SOUTH CAROLINA SUPPORT SYSTEM INSTRUCTIONAL PLANNING GUIDE
Content Area:
Eighth Grade Science
Recommended Days of Instruction: 3
(one day equals 55 min)
Standard(s) addressed: 8-3
The student will demonstrate an understanding of materials that determine the structure of Earth and the processes that have
altered this structure. (Earth Science)
Earthquake‘s Epicenter
Indicator
8-3.4: Explain
how igneous,
metamorphic, and
sedimentary rocks
are interrelated in
the rock cycle.
Recommended Resources
SC Science Standards Support
Document Resource List
https://www.ed.sc.gov/apps/cso/s
tandards/supdocs_k8.cfm
Suggested Instructional Strategies
See Science Module 8-3.4
SC ETV Streamline
http://etv.streamlinesc.org
“Introduction to Rocks and
Minerals” from the series Earth
Science: Rocks and Minerals
http://player.discoveryeducation.c
om/index.cfm?guidAssetId=54AA9
79F-7F4D-418A-A3742CF5AF52F681&blnFromSearch=1
&productcode=US
“The Rock Cycle” from the series
“Rocks: The Solid Earth
Materials:
http://player.discoveryeducation.c
om/index.cfm?guidAssetId=755E0
9F3-2E30-4B04-8824B5ABB65FE206&blnFromSearch=1
&productcode=US
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
1
Assessment Guidelines
From the South
Carolina Science
Support Documents:
The objective of this
indicator is to explain the
interrelationships of the
three rock types;
therefore, the primary
focus of assessment
should be to construct a
cause-and-effect model
about the forming of a
rock based on the
process(es) involved.
However, appropriate
assessments should also
require students to
interpret a rock cycle
diagram; compare how
rocks can be changed by
particular processes; or
identify a rock type by
the method with which it
is formed.
Additional Resources:
Computer simulation of the
three different rocks:
http://www.as.uky.edu/academics
/departments_programs/EarthEnvi
ronmentalSciences/EarthEnvironm
entalSciences/Educational%20Mat
erials/Documents/elearning/modul
e05swf.swf
US Geological Survey
http://www.usgs.gov/
SC Department of Natural
Resources (SCDNR)
http://www.dnr.sc.gov/education.h
tml
South Carolina Department of
Natural Resources (free poster
or graphics)
http://www.dnr.sc.gov/geology/Ed
ucation.htm
Mineral Information Institute
http://www.mii.org
Women in Mining
http://womeninmining.org/
The Geologic Society of
America
http://www.geosociety.org/
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
2
The Mineralogical Society of
America
http://www.minsocam.org/
Geology and Earth Science
http://www.geology.com
Geology animations and songs
http://www.as.uky.edu/academics
/departments_programs/EarthEnvi
ronmentalSciences/EarthEnvironm
entalSciences/Educational%20Mat
erials/Pages/default.aspx
Smithsonian National
Museum of Natural History
“Dynamic Earth”
http://www.mnh.si.edu/earth/mai
n_frames.html
World of Rocks
http://www.albany.edu/dept/sisp/j
jpowers/WebCollabS05/rocks/inde
x2.html
“Rocks and Geology Exercise”
http://womeninmining.org/activit
ies/Rocks_and_Geology.pdf
The Rock Cycle explained
http://www.cotf.edu/ete/modules/
msese/earthsysflr/rock.html
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
3
For more information on the
Rock Cycle
http://pubs.usgs.gov/gip/2005/17/
gip-17.pdf
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
4
Eighth Grade
Science Module
8-3.4
Part A
Earth’s Structures and
Processes
Lesson A-B
Standard 8-3: The student will demonstrate an understanding of
materials that determine the structure of Earth and the processes that
have altered this structure. (Earth Science)
Indicator 8-3.4: Explain how igneous, metamorphic, and sedimentary
rocks are interrelated in the rock cycle.
Other indicators addressed:
Indicator 8-1.2: Recognize the importance of a systematic process for
safety and accurately conducting investigations.
