North Seattle Community College's
@1996 -- The information contained in this document
I. A Tutorial
This lab exercise is designed to introduce you to the essential differences in basic rock types and some clues for classifying rock samples. After reading Chapter 1 of your textbook, you will know that: (1) igneous rocks form from the cooling of molten (liquified) rock to produce crystals or, if violently erupted from a volcano, from magma-derived fragments such as ash, (2) sedimentary rocks form at the Earths surface either from the compaction of loose fragments of other rocks or from crystals precipitated from water or from organic remains, and (3) metamorphic rocks are rocks that were once igneous or sedimentary but have been altered (deformed) physically and/or chemically below the Earths surface by temperature and/or pressure changes.
Rock "texture" is an important clue to rock origin. For example the mixture of solid particles (grains) in a rock may give it a crystalline, clastic (fragmental), glassy, vesicular (having void spaces), or foliated (leaf-like layered) texture. The grains have various characteristics (size, sorting, rounding, orientation) that will be important for precisely classifying rocks (to be utilized in later labs). Generally, rocks with large grains are easier to classify since we can more easily identify the constituent parts, whereas we may have to resort to other properties to correctly identify rocks with very fine grains.
Additional features serving as clues to rock type are the presence of fossils, mudcracks, layering, folds, as well as specific minerals, the rock shape, its density and its weathered (rusty) appearance. The "fizz test" using hydrochloric acid is a very important method for determining the presence of calcium carbonate. This is often one of the first tests performed. The color of the rock (which reflects its chemistry) is an obvious feature and it may sometimes suggest a rock type, but usually color is not as useful a diagnostic property.
Also valuable for interpreting rock types is "regional context" -- that is, where was the rock found and what large structure does the rock appear to be part of (lava columns, for example). Of course, you do not know where the photographed "hand specimens" were collected so this is not a useful diagnostic feature.
Below is a summary table of clues toward identifying a rock as being igneous, sedimentary, or metamorphic in origin. As you can see, each rock type can have a great variety of forms so you usually can't depend on one feature in order to identify its origin. Without the chance to make more conclusive tests, you need to rely on your best observation skills.
II. A Virtual Field Trip to Indiana:
Take this virtual field trip to sunny Indiana to be introduced to igneous, sedimentary, and metamorphic rocks. Note that glaciers from the ice ages have had a significant impact on Indiana's topography and resident rocks just as the glaciers have done in the Pacific Northwest. Try to spot the basic differences between the three main categories of rocks. Don't worry about the rock names or terminology or the detailed textural and compositional differences within each of these categories.
III. Online Rock Exercise:
Below are photographs of 15 rocks from the campus geology lab. Look carefully at each specimen (click on each image for a closer view) and compare your observations with the diagnostic properties listed in the table above. Each specimen may have a combination of features which makes its classification more challenging! In that case, make an educated guess from all your observations. Then complete the following statements for each rock on the "Week 1 -- Lab Exercise" form in your In Box.
Your answers need not be long (a few words will do) and you need not be correct with your identification. Think of this lab exercise as a first step toward the more detailed identifications which you will learn with your own "hand specimens" in following lab exercises. This part of the lab exercise is worth 45 points (3 points per rock).
Please contact me if you have any questions before turning in your answers. Or post your comments in the student cafe to receive feedback and thoughts from your fellow students. A little discussion might clear up any confusing spots.
Rock Identifications: Igneous, Metamorphic, or Sedimentary?
Click on each photographic
"thumbnail" to enlarge the rock image.
All rock photographs by Tom Braziunas @1996
Question 1 Hint: Look at the
clues in the texture and layered structure.