Every Pebble Tells a
Story 1
L. Braile, Purdue University
S. Braile, Happy Hollow
School, West Lafayette, IN
March, 2006
NSTA, Anaheim, 2006
braile@purdue.edu,
http://web.ics.purdue.edu/~braile
Last modified March 13, 2006
1 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.
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
(“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 Origin
Flowchart
(designed
primarily to
determine the
origin of the rock
sample –
igneous,
sedimentary or
metamorphic).
Simplified Rock Identification and Origin** Flowchart
Simplified
Rock
Identification
and Origin
Flowchart
(designed
primarily to
determine the
origin of the
rock sample –
igneous,
sedimentary or
metamorphic).
Visible, coarse grain, foliation (wavy
layers), grains are aligned, often has “salt
and pepper” look, white “veins”, most grains
are harder than a nail
Coarse to fine grain, surface is shiny,
foliation (wavy or flat layers), softer than a
nail
Rock
has
layers*
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
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*
METAMORPHIC, gneiss
METAMORPHIC, schist (coarse
grain), phyllite (medium grain),
slate (fine grain, less shiny,
thin flat layers)
METAMORPHIC, quartzite
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
Soft***, reacts with acid, may
have fossils, usually light gray,
SEDIMENTARY, limestone
Fine grain, no crystals visible
Grains visible or many wavy
lines, METAMORPHIC, marble+
Visible interlocking grains (crystal shapes),
often “salt and pepper” appearance
Hard***, does not react with
acid, no fossils, usually dark
color, IGNEOUS, volcanic
Fine grain, no crystals visible, has vesicles
(holes, like gas bubbles)
Glassy, usually black
IGNEOUS, plutonic, may
contain xenoliths
Rock
has
layers*
Simplified
Rock
Identification
and Origin
Flowchart
(designed
primarily to
determine the
origin of the
rock sample –
igneous,
sedimentary or
metamorphic).
sometimes colored, may show cross
bedding
Fine grain, surface is not shiny, thin flat
layers, usually dark color
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*
fossils, shale, mudstone
SEDIMENTARY, may have
fossils, siltstone, (fine grain),
sandstone (medium grain),
conglomerate (contains
rounded pebbles), breccia
(contains angular pebbles)
SEDIMENTARY, chert, flint
Soft***, reacts with acid, may
have fossils, usually light gray,
SEDIMENTARY, limestone
Fine grain, no crystals visible
Grains visible or many wavy
lines, METAMORPHIC, marble+
Visible interlocking grains (crystal shapes),
often “salt and pepper” appearance
Hard***, does not react with
acid, no fossils, usually dark
color, IGNEOUS, volcanic
Fine grain, no crystals visible, has vesicles
(holes, like gas bubbles)
Glassy, usually black
Fine grain matrix, distributed crystals
(phenocrysts) visible (usually rectangular)
IGNEOUS, plutonic, may
contain xenoliths
IGNEOUS, volcanic
IGNEOUS, volcanic, obsidian
IGNEOUS, volcanic
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.
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 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 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
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)
Hardness
6
6
1-5
2.5
3.5-4
Color
Diagnostic
Features
black
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
none
Density
(g/cm3)
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
Chalcopyrite is a
widespread copper
mineral.
5.2
Mineral Key -- Nonmetallic Minerals (all minerals have nonmetallic Luster)
Hardness
Cleavage
Color
Diagnostic Features
Density
(g/cm3)
Mineral
Comments
7
none
colorless, 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.
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 56o
and 124o
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 75o
and 105o
mostly white, yellowish to
colorless, but many colors
possible
hardness and cleavage
(rhombohedral shape) are
characteristic
2.71
Calcite
Calcite is primary mineral in
limestone. Many crystal forms.
2.5
cubic; three
planes at 90o
white to colorless
tastes like salt
2.16
Halite
NaCl; evaporite; source of table
salt.
2
cleavage
planes at 66o
and 114o
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
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.
NSTA Presentations:




Seismic Waves, Thur., 4/6, 8-9AM, CC 210A
Journey to the Center of the Earth, Fri., 4/7, 89AM, Hilton, San Clemente
Seismographs, Fri., 4/7, 2-3PM, CC 304C
Every Pebble Tells a Story, Sat., 4/8, Hilton, El
Capitan A
CSTA (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 Story
NSTA (March 28, 2007, St. Louis):

Earthquakes – A One Day Workshop for Teachers
Every Pebble Tells a
Story 1
L. Braile, Purdue University
S. Braile, Happy Hollow
School, West Lafayette, IN
March, 2006
NSTA, Anaheim, 2006
braile@purdue.edu,
http://web.ics.purdue.edu/~braile
Last modified March 13, 2006
1 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.