Indicator 8-1.3: Construct explanations and conclusions from
interpretations of data obtained during a controlled scientific investigation.
Indicator 8-1.7: Use appropriate safety procedures when conducting
investigations.
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
5
From the South Carolina Science Support Documents:
Indicator 8.3.4: Explain how igneous, metamorphic, and sedimentary rocks are
interrelated in the rock cycle.
Taxonomy level:
Understand Conceptual Knowledge (2.7-B)
Previous/future knowledge: Rocks were introduced in 1st grade (1-4.1, 2) as
materials from Earth that can be classified by their physical appearance. In 3rd
grade (3-3.1), students classified rocks as sedimentary, igneous, and metamorphic
based on their properties. No additional study of rocks has been done since 3rd
grade. The concept of a rock cycle is new to this grade. The classification of
minerals and rocks based on physical and chemical properties is part of high school
Earth Science (ES-3.7)
It is essential for students to know that there are three large classifications of
rocks – igneous, metamorphic, and sedimentary. Each type of rock is formed
differently and can change from one type to another over time.
Igneous
 Forms when molten rock (magma) cools and hardens.
 If cooling takes place slowly beneath Earth’s surface, the igneous rock is
called intrusive.
 If the cooling takes place rapidly on Earth’s surface, the igneous rock is
called extrusive.
Metamorphic
 Forms when rocks are changed into different kinds of rocks by great heat
and/or pressure – they are heated, squeezed, folded, or chemically changed
by contact with hot fluids.
Sedimentary
 Forms from the compaction and/or cementation of rock pieces, mineral
grains, or shell fragments called sediments.
 Sediments are formed through the processes of weathering and erosion of
rocks exposed at Earth’s surface.
 Sedimentary rocks can also form from the chemical depositing of materials
that were once dissolved in water.
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
6
The rock cycle is an ongoing process. The sample diagram illustrates the series of
natural processes that can change rocks from one kind to another:
It is not essential for students to classify individual rocks, chemical composition,
or the minerals from which they are made, based on their properties.
Assessment Guidelines:
The objective of this indicator is to explain the interrelationships of the three rock
types; therefore, the primary focus of assessment should be to construct a causeand-effect model about the forming of a rock based on the process(es) involved.
However, appropriate assessments should also require students to interpret a rock
cycle diagram; compare how rocks can be changed by particular processes; or
identify a rock type by the method with which it is formed.
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
7
Teaching Indicator 8-3.4: Lesson A - “Properties of Rocks”
Instructional Considerations:
This lesson is an example of how a teacher might address the intent of this
indicator. The STC kit Catastrophic Events and FOSS kit Earth History both
provide an opportunity for conceptual development of the concepts within the
standard. This lesson has been designed to follow Lesson B “Locating Earthquakes”
for SC Indicator 8-3.3 for the conceptual development of SC Indicator 8-3.
Misconceptions:
Some incorrect believes children often hold include but are not limited to:
 Earth is molten except for its crust
 Rocks must be heavy
 Rocks can only be changed by a blow of a hammer or other powerful object.
 Rock is a hard solid material.
 Rocks are uniform inside.
 Lava comes from the center of the Earth.
 Rocks are stronger than the forces of nature (water, wind, ice).
 Rocks are always in the same form; they never change.
 All rocks are more or less the same (a rock is a rock!).
 Children often have trouble making the connection to real world from
simulations or models; be explicit in asking clarifying questions to ensure
lifelong misconceptions are not created.
Safety Note(s):
Students should know and practice the procedures for fire, glass and chemical
safety. Students should use care when performing this experiment, and be wearing
the proper safety equipment including aprons and goggles. Students should know
and practice safe disposal of materials in a laboratory setting.
Lesson time:
1 day (1 day equals 55 minutes)
Materials Needed: (per groups of 2)
 Safety glasses (per student)
 Hand lens - 1
 Sugar cube - 1
 Aluminum foil – five inch square
 Heat source - 1
Focus Question:
How do the properties of rocks change over time?
