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.

This assignment includes a field exercise in which you are expected to collect and describe a piece of 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 print out the assignment and mail everything together.

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).

1. On the diagram to the right, draw the approximate boundaries between the crust and the mantle, and the mantle and the core; 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)

3. The genus Homo has been around for about 3 million years. What does that represent as a percentage of geological time?  (1 point)

4. Name any two of the inert elements, and explain what it means for an element to be “inert.” (2 points)

5. Name the mineral group for the following minerals: dolomite, serpentine, corundum, and sylvite. (2 points)

6. What is the net charge on the silica tetrahedra part of the mineral pyroxene? (1 point)

7. What is the difference between cleavage and fracture in minerals? (2 points)

8. 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)

9. 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)

10. Magma composition has implications for its viscosity. What implications do differences in viscosity have for volcanic eruption styles?  (2 points)

11. Explain how GPS positional information can be used to monitor a volcano that has the potential to erupt. (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 photo or make a sketch of the results, and include it with your assignment.  If you take a photo, It might look a little like the one below, although this was Himalayan rock salt that also has some iron-oxide minerals.  These crystals are up to about 2 mm across.  [SE photo]

© Steven Earle. Used with permission.

B2: Collect and describe a sample of intrusive igneous rock  (20 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 rock from 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.

Describe the sample site

Describe where you found the sample (e.g., name of river, beach, road, nearest town, etc.) and briefly the sample location.  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.  (4 points)

© Steven Earle. Used with permission.

To mark your description, your Open Learning Faculty Member will need to see what the rock looks like.  You have two options.  One would be to take at least 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 the best results.  The other option is to break the sample in half^* and send part of it (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 the remainder of the sample for yourself, so you can understand the comments from your Open Learning Faculty Member.

*There are two good reasons to break the sample.  First, you can keep part of the sample for yourself to help you understand the comments from your Open Learning Faculty Member.  Second, you’ll be able to see a fresh surface of the rock.  You’ll need a heavy hammer to break a rock like granite.  Please wear eye protection.

Describe the sample texture and composition

Describe the overall appearance (colour, texture), range of crystal sizes (in mm), general shapes of the crystals, and any other structures.  You may find that the crystals of one of the mineral types are generally larger or differently shaped than the others, and if so, make a note.  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.) (4 points)

Identify the minerals in the sample and estimate 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 in 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, its 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.  (6 marks)

Provide a rock name for your sample

In other words, what type of rock is it?  (2 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.  (3 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:

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 composition? (2 points)

2. On the scale of mafic-intermediate-felsic, how does that rock compare with the Lower Triassic MPd that shows up on cross-section B between Bootjack Lake and Polley Lake? (2 points)

3. Volcanic rocks are listed with sedimentary rocks under “Layered Rocks.” Identify and provide the rock-type names for two different volcanic layers—one from the Pleistocene, Eocene, or Miocene and one from the late Triassic.  (3 points)

4. The rock labelled EJt is described as a “latite tuff.” What does that tell you about its geological origins?  Describe where you might find some of this rock on the map.  (3 points)

B4: Distinguishing between similar-looking minerals  (20 points)

All minerals have some specific distinguishing features, so the key to mineral identification is to know what characteristics to focus on, especially when trying to distinguish one mineral from another that looks similar.  The following table has a list of three pairs of minerals from your kit that can be easily confused.  For each mineral, describe what you think are its most important diagnostic properties, and then describe what tests you would perform on a rock sample to distinguish the two minerals.  Be very explicit in your answer.  It is not sufficient, for example, just to say that a distinguishing feature is “hardness.”  Also, remember that although colour might seem to be the most obvious distinguishing feature of a mineral, it is not always reliable. (16 points)

B5: Characteristics of igneous rocks  (14 points)

1. All igneous rocks form from cooling magma. Arrange the following rocks in a likely cooling order from fastest to slowest, and describe the composition of the magma from which they might have formed: syenite (8), pumice (9), granite (10), pegmatite (11), scoria (14). (5 points)

2. The following diagram is a hypothetical cross section through a zone of active oceanic crust formation (on the left) and ocean-continental plate convergence. Identify, by number and rock name, which of the igneous rocks from your kit is most likely to have formed at locations (a) through (d).  (4 points)

3. List the following minerals in the sequence in which you would expect them to crystallize as the temperature in a typical magma chamber decreases: potassium feldspar (28), quartz (31), biotite (37), amphibole (39), and pyroxene (40). (5 points)

Part C: Longer question  (10 points in total)

Please answer the following question.  Write as much as you think is necessary to answer the 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, use in-text citations.  Use any referencing style you are comfortable with.

Two common types of volcanoes are shield volcanoes and composite volcanoes.  Compare these two types by focussing on the 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)