Author Question: While hiking with two of your classmates, you observe an outcrop of sandstone rich in quartz and ... (Read 108 times)

evelyn o bentley · **on:** Jul 16, 2018

While hiking with two of your classmates, you observe an outcrop of sandstone rich in quartz and feldspar. Friend A says that the sand grains in the rock were eroded from a nearby basaltic volcano.

Friend B says that the sandstone was derived from a more distant granite outcrop. Which one of your friends is right and why?
What will be an ideal response?

Question 2

Imagine a river that flows through steep mountain valleys and then across a gently sloping coastal plain to the ocean. Write a prediction of how grain size,

rounding, and sorting of the sediment deposited by the river will change from the mountains to the coastline. Explain why these changes will take place.
What will be an ideal response?

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

Answer to Question 1

Friend B is right. Granite is rich in quartz and feldspars. The basalt volcano produces basalt, with no quartz.

Answer to Question 2

Grain size would decrease, rounding would increase and sorting would increase from the mountains to the coastline. As the material is eroded from the source, the energy of the eroding agent (wind, water) would leave behind larger grain sizes and pick up and carry the smaller ones toward the coast. This process would also sort the larger and very small grain sizes, increasing the sorting of the material. The transport of the grains would cause abrasion along the path and round any angular particles from source to site of deposition.