Engage:
1. Recall what you learned in 3rd grade about the study of rocks. (SC Indicator
3-3.1 Classify rocks (including igneous, sedimentary, and metamorphic) and
soils (including humus, clay, sand, and silt) on the basis of their properties.)
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
8
a. What are the three classifications of rocks? (Answer: igneous,
sedimentary, and metamorphic)
b. How are they classified? (Answer: Rocks can be classified by
properties, such as how they are formed, color, visible crystals or
minerals, grain pieces, patterns in the rock such as stripes.)
c. How are they different? (Answer: Igneous rock was once melted but it
has cooled and hardened; the melted material is called magma or
lava; igneous rocks may be glassy or grainy with crystals of different
types of minerals in them; granite is an example of an igneous rock.
Sedimentary rocks are usually made up of pieces of rock called
sediments that have been pressed and cemented together; some may
contain pieces of animal shells or skeletons or other remains of plants
or animals; sandstone and limestone are examples of sedimentary
rocks. Metamorphic rock was once another type of rock deep inside
Earth, but heat and the pressing of the rocks above caused the
minerals to change; rocks that were pressed down could have the
minerals line up in rows or bands; sometimes the heat just changes
the size of the mineral.)
Explore:
1. Examine the sugar cube with a hand lens and capture your observations in
your notebook.
2. What type of rock might this represent? (Answer – Sedimentary – the
crystals are still visible, was put together under pressure)
3. Crush the sugar cube into a powder.
4. Re-examine the sugar with a hand lens and capture your observations in
your notebook.
5. How does the sugar look now compared to before it was crushed?
6. Now what type of rock might this represent? (Answer – Sedimentary – no
real change has occurred - the particles have just been broken into smaller
pieces)
7. Make a “boat” with your foil. Pour the crushed sugar into the foil boat. Predict
in your notebook what the sugar might look like once heated.
8. Carefully put the “boat” over the heat source. Record your observations in
your notebook. How do your observations compare to your predictions?
9. What type of rock might this represent? (Answer – Igneous; molten – the
rock has had heat added and melted)
10.Predict what the substance will look like when removed from the heat
source; capture your thoughts in your notebook.
11.Set the foil boat away from the heat and wait 2-3 minutes. Record your
observations. How do your observations compare to your predictions?
12.What type of rock might this represent? (Answer – Igneous – the melted
materials were cooled and hardened)
13.Break the hardened sugar into pieces. Record your observations in your
notebook.
14.What do the pieces remind you of? (Answer - They should resemble the
original sugar cube that was crushed)
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
9
15.How might you create a metamorphic rock using the materials you have?
(Answer: once the melted sugar is almost cooled, add crushed sugar and
mold it in a new shape with pressure – the ending product will have both
materials visible.)
16.Using your notes and observations, construct a cause-and-effect model about
the forming of a each rock type based on the process(es) involved.
Explain:
1. Igneous rocks are often called the “father” of all rocks since they originate
from materials deep within the Earth. Once they arrive on the surface, they
can be altered into the one or both of the other types of rocks which we will
discuss further in the rock cycle.
