Assignment 1

This assignment is worth 12% of your total course mark. Please type your answers directly into this document and submit the assignment to your Open Learning Faculty Member. Please do not remove the questions, or the number of points for each question, from the document.

This assignment includes a field exercise for which you are expected to collect and describe an intrusive igneous rock. You have the option of mailing a piece of the rock to your Open Learning Faculty Member, or sending a high-quality photograph. If you choose the mail option, please also print out the assignment and mail everything together.

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Part A: Short-Answer Questions (20 points in total)

Answer the following questions as succinctly as you can. None of your answers should be more than a couple of sentences (100 words or less). Please type your answers into the grey boxes.

  1. On the diagram to the right, draw the boundaries between the crust and the mantle, and the mantle and the core in their (approximate) correct proportions, and label the crust mantle and core. (2 points) (You could use the “insert shapes” tool of Word to do this, or else draw it by hand and take a photo and then insert it into your document.)
  2. What is the lithosphere, and what role or importance does it play in plate tectonics? (2 points)
  • The genus Homo has been around for about 3 million years. What does that represent as a percentage of geological time? (1 point)
  • (2 points)
  • Name the mineral group for the following minerals: dolomite, serpentine, corundum, and sylvite. (2 points)
  • What is the net charge on the silica tetrahedra part of the mineral pyroxene?
    (1 point)
  • Explain the difference between cleavage and fracture in minerals. (2 points)
  • When a rock is subjected to partial melting, what is the likely compositional difference between the original rock and the magma produced? Explain why this is the case. (2 points)
  • What is the name for an extrusive igneous rock with 60% plagioclase, 25% amphibole, and 15% pyroxene? What is the name for its intrusive equivalent? (2 points)
  1. Magma composition has implications for its viscosity. What implications do differences in viscosity have for volcanic eruption styles? (2 points)
  1. (2 points)

Part B: Exercises (45 points in total)

B1: Making halite crystals (10 points)

Place about 1/2 teaspoon (~2.5 cm3) of any kind of table salt into a small bowl. Add about 2 teaspoons (~10 mL) of boiling water and swirl it around for a few minutes until all or almost all of the salt has dissolved. (Please be careful not to splash yourself with the hot water.) Place the bowl in a safe place (windowsill, bookshelf), and check back every 24 hours to see what has happened. Each time, describe the crystals in terms of size range (in mm), shapes, colour and any other details that you think are important.

When all of the water has evaporated, take a photograph or make a sketch of the results and include that with your assignment. If you take a photo, it might look a little like the one below, although this is Himalayan rock salt that also has some iron-oxide minerals. These crystals are up to about 2 mm across.

© Steven Earle. Used with permission.


B2: Collect and describe a sample of intrusive igneous rock (25 points)

This exercise involves fieldwork to collect a rock sample and some follow-up research at home. You may think it will not be possible for you to complete this exercise where you live, but rest assured, you can find good examples of intrusive igneous rocks almost anywhere, except the bottom of the ocean. If you’re stuck, please ask your Open Learning Faculty Member for some help.

Collect a sample of igneous intrusive rockfrom an outcrop, stream bed, beach, or other suitable location. Your rock should have visible and identifiable crystals, including feldspar, and probably quartz, amphibole, or mica. Granite and diorite are good examples. The rock shown below is an example of granite.

© Steven Earle. Used with permission.

Describe the sample site

Describe where you found the sample (e.g., name of river, beach, road, nearest town, etc.) and briefly describe the samplelocation. Was the sample collected from a stream bed, beach, forest trail, gravel pit, or someone’s driveway? Was it a loose pebble or boulder lying on the ground, or was it part of the solid rock of an outcrop. Include a sketch or photograph(s) of the sample site with a measure of scale such as a notebook, hammer, or person; and where the sample site is located. It’s very important to show some context in your photo (like the left-hand photo below) or sketch, so your Open Learning Faculty Member can understand the setting. Also, don’t forget to mark on your context photo or sketch where you actually found the sample. (5 points)


© Steven Earle. Used with permission.

To mark the next few questions your Open Learning Faculty Member is going to need to see what the rock looks like. You have two options. One would be to take two good clear photographs and insert those into your assignment. You can choose this option if you know how to take good photos. Remember that strong light (preferably direct sunlight) will give you the best results. Please break your rock[1] so that one of your photos shows a fresh (unweathered) surface. Include something (such as a coin) in the photos to show the scale.

