Question
Homework #3 Rocks and Geologic Process of the Front Range, CO
On the syllabus, one of the goals stated for this course is for you to be able to interpret how a
rock or outcrop (rock exposure) that you observe around Boulder, CO may have formed.
This homework is a review of many of the ideas and concepts covered through the semester
that we covered that are necessary in order to achieve that goal and a practice to apply this skill
on six rock samples from about Boulder, CO.
Tip #1: Compile all of your mastery quizzes, mineral homework and notes in order to complete
this homework. You will be referring back to the rock cycle, characteristics of minerals
(homework#2), igneous rocks, in addition to the current topics of weathering and clastic
sedimentary rocks.
Tip #2: This homework is review a large number of concepts from the past four weeks. DO NOT
wait until the last minute to look at the homework (this homework is a great review for the exam
on Sept 28-29th)
Tip #3: The homework does need to be completed in order. In fact answering what you can first
may be beneficial to work through ideas.
Pre homework questions (1 pt):
Below are statements that reflect some of what you should be getting out of this assignment.
1. Before you complete the assignment, rate how true each of the following statements is for
you using a scale of 1-5, where
.
These answers for the pre-question are graded only for being complete (1 pt all or nothing)
and not what your answers are.

Your rating
(1 through 5)

1. I can identify the common rock forming silicate minerals based
on their physical appearance and provided diagnostic
properties
2. I am comfortable identifying is a rock is igneous, sedimentary or
metamorphic based on appearance and description.
3. I can identify which of the nine common igneous rocks a sample is
based on mineralogy composition and texture.
4. Based on the rock type (e.g. sedimentary, igneous, or metamorphic)
and its appearance, I can infer the conditions under how that rock
formed.

Location #1: Flagstaff Road
Answer the questions to identify each of the rocks that can be seen on Flagstaff Road in
Boulder, and provide a short answer about what were the geologic conditions that created
that rock.
After identifying the rock and it minerals, you are asked to write a sentence describing how
it formed. See below for the information that you will need to include the following
Igneous rocks:
What was the magma composition involved to create the rock?
What the most likely source to create that magma (e.g continental crust/oceanic
crust/mantle)?
Where did the magma cool, at the surface or at depth?
What was the cooling rate, quickly or slowly?
Example: Basalt – A mafic magma was created by partially melting the mantle. This magma
then cooled quickly at the surface to create a basalt.
Sedimentary Rocks:
What are the shape of the sediment?
What is sorting of the sediment?
What is name of the rock?
Based on the grain characteristics, did the sediments experience a long weathering
period or a short weathering period?
Example: A well sorted, well rounded sandstone that implies that the sediments were weathered
a long time before being deposited.

Rock Sample #1

This is a picture of a rock commonly seen up
Boulder Canyon, just west of the city. It as
interlocking, visible crystals of the mineral
Mineral A: Black, shiny, and breaks
into sheet
Mineral B: Pink in color and hard.
Mineral C: Clear in color and very hard,
does not have a cleavage.
5. Mineral A described in the rock is:
6. Mineral B described in the rock is:
7. Mineral C described in the rock is:
8. What is the rock type of this sample? (Sedimentary, Igneous, or Metamorphic):
9. What is the rock name for this sample?
10. How did rock sample #1, which is commonly found in Boulder Canyon, form? Be sure to
refer to the direction to ensure you include all of the information for a complete answer

Rock Sample #2

This is a picture of a rock that makes up the Flatirons.
Loose pieces of varying sizes that touch each other,
but do not interlock.Most of the grain size range
from fine sand to course sand, with some gravel
pieces.
The visible crystals that can be seen are of mainly
two mineral types
Mineral A: Clear, whitish in color. Hard to break but
when it does, it fractures.
Mineral B: Pink in color. Well-formed crystals have a
rectangular

11. What is Mineral A in this rock?
12. What is Mineral B in this rock?
13. What rock type is this sample? Igneous, Sedimentary, or Metamorphic.
14. Based on the picture and the description, how would describe the sorting of the grains in this
sample? Well sorted or poorly sorted?
15. Based on the picture and the description, what is the name of the rock pictured here?
16. How long were the grains that make up Rock Sample #2 exposed to the surface and
weathered? Be sure to refer to the directions to ensure that you include all information for a
complete answer.
Location #2: Lefthand Canyon
Answer the questions to identify each of the rocks that can be seen on Lefthand Canyon,
North of Boulder, and provide a short answer about what were the geologic conditions that
created that rock.
After identifying the rock and it minerals, you are asked to write a sentence describing how it
formed. See below for the information that you will need to include the following
Igneous rocks:
What was the magma composition involved to create the rock?
What the most likely source to create that magma (e.g continental crust/oceanic
crust/mantle)?
Where did the magma cool, at the surface or at depth?
What was the cooling rate, quickly or slowly?
Example: Basalt – A mafic magma was created by partially melting the mantle. This magma
then cooled quickly at the surface to create a basalt.
Sedimentary Rocks:
What are the shape of the sediment?
What is sorting of the sediment?
What is name of the rock?
Based on the grain characteristics, did the sediments experience a long weathering
period or a short weathering period?
Example: A well sorted, well rounded sandstone that implies that the sediments were weathered
a long time before being deposited.