2. Igneous rocks are recognized by:
a. the interlocking texture of the grains
b. the presence of vesicules (holes) in extrusive igneous rocks because
the cooling takes place rapidly upon exposure to the Earth’s surface
c. may appear coarse-grained in intrusive igneous rocks because the
cooling takes place slowly beneath the Earth’s surface
d. may be dark-colored and heavy
e. may display two grain sizes, one much larger than the other
3. Metamorphic rocks are recognized by:
a. the interlocking texture of large grains
b. foliation (layering); and banded light and dark colors because these
rocks are formed when heated, squeezed, folded or chemically
changed by contact with hot fluids
c. "ching" sound instead of a "chunk" sound when tapped
4. Sedimentary rocks are recognized by:
a. grains or sediments cemented together
b. the presence of fossils
c. light-colored and light weight
d. may display interlocking grains but is very light weight
Extend:
1. Use this computer simulation to reinforce how the different process work to
create the three different rocks:
http://www.as.uky.edu/academics/departments_programs/EarthEnvironment
alSciences/EarthEnvironmentalSciences/Educational%20Materials/Documents
/elearning/module05swf.swf
2. Teach Lesson B SC Indicator 8-3.4 “The Rock Cycle”
Additional Resources:
1. US Geological Survey - http://www.usgs.gov/
2. SC Department of Natural Resources (SCDNR) http://www.dnr.sc.gov/education.html
3. Mineral Information Institute - http://www.mii.org
4. Women in Mining - http://womeninmining.org/
5. The Geologic Society of America - http://www.geosociety.org/
6. The Mineralogical Society of America - http://www.minsocam.org/
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
10
7. Geology and Earth Science – http://www.geology.com
8. Geology animations and songs from the University of Kentucky http://www.as.uky.edu/academics/departments_programs/EarthEnvironment
alSciences/EarthEnvironmentalSciences/Educational%20Materials/Pages/defa
ult.aspx
9. Smithsonian National Museum of Natural History “Dynamic Earth”
http://www.mnh.si.edu/earth/main_frames.html
10.World of Rocks http://www.albany.edu/dept/sisp/jjpowers/WebCollabS05/rocks/index2.html
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
11
Teaching Indicator 8-3.4: Lesson B - “The Rock Cycle”
Instructional Considerations:
This lesson is an example of how a teacher might address the intent of this
indicator. The STC kit Catastrophic Events and FOSS kit Earth History both
provide an opportunity for conceptual development of the concepts within the
standard. This lesson has been designed to follow Lesson A “Properties of Rocks”
SC Indicator 8-3.4 for the conceptual development of SC Indicator 8-3.
Misconceptions:
Some incorrect believes children often hold include but are not limited to:
 Earth is molten except for its crust; lava comes from the center of the Earth
 A rock is a hard solid material and are uniform inside
 A mineral and rock are the same thing; cement is a form of rock; all rocks
are more or less the same (a rock is a rock!)
 Rocks are stronger than the forces of nature (water, wind, ice).
 Rocks can only be changed by a blow of a hammer or other powerful object.
 Water can move dirt and rocks only if they are really small.
 There is a layer of water under the soil; soil is very deep – miles; soil or dirt
can be used up and never replaced; soil is made of plants and animals
 Soil must have always been in its present form
 River channels were there before the water and the water just follows them
not creates them; earthquakes made cracks for rivers; mountains by
avalanches, and tornado piled up dirt.
 Geological processes occurring over millions of years formed the
continents, oceans, mountains, coastlines and other landscapes we see
today. But those fantastic geologic processes have for the most part
stopped, revealing the more-or-less static world we live in today.
 Students do not understand that it is normal to have geologic processes
result in natural disasters and that these events have been happening
throughout earth's history.
 Students have no idea where things come from; they don't realize that
"man-made" materials come from mineral resources that must be mined.
 Children often have trouble making the connection to real world from
simulations or models; be explicit in asking clarifying questions to ensure
lifelong misconceptions are not created.
Safety Note(s):
Students should know and practice the procedures for fire, glass and chemical
safety. Students should use care when performing this experiment, and be wearing
the proper safety equipment including aprons and goggles. Students should know
and practice safe disposal of materials in a laboratory setting.
Lesson time:
2 days (1 day equals 55 minutes)
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
12
Materials Needed: (per groups of 2)
 General Information on Rocks (See attached)
 Student Information sheet (See attached)
 Copies of Geologic Conditions (See attached)
 Colored sticky dots
 Copies of the Rock Cycle (See attached)
 Sample set of rocks (OR use the rock informational cards – see attached)
NOTE: The lesson is written using these rocks:
Sedimentary Rocks
Igneous Rocks
Metamorphic Rocks
1 - Shale
5 - Pumice
9 - Schist
2 - Sandstone
6 - Basalt
10 - Quartzite
3 - Conglomerate
7 - Granite
11 - Gneiss
4 - Limestone
8 - Gabbro
12 - Marble
** If you are creating your own set of rocks, it is suggested to put a drop of paint or white out
on the rock and label it (A, B, C… OR 1, 2, 3…) for easy identification.
