Every Pebble Tells a Story 1 L Braile
Every Pebble Tells a Story 1 L. Braile, Purdue University S. Braile, Happy Hollow School, West Lafayette, IN October, 2005 braile@purdue. edu, http: //web. ics. purdue. edu/~braile Last modified November 14, 2005 1 The web page for this document is: http: //web. ics. purdue. edu/~braile/edumod/pebble. htm. This Power. Point file: http: //web. ics. purdue. edu/~braile/edumod/pebble. ppt. Partial funding for this development provided by the National Science Foundation. Copyright 2005. L. Braile. Permission granted for reproduction for non-commercial uses.
Objectives: Make inferences about the geological history of a pebble by observing its characteristics and by utilizing geological principles and concepts. The Every Pebble Tells a Story* activity is an excellent follow-up to the study of mineral and rock identification. However, extensive experience with mineral and rock identification is not required. This activity provides practice in observing and critical thinking and opportunities for sketching and creative writing. Because students select, analyze and write about their own pebble, we have found that the activity is very engaging for students. Additional materials (http: //web. ics. purdue. edu/~braile/edumod/pebble. AM. htm) that accompany this activity include resources for the teacher, for helping the student with the interpretation of their rock sample, and for mineral and rock identification.
Rock Samples of pebble to cobble size are used in the Every Pebble Tells a Story activity.
Limestone Sandstone Quartzite pebbles Basalt Pebbles with cross-bedding Conglomerate
Mineral identification is an excellent preparatory exercise for the Every Pebble Tells a Story activity. Quartz Hematite Olivine
As a practice or review exercise, try to identify one or more mineral samples. 1. List two distinctive properties of your mineral sample. 2. Use the following Mineral Identification Flowchart to identify your mineral sample. 3. What is the name of your mineral sample? An online mineral identification quiz can be found at: http: //www. soes. soton. ac. uk/resources/collection/minerals/min-quiz/index. htm
Mineral Identification Flowchart
Pebbles for “Every Pebble Tells a Story” activity
Observing pebble features
1. The Rock Cycle: The rock cycle schematically illustrates the processes of rock formation through surface and internal processes (represented by the blue and red arrows) in the Earth. Most of these processes, especially at large scale, are the result of plate tectonics.
As the diagram shows, sedimentary rocks are formed by erosion and deposition (and subsequent compaction and cementation of the sediments into rock; usually by burial by additional accumulation of sediments) of igneous, metamorphic and previously-existing sedimentary rock by surface processes (mechanical and chemical erosion by wind and water).
Igneous rocks are formed by melting (and subsequent cooling and crystallization into solid rock) of sedimentary, metamorphic or pre-existing igneous rocks. Because the melting point of typical Earth materials ranges from about 600 and 1300 degrees Celsius, the molten material that forms igneous rocks comes from depths in the Earth where these high temperatures exist (tens to hundreds of kilometers). Volcanic igneous rocks are the result of liquid rock materials (called magma; molten rock; or lava, when it reaches the surface) rising from depths within the Earth (typically 20 to 150 km depth or more) and erupting on the surface where they cool rapidly.
Metamorphic rocks are formed by re-crystallization (without large scale melting of the material) caused to heat and pressure, of sedimentary, igneous or pre-existing metamorphic rocks. Metamorphic processes occur at depths of a few kilometers below the surface to a many tens of kilometers depth. Because igneous and metamorphic processes occur at significant depth within the Earth, the exposure of igneous and metamorphic rocks at the surface means that these rocks have been uplifted to the surface after formation, often by plate tectonic processes.
Common surface sedimentary processes and rock characteristics: a. Mechanical and chemical erosion b. Rounding of rock fragments and particles c. Sorting of sediments during transport d. Deposited in distinct layers. e. Mineral sorting f. Chemical sedimentary rocks (precipitation) g. Clastic sedimentary rocks contain rock or mineral fragments. h. Veins, weathering, staining.
Common igneous processes and rock characteristics: a. Melting, subsequent cooling resulting in crystals of distinct mineral types. b. Plutonic igneous rocks often show a “salt and pepper” appearance and interlocking crystals. c. Volcanic igneous rocks often have gas bubbles (vesicles) visible. d. Plutonic igneous rocks often contain xenoliths. e. Volcanic igneous rocks often include phenocrysts.
Common surface metamorphic processes and rock characteristics: a. Metamorphic rocks are igneous, sedimentary or previouslymetamorphosed rocks that are transformed into new metamorphic rock by heat and pressure; the heat and pressure cause recrystallization of the original minerals in the parent rock and development of metamorphic texture. b. Metamorphic textures are alignment of rectangular, platy, and elongated crystals; development of shiny surfaces by the conversion of clay minerals to mica; the generation of metamorphic layering called foliation c. (“wavy” layering); injection of minerals d. (usually quartz and feldspar) in veins e. and dikes that often cut across the f. foliation; and the development of large g. crystals (called porphoroblasts, such h. as garnet or staurolite) in the i. metamorphic mineral matrix.
Simplified Rock Identification and Origin Flowchart (designed primarily to determine the origin of the rock sample – igneous, sedimentary or metamorphic).
