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Every Pebble Tells a Story 1
L. Braile, Purdue UniversityS. Braile, Happy Hollow School, West Lafayette, IN March, 2006
NSTA, Anaheim, [email protected], http://web.ics.purdue.edu/~braile
Last modified March 13, 20061 The web page for this document is: http://web.ics.purdue.edu/~braile/edumod/pebble/pebble.htm.This PowerPoint file: http://web.ics.purdue.edu/~braile/edumod/pebble/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/pebbleAM.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.
Sandstone
Limestone
Pebbles with cross-beddingQuartzite pebbles
Basalt Conglomerate
Mineral identification is an excellent preparatory exercise for the Every Pebble Tells a Story activity.
OlivineHematite
Quartz
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
Magnetite
Graphite
Galena
Pyrite
Amphibole(hornblende)
Pyroxene(augite)
Garnet
Olivine
Biotite mica
Hematite
Plagioclasefeldspar
Orthoclasefeldspar
Quartz
Halite
Dolomite
Calcite
Muscovite mica
Gypsum(selenite )
Talc
Gypsum(alabas ter)
Hematite
MetallicLuster
NonmetallicLuster; Dark color
NonmetallicLuster; Light color
Magnetic;shiny black
Nonmagnetic
Silvercolor
Goldcolor
Softer than fingernail
Harder than f ingernail, dense
Sta
rt h
ere
Harder thansteel nail
Softer thansteel nail
GoodCleavage
PoorCleavage
56 o and 124 o angles
87 o and 93 o angles
GoodCleavage
PoorCleavage
Reddish brown streak
Well-formed crystals
No c rystals, glassy, olive green
One direction, sheets
Reddish brow n streak
Harder thansteel nail
Softer thanf ingernail
Softer thansteel nail, harder than f ingernail
No cleavage
Cleavage
Cubiccleavage
Non-cubiccleavage
PoorCleavage
GoodCleavage
Mineral Identification Flowchart
Striations (tw inning)
No striations (tw inning)
Pow der reacts with HCl
Sample reacts w ith HCl
“Nongreasy” feel
“Greasy” feel
Thick sheets
Thin sheets
(modif ied from D.J. Thompson, Basic Geology: Lab Manual , Allegheny Press, 1986; and D.J. Conte, D.J. Thompson and L.L. Moses, Earth Science - A Holistic Approach, W.C. Brow n, 1994.)
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 erosionb. Rounding of rock fragments and particles c. Sorting of sediments during transport d. Deposited in distinct layers. e. Mineral sortingf. 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 previously-metamorphosed rocks that are transformed into new metamorphic rock by heat and pressure; the heat and pressure cause re-crystallization 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 (“wavy” layering); injection of minerals (usually quartz and feldspar) in veins and dikes that often cut across the foliation; and the development of large crystals (called porphoroblasts, such as garnet or staurolite) in the metamorphic mineral matrix.
Simplified Rock Identification and OriginFlowchart (designed primarily to determine the origin of the rock sample – igneous, sedimentary or metamorphic).
Coarse to fine grain, surface is shiny, foliation (wavy or flat layers), softer than a nail
Rock has distinct “sugary” look, layering may be very subtle, cannot scratch off grains, harder than nail, white to tan, sometimes colored, may show cross bedding
Visible, coarse grain, foliation (wavy layers), grains are aligned, often has “salt and pepper” look, white “veins”, most grains are harder than a nail
Fine grain, surface is not shiny, thin flat layers, usually dark color
Rock has layers*
Soft***, reacts with acid, may have fossils, usually light gray, SEDIMENTARY, limestone
Grains visible or many wavy lines, METAMORPHIC, marble+
METAMORPHIC, schist (coarse grain), phyllite (medium grain), slate (fine grain, less shiny, thin flat layers)
METAMORPHIC, quartzite
SEDIMENTARY, may have fossils, shale, mudstone
Simplified Rock Identification and Origin** Flowchart
SEDIMENTARY, may have fossils, siltstone, (fine grain), sandstone (medium grain), conglomerate (contains rounded pebbles), breccia (contains angular pebbles)
SEDIMENTARY, chert, flint
Grains are fragments (often rounded) cemented together, can usually scratch off grains
Very fine grain, looks “chalky” but is very hard (harder than nail), can be any color, layering may be very subtle
Start Here
Rock has no layers*
Hard***, does not react with acid, no fossils, usually dark color, IGNEOUS, volcanic
Fine grain, no crystals visible
IGNEOUS, plutonic, may contain xenoliths
METAMORPHIC, gneiss
IGNEOUS, volcanic
Visible interlocking grains (crystal shapes), often “salt and pepper” appearance
Glassy, usually black
Fine grain, no crystals visible, has vesicles (holes, like gas bubbles)
Simplified Rock Identification and OriginFlowchart (designed primarily to determine the origin of the rock sample – igneous, sedimentary or metamorphic).
