ho_{m}}h$. Since $
ho_{c} = 2.9\ g/cm^{3}$ and $
ho_{m}=3.3\ g/cm^{3}$, $h_{2}=\frac{2.9}{3.3}h\approx0.88h$.

Step3: Calculate the height above the surface

The height above the surface $h_{1}=h - h_{2}$. Substituting $h_{2}=\frac{2.9}{3.3}h$ into the equation, we get $h_{1}=h-\frac{2.9}{3.3}h=\frac{3.3 - 2.9}{3.3}h=\frac{0.4}{3.3}h$. The percentage of the block above the surface is $\frac{h_{1}}{h}\times100=\frac{0.4}{3.3}\times100\approx 12\%$, which is closest to 9% considering possible rounding - off and approximations.

For Question 36:
The mantle is not entirely molten. The upper - mantle is composed of semi - molten (plastic) material that can undergo plastic deformation over geologic timescales. The lower mantle is more rigid but still can flow very slowly. So the statement "The mantle is composed of molten material that undergoes plastic deformation over geologic timescales" is False.

Answer:

Question 35: C. 9%
Question 36: B. False