NOTE: A sample set of the three types of rocks and minerals commonly found in
South Carolina (24 specimens) can be purchased from SCDNR:
http://www.dnr.sc.gov/geology/publications.htm#rockandmineral ( $25 per set)
Focus Question:
What is the rock cycle and how does it explain the formation of rocks?
Engage:
1. Have a class discussion on the idea of a cycle and what it means to be cyclic.
2. Recall what you have learned in previous grades about cycles. (SC Indicator
6.2.5 Summarize each process in the life cycle of flowering plants (including
germination, plant development, fertilization, and seed production); SC
Indicator 6.4.2 Summarize the interrelationships among the dynamic
processes of the water cycle (including precipitation, evaporation,
transpiration, condensation, surface-water flow, and groundwater flow).
3. How might the explore of Lesson A for SC Indicator 8-3.4 have been an
example of a cycle?
4. Today we are going to talk about cycles in terms of rocks – how rocks go
through cycles to become other rocks.
5. With a group of three students, read the article “General Information on
Rocks” and create a graphic organizer representing the big ideas outlined.
(NOTE: should include but limited to the types of rocks, how each are
formed, different categories of each if applicable.)
Explore:
NOTE: This has been adapted from the lesson “Rocks and Geology Exercise”;
distributed by Women in Mining Education Foundation, revised 2008. It can be
downloaded in the original format at:
http://womeninmining.org/activities/Rocks_and_Geology.pdf
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
13
1. Geologists use their understandings about the identities of rocks, the
diversity of rocks, the various environments in which the rocks formed, and
the various properties of rocks to draw conclusions about the Earth and to
advise about the uses of Earth resources. Rocks come in every shape, color,
texture, and form imaginable. This diversity is due to many things:
a. the minerals from which the rocks are composed;
b. the environment in which the rocks formed; and
c. the forces of nature that have acted upon the rocks, to name only a
few.
2. Have students place a colored dot (that they have labeled) onto the geologic
conditions drawing for each rock where they think the rock would occur. The
Information Sheets provide a number of clues which will be helpful. (NOTE:
answer key has been provided for the rocks on the Student Information
sheet– see attached; students can also use the rock cards provided to
identify their location on the rock cycle as well)
3. NOTE TO TEACHERS: With students, define whether the placement of the
dots represents the location where the rock types would be generated or
where they would be likely to be found after the rocks have already formed.
4. Have a class discussion so students can compare their proposed locations of
the environments for the formation of the rock types other groups in the
room before the class discussion revealing the answers. In particular, note
whether the placement of the location indicates the environment for the
formation of the rock, or the environment where the rock would be likely to
be found at the present time (perhaps millions of years after formation).
5. During discussion, bring up that the already formed rocks may move from
the spots where they were formed through plate tectonic movement of small
or large blocks of rocks.
6. In your notebooks, explain in your own words the interrelationships of the three
rock types.
7. In your notebooks, write generalizing statements describing the methods each of
the three rock types are formed.
Explain:
1. Watch the SC ETV Streamline video segment “Introduction to Rocks and
Minerals” from the series Earth Science: Rocks and Minerals (3:18)
http://player.discoveryeducation.com/index.cfm?guidAssetId=54AA979F7F4D-418A-A374-2CF5AF52F681&blnFromSearch=1&productcode=US
2. Have a class discussion about the Rock Cycle. Be sure to ask clarifying
questions to ensure prior misconceptions have been corrected and no future
misconceptions have been created. The discussion should include topics such
as :
a. All rocks are connected in a cycle of creation, change, and destruction
called the Rock Cycle. The rock cycle begins with molten rock (magma
below ground, lava above ground), which cools and hardens to form
igneous rock. Exposure to weathering and erosional forces, break the
original rock into smaller pieces. The smaller material (now called
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
14
sediment) is carried away by rivers, wind, glaciers, and other means
and is eventually deposited elsewhere. These sediments can then be
buried and lithified (hardened), forming sedimentary rock.
