Reference no: EM132785104

Exam : The Precambrian in Wisconsin

Collaboration:
This assignment needs to reflect your understanding and knowledge of the questions below. While you may discuss these questions generally with your classmates, TAs, and me, the work you submit must be your own. You must reaffirm the honor code on this and all assignments.

Photosynthesis and life in the Precambrian
Photosynthesis is pretty important! If you need a wonderful review of photosynthesis, They Might Be Giants has you covered.

Photosynthesis can be explained by the following chemical reaction:

6CO₂ + 6H₂O + ï(sunlight) C₆H₁₂O₆ (sugar) + 6 O₂

Question 1. Consider the chemical reaction, why is O₂ considered a "waste product" in this biochemical process? What is the product that photosynthetic organisms are making in this reaction?

Question 2. As aerobic photosynthesis took off, the waste product of O2 gas became widespread and had drastic impacts on both life and rocks on Earth.

a. What is an example of how this waste product impacted life on Earth? Provide two sentences explaining your answers.

b. What is an example of how this waste product impacted rocks on Earth? Provide two sentences explaining your answers.

Question 3. As oxygen breathing organisms, we should be pretty happy that this waste gas built up in our atmosphere! That said, those organisms that were thriving in a reducing, O2 free world were likely less pleased with this O2 "pollution." Today, we have a similar process going on around us, with another, seemingly innocuous waste gas building up in our atmosphere. I am, of course, referring to rising CO2-levels as a result of the burning of fossil fuels. Write a short paragraph considering how these two processes are similar and dissimilar.

Cactus Rock:

Cactus Rock is the closest exposure of Precambrian rocks to Appleton, these rocks are from the Proterozoic. Why is it called Cactus Rock? Well, because there are cacti there! This outcrop is too small, and does not show up on the geologic map of Wisconsin. This rock is similar in composition to some igneous rocks exposed in the Baraboo Range in the southern part of our state, dated at 1.7 Ga. On the Moodle page, there is a PowerPoint file and a series of videos you should watch to help you answer the following questions. The goal of this section will be for you to apply some of the skills you have been developing in this unit to interpret these very cool rocks.

Question 4: One of the most important skills for a geoscientist is making careful observations and sketches. Figures 1-3 show the rocks at Cactus Rock. One is a broader view, the second two are close-ups of a sample of rock collected at Cactus Rock. Use these figures, along with the videos and PowerPoint slides to help you. Your job is to describe the rock. Your description can be a list of bullets. You should be sure to describe what you see. Use the following questions to guide your observations.

• What does Cactus Rock look like?
• What does the rock look like from afar?
• How does it look close-up?
• What color is the rock?
• How big are the grains?
• Are they all the same size?
• What colors do you see when you look up close?
• What are the relative proportions of colors?
• What do the boundaries between the grains look like?

Question 5: X-ray diffraction (XRD) is a technique to identify the minerals in a sample. XRD analysis of this rock shows that it is made up of quartz, orthoclase feldspar (a potassium feldspar), albite (sodium feldspar), and smaller amounts of biotite. Each of these minerals can be positively identified based on their color in this rock. For each of the four minerals, identify their color in the rock:
Quartz: ___________
Orthoclase feldspar: ___________
Albite feldspar: ___________
Biotite: ___________

Question 6: Using section 4.1 in the textbook, your observations, and the mineralogical data, identify what kind of rock we see at Cactus Rock. You will want to be sure that you consider both the chemistry/mineralogy of the rock that is given as well as the grain size. The "What is an igneous rock?" video posted on Moodle has a good discussion of this (around the 3:20 mark in particular).
What kind of rock is this?

What is the basis for your identification?

What two rocks might this be intermediary between?

Question 7: Based on the type of rock that makes up Cactus Rock, and your interpretations of the tectonic setting, what can we say about Lawrence's "here" 1.7 billion years ago?
What tectonic setting likely lead to the volcanism that would produce this rock?

How do you know, and how would these tectonic processes lead to the rock you identified? Section section 4.3 in the textbook, might be helpful here.

Would cactus rock have been in the middle of a continent (as it is today) or not? How do you know?

If you wanted to visit a modern proxy for Cactus Rock, where might you go? (In other words, where might you go today to see a process similar to the processes that were active here 1.7 billion years ago.)

Question 8: Cactus Rock is quite old. As a result, it is much smaller than it was at the time of its formation. Where did all the missing mountain go?

Short Essay Question:

Select one of the two prompts listed below. You will then write a ~200-250 word response to the prompt. I encourage you to discuss these prompts with your lab group and others.

Question 1: A number of the readings and videos draw connections between geological and biological processes. Geological processes allow for the origin and evolution of life. The changing life on our planet had drastic impacts the rocks and minerals we see on Earth today. What were some specific connections between geological and biological processes that you found particularly interesting? Why? What do you think of the idea that life and geology are in fact co-dependent?

Question 2: The Principle of Uniformitarianism states that the present is the key to the past. This elegantly simple idea is founded on the idea that to understand even the geological processes of deep time we should begin by observing the processes in action on Earth today. On some level, it feels surprising that this holds true even when we think about the Precambrian, a time when our planet would have been looked very different than it does today. To you, what are the strengths, and perhaps weaknesses, of this idea as we try to better understand what was happening here in Wisconsin billions of years ago? Does thinking of Wisconsin billions of years ago with majestic mountains and oceans with early life forms make that geological past seem more distant or perhaps a bit closer?

The Geology of your "here"

During Unit 3, you will investigate and report on some basic geology of your "here." For this checkpoint assignment, answer the following questions:

1. What is the bedrock geology of your "here"? This should include a rock type and an age.

2. Can you interpret what type of environment these rocks were deposited in?

Attachment:- The Precambrian in Wisconsin.rar