Rock samples of pebble to cobble size are used in the every pebble tells a story activity

Magnetite GraphiteGalena Pyrite Amphibole hornblende Pyroxene augite Garnet Olivine Biotite mica Hematite Plagioclase feldspar Orthoclase feldspar Quartz Halite Dolomite Calcite Muscovit

Every Pebble Tells a

Story 1

L Braile, Purdue University

S Braile, Happy Hollow

School, West Lafayette, IN

March, 2006

braile@purdue.edu,

http://web.ics.purdue.edu/~braile

Last modified March 13, 2006

http://web.ics.purdue.edu/~braile/edumod/pebble/pebble.htm.

Partial funding for this development provided by the National Science Foundation.

 Copyright 2005 L Braile Permission granted for reproduction for non-commercial uses.

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://

Rock Samples of

pebble to cobble

size are used in

the Every Pebble

Tells a Story

activity.

Mineral identification is an excellent preparatory

exercise for the Every Pebble Tells a Story activity.

Olivine Hematite

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 Plagioclase feldspar Orthoclase feldspar Quartz Halite Dolomite Calcite Muscovite mica Gypsum (selenite ) Talc Gypsum (alabas ter)

Hematite

Metallic Luster

Nonmetallic Luster; Dark color

Nonmetallic Luster; Light color

Magnetic;

shiny black

Nonmagnetic

Silver color Gold color

Softer than fingernail

Harder than f ingernail, dense

Softer than steel nail

Good Cleavage

Poor Cleavage

56 o and 124 o angles

87 o and 93 o angles

Good Cleavage Poor Cleavage

Reddish brown streak

Well-f ormed crystals

No c rystals, glassy, olive green

One direction, sheets Reddish brow n streak

Harder than steel nail

Softer than

f ingernail

Softer than steel nail, harder than

f ingernail

No cleavage Cleavage

Cubic cleavage Non-cubic cleavage

Poor Cleavage

Good Cleavage

Mineral Identification Flowchart

(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 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)

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.

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

Glassy, usually black

Fine grain, no crystals visible, has vesicles (holes, like gas bubbles)

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

Marble (M)

Gneiss (M)*** Schist (M)

Granite (I) Basalt (I)

Diorite (I)

stone (S)

Sand-Shale (S)

Slate (M)

Pumice (I) Scoria (I) Basalt (I)

Obsidian (I)

erate (S)

Conglom-Rock has crys tals (“coars e grained”)

Rock has

no crys tals (“fine grained”)

Rock has

no layers

Layers are wide bands Layers are thin; shiny

Rock is often hard*

Rock is light colored**

Rock is dark colored**

Fine grained; often soft*

Soft*; usually white, pink

stone (S)

Sand-Quartzite (M)

Basalt (I)

stone (S)

Lime-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 grains visible; 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 s ponge Dark

colored**

Glassy; like cinder Not glassy, black, gray Light gray

Glass; black, black with white,red

Not glass

Can scrape sand off rock

Cannot scrape sand off rock

Contains pebbles

No pebbles Hard*

Soft*; usually gray, white or tan

Various colors;

“sugary”

Black,dk gray Light 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 = mentary rock ; M = Meta- morphic rock.

Sedi-Various colors;

dull s mooth surface

Chert (S)

Andesite (I) Andesite (I)

1

2 1

1 2 3

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 Horizontality

Time

“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

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

2 3

1

2 6

3 7

1

3

1

2 6

3 7

“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 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…)