When teaching about igneous rocks, one trick instructors often ...

[mspears3]

When teaching about igneous rocks, one trick instructors often implement to help students determine the igneous composition of the samples is to use the color of the rocks as a guide.
 
  Explain how the color of an igneous rock can often be used as an indicator of its igneous composition. Be sure to reference specific minerals for each composition.
  What will be an ideal response?

Question 2

Refer to the scenario above. Assuming a temperature of 20C at the ground surface, what is the temperature at a depth of 2 miles?
 
  Scenario: In the pursuit of scientific knowledge, you decide to bore a hole into the crust to see how the temperature changes. You start your borehole in Davenport, Iowa, where the geothermal gradient is 20C/km.
  What will be an ideal response?

[vickyvicksss]

Answer to Question 1

Answer: Generally speaking, mafic rocks contain ferromagnesian minerals like pyroxene and amphibole and are therefore very dark because of the high iron and magnesium content. Felsic rocks, which contain light-colored silicate minerals such as quartz and potassium feldspar, are very light in color. Intermediate rocks, which are a combination of mafic and felsic materials, are somewhere between mafic and felsic on the color spectrum. Common intermediate minerals include plagioclase feldspar, amphibole, pyroxene, and biotite mica.

Answer to Question 2

Answer: 2 miles  1.609 = 3.2 km
3.2 km  20C/km = 64C

[mspears3]

Excellent

[okolip]

Thanks for the timely response, appreciate it