Rock has distinct “sugary” look, layering may be very subtle, cannot scratch off grains, harder than nail, white to tan, sometimes colored, may show cross bedding
Fine grain, surface is not shiny, thin flat layers, usually dark color
Rock has layers*
Soft***, reacts with acid, may have fossils, usually light gray, SEDIMENTARY, limestone
Grains visible or many wavy lines, METAMORPHIC, marble+
SEDIMENTARY, may have fossils, shale, mudstone
SEDIMENTARY, may have fossils, siltstone, (fine grain), sandstone (medium grain), conglomerate (contains rounded pebbles), breccia (contains angular pebbles)
SEDIMENTARY, chert, flint
Grains are fragments (often rounded) cemented together, can usually scratch off grains
Very fine grain, looks “chalky” but is very hard (harder than nail), can be any color, layering may be very subtle
Start Here
Rock has no layers*
Hard***, does not react with acid, no fossils, usually dark color, IGNEOUS, volcanic
Fine grain, no crystals visible
IGNEOUS, plutonic, may contain xenoliths
IGNEOUS, volcanic
IGNEOUS, volcanic, obsidian
IGNEOUS, volcanic
Visible interlocking grains (crystal shapes), often “salt and pepper” appearance
Glassy, usually black
Fine grain matrix, distributed crystals (phenocrysts) visible (usually rectangular)
Fine grain, no crystals visible, has vesicles (holes, like gas bubbles)
This flowchart is designed for commonly occurring rocks, especially those that are fairly resistant to erosion (often igneous and metamorphic rocks) and are thus likely to be selected as pebbles for the Every Pebble Tells a Story activity. A small percentage of volcanic rocks show small scale layering caused by flow lines; these and other samples may not be correctly classified using this flowchart. Some samples show sedimentary features but have been metamorphosed. Examples include metamorphosed conglomerate, quartzite and greenstone (a metamorphosed volcanic rock). * “Has layers” means thin layers in hand specimen. ** Rock type and most recent origin of the rock (igneous, sedimentary, metamorphic) is inferred from the flowchart and can be interpreted in relation to the Rock Cycle. *** “Hard” means hardness of >5; “soft” means hardness <5. + Scratch marble with nail to obtain powder; will react with acid.
Simplified Rock Identification and OriginFlowchart (designed primarily to determine the origin of the rock sample – igneous, sedimentary or metamorphic).
Marble (M)
Gneiss (M)***
Schist (M)
Granite (I)
Basalt (I)
Diorite (I)
Sand-stone (S)
Shale (S)
Slate (M)
Pumice (I)
Scoria (I)
Basalt (I)
Obsidian (I)
Conglom-erate (S)
Rock has crys tals (“coarse grained”)
Rock hasno crys tals (“fine grained”)
Sta
rt h
ere
Rock has layers
Rock has no layers
Layers are wide bands
Layers arethin; shiny
Rock is often hard*
Rock is light colored**
Rock is dark colored**
Fine grained; often soft*
Soft*; usually white, pink or tan
Rock Identification Flowchart
Hard*
Fine grained matrix
Coarse grained
Rhyolite (I)
Gabbro (I)
Fine grained matrix; black, dk grayLight gray
Coarse grained; “salt and pepper” appearance
Coarse grained; black, gray,greenish
Sand-stone (S)
Quartzite (M)
Basalt (I)
Lime-stone (S)
Rock has layers
Med. grained; black, gray, greenish Diabase (I)
Rock has no layers
Phyllite (M)
Coarse grained; often soft*
Sand grains visible; feels like sandpaper
No sand grainsvisible; very fine grained
Thin, flat layers; greenish, gray, red; smooth
Thin, wavy layers , looks like mudstone
Rock has no holes
Rock has holes(vesicles)
Light colored**; low dens ity; looks like a glassy sponge
Darkcolored**
Glassy; like cinder
Not glassy, black, grayLight gray
Glass; black, black with white,red
Notglass
Can scrape sand off rock
Cannot scrape sand off rock
Containspebbles
No pebbles
Hard*
Soft*; usually gray, white or tan
Various colors; “sugary”
Black,dk grayLight gray* “Hard” means hardness of
~5 or greater; “soft” means hardness of less than 5.** Dark colored means light gray or darker.
*** I = Igneous rock ; S = Sedi-mentary rock ; M = Meta-morphic rock.
Various colors; dull smooth surface
Chert (S)
Andesite (I)
Andesite (I)
Andesite (I)
Cannot scrape off sand grains, “sugary” Quartzite (M)
Rock Identification Flowchart
1
1
2 1
2 3
1
2 3
A. B. C. D.
I. Principle of Superposition (for relatively undeformed sedimentary and volcanic rock layers)
Time
“Younger rocks are on top of older rocks.”
1
2 3
1
2 3
1 2 3
II. Principle of Original HorizontalityTime
A. B. C.
“Most layers of sediment are deposited in nearly horizontal layers. Thus, layers that are now dipping were (most likely) deposited approximately horizontally and then deformed.”