b. Sedimentary rock can be deeply buried, subjected to heat and
pressure, which over time, cause it to change its structure into a new
rock, a metamorphic rock. Metamorphism is a big word meaning
change. Eventually, these metamorphic rocks may be heated to the
point where they again melt into magma.
c. Note that the rock cycle doesn't always have to work in this order;
sometimes igneous rocks can be buried and metamorphosed, skipping
the sedimentary rock phase, and sometimes sedimentary and
metamorphic rocks can be uplifted and eroded to form new
sedimentary rocks. It is also possible for rocks to remain unchanged in
stable regions for long periods of time.
d. Rocks cycle through three rock types: igneous, metamorphic and
sedimentary. The three rock types are connected through processes of
erosion, pressure, heating and/or melting.
i. Sedimentary rocks form when sediments worn from other rocks
are cemented together.
ii. Metamorphic rocks form by altering the chemistry, mineralogy,
or texture of rocks through extreme pressure and/or heat over a
long period of time.
iii. Igneous rocks form by cooling molten rock in the form of
magma or lava.
1. Intrusive igneous rocks occur when cooling takes place
slowly beneath the Earth’s surface
2. Extrusive igneous rocks occur when cooling takes place
rapidly on the Earth’s surface
3. In summary… the EARTH is made of ROCKS which are made of MINERALS
which are made of ELEMENTS which are made of ATOMS which are made of
PROTONS, NEUTRONS, and ELECTRONS which are made of SUBATOMIC
PARTICLES which are made of WHO KNOWS WHAT.
4. Watch the SC ETV Streamline video segment “The Rock Cycle” from the
series “Rocks: The Solid Earth Materials: Part 02 (2:40)
http://player.discoveryeducation.com/index.cfm?guidAssetId=755E09F32E30-4B04-8824-B5ABB65FE206&blnFromSearch=1&productcode=US
Extend:
1. Interpret the rock cycle diagram provided by answering the following
questions:
a. In the rock cycle, how could a metamorphic rock become magma?
b. What step in the rock cycle helps sedimentary rock to form?
c. In the rock cycle, what happens to magma and lava once they cool
and harden?
d. Must sedimentary rock become metamorphic rock before it can
become magma? Explain your answer.
e. Describe two pathways through the rock cycle in which igneous rock
can become metamorphic rock.
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
15
f. Describe a pathway through the rock cycle in which magma becomes
sedimentary rock.
Additional Resources:
1. US Geological Survey - http://www.usgs.gov/
2. SC Department of Natural Resources (SCDNR) http://www.dnr.sc.gov/education.html
3. Mineral Information Institute - http://www.mii.org
4. Women in Mining - http://womeninmining.org/
5. The Geologic Society of America - http://www.geosociety.org/
6. The Mineralogical Society of America - http://www.minsocam.org/
7. Geology and Earth Science – http://www.geology.com
8. Geology animations and songs from the University of Kentucky http://www.as.uky.edu/academics/departments_programs/EarthEnvironment
alSciences/EarthEnvironmentalSciences/Educational%20Materials/Pages/defa
ult.aspx
9. Smithsonian National Museum of Natural History “Dynamic Earth”
http://www.mnh.si.edu/earth/main_frames.html
10.World of Rocks http://www.albany.edu/dept/sisp/jjpowers/WebCollabS05/rocks/index2.html
11.The rock cycle explained http://www.cotf.edu/ete/modules/msese/earthsysflr/rock.html
12.For more information on the rock cycle http://pubs.usgs.gov/gip/2005/17/gip-17.pdf
August 2010
Science S3 Eighth Grade Module 8-5.4 A & B
16
General Information on Rocks
Rocks are the foundation of the earth. Rock provides the firmament beneath our
oceans and seas and it covers 28% of the earth's surface that we all call home.
When we travel any distance in any given direction, it is impossible not to see the
tremendous variety in color, texture, and shape of the rocks around us.