Rock Sample #3

Scattered larger crystals in a fine crystalline gray matrix
(also known as groundmass). Contains way more Na-rich
plagioclase feldspar than potassium feldspars for other
rocks typically from this
Mineral A: Clear in color, no fracture
Mineral B: White and light colored, with two planes of
cleavage
Mineral C: Black in color, shiny, layered in fine sheets

17. What is mineral A in this sample?
18. What is mineral B in this sample?
19. What is mineral C in this sample?
20. Based on the three mineral identifications and descriptions of the rock, what is the
classification of the chemical composition?
21. What was the rate or crystallization/rate of cooling for this sample?
22. How did Rock Sample #3, a dacite found in Lefthand Canyon, form? Be sure to refer to the
directions to ensure that all information is included for a complete answer.

Rock Sample #4

This sample is found close Highway 36 on
Lefthand Canyon Road and is also seen while
hiking Mt. Sanitas. It contain clasts of only one
mineral and feels sandy when touched. The
mineral is light to clear in color, with a glassy
appearance, very hard, and no cleavage planes.

23. What rock type is this sample? Igneous, Sedimentary, or metamorphic?

24. What is the mineral present in this sample?
25. What is the sorting present in this rock? Well sorted or poorly sorted?
26. What is the level of rounding in this rock sample? Angular or well rounded?
27. What is the name of this rock?
28. How long were the grains that make up Rock sample #4 exposed to the surface and
weathered? Be sure to refer to the directions to ensure that you include all information for a
complete answer.

Location #3 Valmont Butte
Answer the questions for the rock that makes up the Valmont Butte, in East Boulder, and
provide a short answer about what were the geologic conditions that created that rock.
After identifying the rock and it minerals, you are asked to write a sentence describing how it
formed. See below for the information that you will need to include the following
Igneous rocks:
What was the magma composition involved to create the rock?
What the most likely source to create that magma (e.g continental crust/oceanic
crust/mantle)?
Where did the magma cool, at the surface or at depth?
What was the cooling rate, quick or slow? Slow than quick?
Example: Basalt – A mafic magma was created by partially melting the mantle. This magma
then cooled quickly at the surface to create a basalt.
Sedimentary Rocks:
What are the shape of the sediment?
What is sorting of the sediment?
What is name of the rock?
Based on the grain characteristics, did the sediments experience a long weathering
period or a short weathering period?
Example: A well sorted, well rounded sandstone that implies that the sediments were weathered
a long time before being deposited.

Rock Sample #5

Dark colored rock, finely crystalline but no visible crystals.
Igneous cuts through pre-existing continental sedimentary
rock as a dike intrusion.
29. What is the rock type of this sample? Igneous,
Sedimentary, or metamorphic?
30. Based on crystal size, what is the name of the type of
texture for this sample?
31. What is the rate of crystallization/rate of cooling for this rock?
32. What is the likely chemical composition of this rock sample?
33. What is the name of this rock sample?
34. How did sample #5 from Valmont Butte form? Be sure to refer to the directions to ensure
that all information is present for a complete answer.

Post homework questions

Below are statements that reflect some of what you should be getting out of this assignment.
After you completed the assignment, rate how true each of the following statements is for you
using a scale of 1for the post-question are graded only for being complete (2 pts) and not what your answers are.

Your rating
(1 through 5)
35. I can identify the common rock forming silicate minerals based on
their physical appearance and provided diagnostic properties
36. I am comfortable identifying if a rock is igneous, sedimentary, or
metamorphic based on appearance and description.

37. I can identify which of the nine common igneous rocks a sample is
based on mineralogy composition and texture..
38. Based on the rock type (e.g. sedimentary, igneous, or metamorphic)
and its appearance, I can infer the conditions under how that rock
formed.
39. In the space down below, write a sentence on what skills or knowledge you are feeling more
confident on now after completing this exercise OR what questions you may still have after
completing this exercise.

Tip#4: Use your answers to the pre and post homework questions to help determine (a) the
ideas/concepts you know and are comfortable with and (b) which ones you feel you need to
study more before the final.