Lesson 3: Rocks and Minerals in California
This is the second of three lessons that will introduce you to some basic geologic concepts we'll be applying throughout the semester. Rocks and minerals are the "building blocks" of our planet, and in addition to preserving a record of California's geologic history they are also valuable commodities that have made major contributions to the state's economic development. As an indication of how much California has changed since the 1850s it's sobering to realize that the most valuable mineral commodity mined here today is not gold (shown at right in a piece of quartz from the Mother Lode) but, rather, aggregate (sand and gravel) that is used for construction (see Table 19-1).
In chapter 2 Harden introduces rocks and minerals, and highlights those that are particularly common or geologically significant in California. Although minerals are characterized by their unique compositions and atomic structures, we'll see that their basic physical properties (luster, density, hardness, etc.) can be used to recognize them. Rocks, on the other hand, are aggregates of minerals that are formed either by the solidification of magmas, the deposition of weathered materials at Earth's surface, or the recrystallization of older rocks due to heat and pressure within the Earth. Because a rock's composition and texture reflect the conditions under which it formed, we'll learn how to use these characteristics to interpret a sample's origin and history.
As you read about rocks and minerals in our text and online it will be helpful to take detailed notes. Writing concepts out in your own words or making neatly labeled drawings will help you better understand key points and also recognize any gaps in your knowledge. Having good notes will also make it easier for you to review for this week's quiz and and access what you've learned when you need it for future assignments. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the quiz at the bottom of the page.
Weekly Learning Objectives
Upon successful completion of this week's lesson, a student is expected to be able to:
- Determine whether a sample is a mineral or a rock based on its characteristics, and explain what the basic difference is between these two types of materials.
- Identify the likely setting in which an igneous rock crystallized (deep underground, at Earth's surface, or at two different depths) based on its texture (coarse-grained, fine-grained, porphyritic or glassy).
- Identify which part of the Earth (crust or mantle) was likely the source of the magma that formed a given igneous rock based on the rock's composition or color. (Magmas that crystallize to form felsic or light-colored rocks are typically produced by partial melting of the crust, whereas those that form mafic or dark-colored rocks are produced by partial melting of the mantle.)
- Distinguish between a clastic sedimentary rock and a chemical or biological sedimentary rock on the basis of texture.
- Outline a simple history of weathering and transport for the sediment that forms a clastic sedimentary rock from observations of the sediment's rounding, sorting, and grain size.
- Determine whether a metamorphic rock has a foliated or nonfoliated texture and explain what this tells us about the likely state of stress in the crust during the rock's formation.
- Contrast the pressure-temperature "paths" that rocks follow when they undergo: (1) blueschist metamorphism versus "normal" regional metamorphism, and (2) contact metamorphism versus "normal" regional metamorphism (see Fig. 2-15).
Reading and Browsing Assignment
- Read Chapter 2 of our text, and pay particular attention to the topics covered in the learning objectives above.
- In order to learn how physical properties (color, luster, etc.) are used to identify common minerals go to Mineral Physical Properties and Identification on Richard Harwood's Physical Geology course website. Read through the introductory information Harwood provides and then identify minerals 4, 9, and 17 and record their names in your notes. Here are some tips to bear in mind as you work on this website.
- To perform a test or make an observation on a sample just click the appropriate gray button.
- Hardness is determined by working up the scale to find the lowest value at which your unknown is scratched; the unknown sample's hardness is one hardness unit less than this value.
- Record your observations by clicking the round "radio buttons" in the table below the picture.
- After you've recorded all of a sample's properties, use this mineral identification key to determine its name. The key is a "decision tree"; you decide which path to follow based on your observations. For example, if your unknown sample has a non-metallic luster and is green, you would go to the "Non-metallic, dark colored" page of the key and then look at the sample's hardness to narrow down your choices further. Just keep narrowing down choices until only one is left.
- As you record your observations be sure that one radio button is clicked for each property and that a name is chosen before you ask the program to check your identification. If anything is incorrect it will tell you which property(ies) you need to go back and look at again.
- Based on your reading of our text be sure that you understand what makes each of the three major families of rocks distinct and which criteria are used to distinguish rocks within each family. Then, try your hand at rock identification by determining the names of the following samples on Harwood's rock identification pages.
- Igneous Rock Identification, samples 4 and 9;
- Sedimentary Rock Identification, samples 1 and 8;
- and Metamorphic Rock Identification, samples 1 and 3.
Exercise 3: Map Reading (Due by 9:00 AM on 31-Jan-2011)
The browsing assignment you worked on above introduces rock and mineral identification. Because we cannot actually go on to do a "hands on" lab or field activity with earth materials, however, I would like you to learn a little about map reading for this week's exercise. The activity I've chosen is from your Hazard City CD, and will provide useful background information for our discussion of geologic maps next week.
To begin, load your CD, click on the "Map Reading" exercise, and then on "Version 1". Like all of our Hazard City exercises this one is pretty self-explanatory, but it does require careful reading and attention to detail. After you've read the introduction click through the different parts of the exercise using the buttons at the bottom of the screen. It will be useful to take notes on any concepts that you are unfamiliar with so that you'll have them handy as you work on the map and report form. When you are ready to begin writing down your answers click on the "Report" button and print out a copy of the form so that you'll know exactly what you need to do. With this week's exercise it will also be a good idea to print a copy of the map so that you can make accurate distance measurements (this is tough to do on your computer screen).
When you have answered all of the questions on the form and feel that you understand how to read a topographic map pretty well, point your browser to the "Assignments, Tests and Surveys" tool on our Etudes site and click on Exercise 3. The first five questions will be the ones you've answered on the report form, and the last five will be questions I've written that are directly related to what you've done.
Quiz 3: Earth Materials (Due by 9:00 AM on 31-Jan-2011.)
After you feel you have met the learning outcomes outlined above, please complete Quiz 3 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about earth materials, each worth one point. If you can answer all of them correctly it means that you know your way around the basics of California's rocks and minerals pretty well and are ready to start learning about geologic maps and dating next week. Like all of our weekly quizzes, this one is timed (you'll have 30 minutes) and must be completed in one "sitting". (That is, you will only be granted access once.) So, be sure you're ready to complete your quiz when you start it—and be sure you're using Firefox. Good luck.