The other option is  send part of your sample (about 2 x 2 x 1 cm) by mail to your Open Learning Faculty Member along with the rest of your assignment. If you have doubts about your ability to take a good photo, it’s best to send the sample. You will likely lose marks if your Open Learning Faculty Member is unable to evaluate your answers due to an unclear photo. Keep a piece of the sample for yourself, so you can understand the comments from your Open Learning Faculty Member.

Describe the sample texture and composition

Describe the overall appearance (colour, texture), range of crystal sizes (in mm), the general shapes of the crystals, and any other structures. You may find that the crystals of one of the minerals are generally larger or differently shaped than the others, and if so, make a note of that. Test its strength and hardness by scratching with a knife. (The Mohs hardness scale doesn’t apply to rocks—only to minerals—but you can describe rocks as being soft, hard, very hard, etc.) (5 marks)


Identify the minerals in the sample and estimate of their percentage proportions

This task may seem almost impossible at first, but if you work at it systematically, it won’t be that difficult. Using Figure 3.17 your textbook as a guide, estimate the proportion of dark minerals. If the dark minerals are flaky, they are likely biotite; if they are more prismatic (long and thin), they are likely amphibole. (Both could be present.) Feldspar tends to be dull white, whereas quartz is typically glassy. Try estimating the quartz content next (using Figure 3.17 again). In most cases, everything else should be feldspar. If some of the feldspar is pink, it’s likely to be potassium feldspar, and the rest is likely plagioclase, but you don’t have to try to distinguish the two.The percentage proportions must add up to 100. (7 marks)


Provide a rock name for your sample

In other words: what type of rock is it?(3 points)


Briefly outline the geological history of your rock

Briefly describe how you think your rock formed, and in what geological setting. If it wasn’t part of an outcrop, it could have come from 100s of km away. Describe how you think it got to where you collected it. (5 points)


B3: Understanding the Mt. Polley area geological map (10 points)

You have been provided with a copy of the geological map of the area around Mt. Polley in central British Columbia. (Mt. Polley is about 75 km SE of Quesnel and 60 km NE of Williams Lake.) The following questions are based on information in the map legend and on the map itself. You don’t need to look elsewhere for the answers to these questions, but it will help if you’ve read the assigned parts of the text, and you may need to look up some of the terms you encounter.

Many of the answers to these questions can be found in the map legend. It’s important to be aware that the legend is divided into two parts, with intrusive igneous rocks in the top part and layered (sedimentary, volcanic and metamorphic) rocks in the bottom part. Within each part the units are arranged by age, with the youngest at the top.

  1. What is the rock-type name for an Early Jurassic intrusive igneous rock in this area, and what does that name tell you about its mineral composition?
    (2 points)
  • On the scale of mafic-intermediate-felsic, how does the rock in question 1 compare with the Lower Triassic unit MPd? (2 points)
  • The rock unit EJt is described as a “latite tuff.” What does that name tell you about its texture and geological origins? Describe where this rock is located on the map. (3 points)

Part C: Longer Questions (35 points in total)

Please answer the following questions. Write as much as you think is necessary to answer each question, but don’t forget that someone has to read what you write, so be as concise and clear as possible. You do not need to reference the textbook or the material in the Course Units, but if you use any outside sources, provide in-text citations. Use any referencing style that you are comfortable with.

  1. Silicates are by far the most important minerals in the crust and mantle, and they have a wide range of internal structures and properties. Describe some of the more important silicate minerals by completing the following table.
    (15 points)
Mineral Tetrahedron configuration Typical elements other than Si and O Hardness and typical colour Type of cleavage
  • Most magmas that form the igneous rocks that we see at surface originate from partial melting of mantle rock, and then move up into the crust, sometimes remaining within a magma chamber in the crust until entirely cooled. Magma can change quite significantly while present in a crustal magma chamber. Describe some of the processes that occur that lead to those changes, and explain how the resulting magma might differ from the original mantle melt. (10 points)
  • Two common types of volcanoes are shield volcanoes and composite volcanoes. Compare these two types by focussing on plate tectonic setting, shape and size, compositions of magma, and variations in eruption style and frequency. Give an example of each type of volcano. You can use a table if you wish. (10 points)
Feature Shield volcanoes Composite volcanoes
Plate tectonic setting    
Shape and size    
Magma composition    
Typical eruption styles    
Typical eruption frequency    

[1] You’ll need a heavy hammer to break a rock like granite. Please wear eye protection.

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