Rock Identification Flowchart
Sample Reporting Pages: An example of several of these steps for a particular rock sample is shown below: 1. Observations and description (size, shape, smoothness, texture, layering, colors, crystals or rock fragments visible, etc. ) of your rock sample (pebble): - elongated, approximately oval shape; ~14 cm x 6 cm x 3. 5 cm - has dark and light alternating layers, ~2 mm thick - medium grain size (individual grains or crystals are visible) - layers are slightly wavy (foliation) - hardness of minerals is ~5 or greater - surface is rounded and smooth - white veins that cut across the layers are present - some small cracks are visible - a small piece was fractured off after the rounding
2. Sketch of your sample (annotate your sketch by pointing out distinctive features or characteristics):
3. Rock type or classification (igneous, sedimentary or metamorphic; include evidence, rationale and observations which justify your classification) and rock name (if possible; such as granite, basalt, sandstone, limestone, gneiss, etc. ): - metamorphic; grains have re-crystallized and are interlocking with virtually no pore spaces and foliation visible - rock is a gneiss 4. Inferred geologic history of your sample (oldest to youngest): Oldest Sediments accumulate in layers Rock forms by burial and lithification Deeper burial and high temperature and pressure cause metamorphism Heating and pressure cause foliation Rock fractures and veins are injected Rock unit is uplifted or brought to surface Rock breaks (erodes) into small fragment Rounding occurs - probably by river transport Small piece of rock breaks off during transport Youngest A few scratch marks occur on upper surface
6. The story of your rock. (Write a creative story of the "lifetime", “history”, or "adventures" of your rock. Use opposite side of paper or additional sheets of paper to continue your story): Gneiss Rock Hello! My name is Gneiss Rock. I’m a metamorphic rock. I didn’t start out that way. No. I began as part of a very large accumulation of sediments millions of years ago somewhere beneath the ocean. There were sand shale deposits and the sediments just kept accumulating until I was buried very deeply and compacted into part of a thick sequence of layers of sedimentary rocks. I really don’t know what happened next except that I began to feel hot and feel increased pressure from the rock around me. Something must have been pushing on the sedimentary rock layers. The heat and pressure kept increasing until I noticed that my sand silt particles were changing into larger mineral crystals. Also, the flat boundaries separating the different sedimentary materials were now more wavy layers. Later, some cracks formed in me and they were quickly filled with a hot fluid that became solid and formed white veins that cut though some of my layers. It had taken a long time, but I had become a metamorphic rock. (continued…)
Try the Every Pebble Tells a Story activity with your own pebble. Use the procedure and sample pages from the Every Pebble… activity description and the flowcharts and other materials in the “Additional Materials” handout.
Mineral identification tables for more information on common mineral properties.
Mineral Key -- Metallic Minerals (all minerals have metallic Luster) Hardness 6 6 1 -5 2. 5 3. 5 -4 Color Diagnostic Features none black color and strong magnetic character, black streak poor brassyellow hardness, color and cubic crystal habit, black streak not prominent dark metallic silver; also reddish brown; red streak reddish brown streak is characteristic cubic silver gray prominent cubic cleavage; gray to black streak, high density sometimes brassyellow softer than pyrite, often tarnished slightly iridescent Cleavage Density (g/cm 3) Mineral Comments Magnetite The most common of a small number of minerals that are magnetic. 5. 2 Pyrite Fool's gold. Shiny or dull gold appearance; much harder than gold. 4 -5 Hematite Iron oxide, often appears rust colored. 7. 6 Galena Most important lead mineral in the Earth's crust. 4. 2 Chalcopyrite is a widespread copper mineral. 5. 2
Mineral Key -- Nonmetallic Minerals (all minerals have nonmetallic Luster) Hardness Cleavage Color Diagnostic Features Density (g/cm 3) Mineral Comments 7 none colorless, white, pink, smoky, amythest (violet) glassy, breaks like glass (conchoidal fracture) 2. 65 Quartz Common mineral; made of Si. O 2. Earth's crust is ~50 -70% Si. O 2. 6 cleavage planes at ~ right angles pink, white, gray hardness similar to glass, color 2. 6 Feldspar Common rock forming mineral in the Earth's crust. 5 -6 cleavage planes at 56 o and 124 o dark green, dark brown, black cleavage angles and six sided crystals ~3. 2 Amphibole Hornblende is the most common mineral of the amphibole group 5 -6 cleavage planes at ~ right angles usually dark green to black but can be white to pale green cleavage and dark color 3. 2 -3. 8 Pyroxene Many different minerals make up the pyroxene group. 3 cleavage planes at 75 o and 105 o mostly white, yellowish to colorless, but many colors possible hardness and cleavage (rhombohedral shape) are characteristic 2. 71 Calcite is primary mineral in limestone. Many crystal forms. 2. 5 cubic; three planes at 90 o white to colorless tastes like salt 2. 16 Halite Na. Cl; evaporite; source of table salt. 2 cleavage planes at 66 o and 114 o white to transparent hardness and cleavage are characteristic 2. 32 Gypsum Tabular crystals and rhombic fragments are common. ~2. 5 1 direction colorless, yellow brown, smoky, black very thin sheets, excellent cleavage in one direction 2. 8 -3. 4 Mica Brown to black is biotite. Nearly colorless mica is muscovite. 1 poor pale green, white or gray hardness and greasy feel are characteristic 2. 82 Talc is used to make talcum powder. 1 -5 not prominent reddish brown, also dark metallic silver; red streak reddish brown streak is characteristic 4 -5 Hematite Iron oxide; often appears rust colored. 6. 5 -7 poor olive green glassy, olive green crystals ~3. 3 Olivine An iron-magnesium silicate, an important part of Earth's mantle.
Every Pebble Tells a Story 1 L. Braile, Purdue University S. Braile, Happy Hollow School, West Lafayette, IN October, 2005 braile@purdue. edu, http: //web. ics. purdue. edu/~braile Last modified November 14, 2005 1 The web page for this document is: http: //web. ics. purdue. edu/~braile/edumod/pebble. htm. This Power. Point file: http: //web. ics. purdue. edu/~braile/edumod/pebble. ppt. Partial funding for this development provided by the National Science Foundation. Copyright 2005. L. Braile. Permission granted for reproduction for non-commercial uses.
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