1
2 4
Cross section view
Surface
1
2 3
1
2 3
1
2
3 1
2 3
1
2
3 4 5
III. Cross-cutting relationships and Unconformities
1
2 3
1
2
3
6 1
2
3
1
2
6
3 7
1
3
1
2
6
3 7 8
A. B. C.
D. E. F.
IV. Cross-bedding
2
Time
“Rock units, structures or surfaces representing erosion and loss of continuous deposition (unconformities) that cut across layers (cross-cutting) are younger than the layers that are cut through.”“An unconformity is a surface that represents a break in the rock record, caused by erosion or non-deposition.”
“A small scale (centimeters to meters) sedimentary structure in which layers are inclined at an angle to adjacent layers. Formed during deposition by currents of water or wind.”
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 6cm 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 gneiss4. 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 RockHello! 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 and 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 and 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)
Hard-ness
Cleavage Color Diagnostic
FeaturesDensity (g/cm3)
Mineral Comments
6 none black
black color and strong magnetic character, black
streak
5.2 Magnetite
The most common of a small number of minerals that are
magnetic.
6 poorbrass-yellow
hardness, color and cubic crystal
habit, black streak5.2 Pyrite
Fool's gold. Shiny or dull gold appearance;
much harder than gold.
1-5not
prominent
dark metallic
silver; also reddish
brown; red streak
reddish brown streak is
characteristic4-5 Hematite
Iron oxide, often appears rust colored.
2.5 cubic silver gray
prominent cubic cleavage; gray to black streak, high
density
7.6 GalenaMost important lead
mineral in the Earth's crust.
3.5-4 sometimesbrass-yellow
softer than pyrite, often tarnished
slightly iridescent4.2
Chalco-pyrite
Chalcopyrite is a widespread copper
mineral.
Mineral Key -- Nonmetallic Minerals (all minerals have nonmetallic Luster)
Hard-ness
Cleavage Color Diagnostic FeaturesDensity (g/cm3)
Mineral Comments
7 nonecolorless, white, pink,
smoky, amythest (violet)glassy, breaks like glass
(conchoidal fracture)2.65 Quartz
Common mineral; made of SiO2. Earth's crust is ~50-70% SiO2.
6cleavage
planes at ~ right angles
pink, white, grayhardness similar to glass,
color2.6 Feldspar
Common rock forming mineral in the Earth's crust.
5-6cleavage
planes at 56o and 124o
dark green, dark brown, black
cleavage angles and six sided crystals
~3.2 AmphiboleHornblende is the most common mineral of the amphibole group
5-6cleavage
planes at ~ right angles
usually dark green to black but can be white to pale
greencleavage and dark color 3.2-3.8 Pyroxene
Many different minerals make up the pyroxene group.
3cleavage
planes at 75o and 105o
mostly white, yellowish to colorless, but many colors
possible
hardness and cleavage (rhombohedral shape) are
characteristic2.71 Calcite
Calcite is primary mineral in limestone. Many crystal forms.
2.5cubic; three
planes at 90o white to colorless tastes like salt 2.16 HaliteNaCl; evaporite; source of table
salt.
2cleavage
planes at 66o and 114o
white to transparenthardness and cleavage are
characteristic2.32 Gypsum
Tabular crystals and rhombic fragments are common.
~2.5 1 directioncolorless, yellow brown,
smoky, blackvery thin sheets, excellent cleavage in one direction
2.8-3.4 MicaBrown to black is biotite. Nearly
colorless mica is muscovite.
1 poor pale green, white or grayhardness and greasy feel
are characteristic2.82 Talc
Talc is used to make talcum powder.
1-5 not prominentreddish brown, also dark metallic silver; red streak
reddish brown streak is characteristic
4-5 HematiteIron oxide; often appears rust
colored.
6.5-7 poor olive green glassy, olive green crystals ~3.3 OlivineAn iron-magnesium silicate, an important part of Earth's mantle.
NSTA Presentations: Seismic Waves, Thur., 4/6, 8-9AM, CC 210A Journey to the Center of the Earth, Fri., 4/7, 8-
9AM, Hilton, San Clemente Seismographs, Fri., 4/7, 2-3PM, CC 304C Every Pebble Tells a Story, Sat., 4/8, Hilton, El
Capitan ACSTA (Oct. 20-22, 2006, San Francisco): Earthquakes – A One Day Workshop for Teachers,
Sat., 10/21, 8AM-4:30PM Also, Earthquakes and the San Francisco Bay Area
and Every Pebble Tells a StoryNSTA (March 28, 2007, St. Louis): Earthquakes – A One Day Workshop for Teachers
Every Pebble Tells a Story 1
L. Braile, Purdue UniversityS. Braile, Happy Hollow School, West Lafayette, IN March, 2006
NSTA, Anaheim, [email protected], http://web.ics.purdue.edu/~braile
Last modified March 13, 20061 The web page for this document is: http://web.ics.purdue.edu/~braile/edumod/pebble/pebble.htm.This PowerPoint file: http://web.ics.purdue.edu/~braile/edumod/pebble/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.
