Author Question: Mountain building and erosion near convergent plate boundaries commonly expose rocks that formed in ... (Read 56 times)

CharlieWard · **on:** Jul 16, 2018

Mountain building and erosion near convergent plate boundaries commonly expose rocks that formed in the middle crust at depths of 15 to 20 km below the surface. These rocks include plutonic-igneous rocks and metamorphic rocks.

The metamorphic rocks formed under both regional and contact metamorphic conditions. What combination of field and laboratory observations could you make to distinguish the effects of the two sets of conditions? In developing your answer, assume that the rocks contain aluminum silicate polymorphs, and use the graphs in Figures 6.5 and 6.18 to guide your thinking.
What will be an ideal response?

Question 2

Define recrystallization. Describe the changes a crystal undergoes during recrystallization. How does recrystallization differ from cementation of sedimentary particles? How might you distinguish between cementation and recrystallization?

What will be an ideal response?

vickyvicksss · **Reply #1 on:** Jul 16, 2018

Answer to Question 1

The pressures for depths between 1520 km equate to between 47 kb (Figure 6.2). In this pressure range, we would expect to see kyanite and/or sillimanite in the metamorphic rocks (Figure 6.18). With a normal geothermal gradient of 25oC/km (Figure 6.5), the temperature at 1520 km would be between 375o and 500oC, which is in the kyanite stability field (Figure 6.18). So, the regional metamorphic rocks would contain kyanite but contact metamorphic rocks, which should be closer to the outcrops of igneous rocks, could contain sillimanite because they formed at higher temperatures (Figure 6.5 and 6.18).

Answer to Question 2

The transfer of atoms from one part of a crystal to another part of the crystal or an adjacent crystal. It changes the size and shape of existing minerals but does not produce new ones. Large grains are able to grow and fit tightly together, often suturing to form a mosaic of large grains. In cementation, grains are not greatly altered by the precipitation in the pore spaces, and you would expect to see a difference between sedimentary grain size and precipitated crystal size.