North Seattle Community College's
PHYSICAL GEOLOGY 101
Designer:  Tom Braziunas
Instructor:  Gwyneth Jones

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Rock Identification Exercise

@1996 -- The information contained in this document is copyrighted.
No reproduction may be made without prior approval from the author.

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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.  There are three rock types on Earth, they include igneous rocks which form from the cooling of molten rock to produce crystals, sedimentary rocks which form at or near Earth’s surface either from the compaction of loose fragments of other rocks or from crystals precipitated from water, and metamorphic rocks which form from other rocks that have been altered physically and/or chemically below Earth’s surface.

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, specific minerals, 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.

ROCK TYPE / ORIGIN

DIAGNOSTIC PROPERTIES

IGNEOUS
(from molten rock or volcanic fragments)

Some Possible Textures: Glassy; or vesicular (full of bubble holes); or randomly oriented small crystals ("aphanitic"); or randomly oriented large crystals ("phaneritic"); no fossils or organic material.

Some Possible Structures and Shapes: Tear drops; or glass shards; or ropey streamlined lava flow shapes; or massive crystalline shapes.  

Some Possible Chemistries: Dark colors (rich in magnesium or iron); multicolored (several kinds of mineral crystals are present); light colors (rich in quartz or feldspars); sometimes distinctive minerals are present such as olivine (green) or alkali feldspar (pink).

SEDIMENTARY
(from sediments or precipitation of crystals in water at or near Earth's surface)

Some Possible Textures: Clastic (made of rock fragments either large or small such as mud, silt, sand, gravel or pebbles); layered; fragments of life forms (fossils); crystalline (usually one type of crystal). 

Some Possible Structures and Shapes: Icicle shapes (stalactites); delta and sand bar shapes; extensive flat layers (also called "strata").

Some Possible Chemistries: Often one color (if fine grained); multicolored if composed of large rock fragments cemented together; sometimes composed of calcite (precipitated by corals or shells) which fizzes when acid is applied; sometimes composed of halite or gypsum (soft light-colored crystalline rocks that form when salty water bodies dry up).

METAMORPHIC
(from physical or chemical alteration of other rocks by heating or intense pressure beneath Earth's surface)

Some Possible Textures: Foliated (folds such that minerals lie parallel to one another); banded (separate bands of light and dark minerals make wavy folds in the rock -- not like the flat layers in sedimentary rocks); an especially shiny metallic reflection from crystals aligned parallel to one another; sometimes very large crystals of equal size; sometimes deformed fossils (stretched or crushed).

Some Possible Structures and Shapes: Can be fractured into pieces.

Some Possible Chemistries: Often contain "metamorphic minerals" such as garnets (reddish-brown 12-sided crystals), mica (flat and shiny gray), and epidote (light green).

 

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II. A Virtual Field Trip to Indiana:

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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.

 

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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.

  • (1) List any of the diagnostic properties above which you believe can be observed in the photograph of the rock "hand specimen"
  • (2) Classify each rock as igneous, sedimentary, or metamorphic based on your observations.

Your answers need not be long (a few words will do).  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. 

Please contact me if you have any questions before turning in your answers. Or -- even better -- 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.
(NOTE: These rocks do NOT correspond to the specimens in your rock kit.)

(NOTE: Also, be sure you use the CAMPUS SAMPLE #'s (in red below), NOT the sample numbers within the photos.)

All rock photographs by Tom Braziunas @1996

Campus Sample #1
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Hint:
Look at the clues in the texture and layered structure.

Campus Sample #2  
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Hint: Observe at the random arrangement of the crystals.

Campus Sample #3
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Hint:  This rock has a distinctive texture.

Campus Sample #4
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Hint:  This rock has both fragments and mineral crystals.

Campus Sample #5
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Hint: This rock has large crystals of a mineral especially associated with one of the three rock types.

Campus Sample #6
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Hint: This rock has a distinctive texture.

Campus Sample #7
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Hint: Pumpkin pie? No, just a "sandy" appearance.

Campus Sample #8
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Hint: This rock texture is "massive" in appearance and rich in iron (the reason for the rusty surface).

Campus Sample #9
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Hint: Two photographs of the same rock type (the second is a core drilled from the rock).

Campus Sample #10
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Hint: This rock is crystalline. What does the crystal orientation suggest?

Campus Sample #11
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Hint: This rock fizzes when a drop of dilute hydrochloric acid is applied. When viewed with a hand lens, this rock is seen to have a network of interlaced crystals distinctive in many marine organisms.  What creature do you think belonged to this "skeleton" now worn down and smoothed?

Campus Sample #12
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Hint: This rock contains two distinctive forms in the facing surface. 

Campus Sample #13
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Hint: Does this rock have crystalline or fragmental texture?

Campus Sample #14
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Hint: Can you recognize some features in the dark brown mass on the rock surface?

Campus Sample #15
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Hint: Do the features of this rock remind you of something which is not a rock?


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