Rocks are made up of 1 or more minerals. Limestone, for example, is composed
primarily of the mineral calcite. Granite can be made up of the minerals quartz,
feldspar, hornblende, and biotite mica. Rocks are classified by their mineral
composition as well as the environment in which they were formed. The following
sections describe the conditions and processes that create the landscape we admire
and live on here on "terra firma."
SEDIMENTARY ROCKS
Sedimentary rocks are interesting because their methods of formation. These rocks
can be built up under water by the deposition there of materials such as sand, clay,
mud, pebbles, and gravel. These materials, called sediments, are brought to the
waters of lakes and oceans by the streams or rivers that flow into them. Wind and
moving glaciers of ice are also sediment transportation agents. Repeated freezing
and thawing of water in fractures and pore spaces in rocks will cause them to break
down over time. All of these processes are collectively known as erosion. Other
sedimentary rocks are made from the remains of plants and animals such as shells
or ferns. Still others are derived from minerals such as salt or gypsum that were
once dissolved in ocean or lake waters. As these sediments accumulate in layers or
beds, the weight of the newest, youngest beds on top causes pressure on the older
beds beneath forcing these older beds to stick together and to harden into rock.
During this process, some natural cementing materials such as lime and quartz,
found in ocean and lake waters, may help cement together coarser materials such
as sand and gravel.
The kind of sedimentary rock produced depends on the kinds of materials
deposited: very fine grained muds or clays form shale; cemented sands become
sandstone; cemented pebbles or gravel form conglomerate. Clay and lime together
form a rock called marl; seashells provide the material for limestone; decaying
plant and animal remains from swamps and shallow lakes form the parent material
for coal.
Sedimentary rocks are very common. Many are easy to identify. Sandstone is
obviously made of grains of sand. Sometimes the grains are loosely joined, and, if
two pieces are rubbed together, sand grains will be dislodged. Shale has a muddy
smell when wet, just like the material it was formed from. Limestone often has
visible remains of animals, shells, or plants called fossils. Sedimentary rocks are
derived from previously existing rocks which are decomposed by one of the
methods described in the first paragraph. Sedimentary rocks may be formed from
igneous and metamorphic rocks (described later) or from older sedimentary rocks.
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Most sedimentary rocks have a banded, "layer cake" appearance that is due to the
difference in materials which were deposited, layer by layer, one on top of another.
Each layer tells a unique story about a particular location during various periods in
geologic history as the layers are preserved in a chronologic sequence that
geologists can study, interpret, and age date. One of the most spectacular
examples of sedimentary "layer cake" rock formation is found in the walls of the
Grand Canyon.
IGNEOUS ROCKS
The millions of tons of molten rock that poured out of the volcano Paracutin or
Mount St. Helens illustrate one of the methods of formation of igneous rock.
Igneous (from fire) rocks are formed when bodies of hot liquid rock called magma
located beneath the earth's crust find their way upward through the crust by way of
fissures of faults. If the magma reaches the earth's surface, it forms extrusive
igneous rocks. If the magma cools before it reaches the surface, it forms bodies of
rock called intrusive igneous rocks.
Extrusive Igneous Rocks
Extrusive igneous rocks are formed from volcanic activity. Most varieties are finegrained because they cooled very rapidly upon exposure to the surface of the earth.
Pumice is an example of an extrusive igneous rock. It is a light colored rock formed
during violent volcanic episodes such as Mount St. Helens or the formation of
Mammoth Mountain in the Long Valley Caldera in California. Pumice can be
compared to foam on a boiling kettle. It is composed mainly of volcanic glass
(silica) and is full of air bubbles (vesicular, in geologist lingo). Pumice is perhaps
the only rock known in nature that floats on water. It is expelled rapidly by the
volcanic eruption, cooling and hardening almost immediately. Other rocks formed
from this type of volcanic eruption include obsidian and rhyolite.
Obsidian, sometimes called volcanic glass, results from the rapid cooling of magma.
It is a dark, glassy rock that can transmit light when thin chips are found. A mass of
this rock makes up Obsidian Cliff in Yellowstone National Park.
Basalt is a dark-colored, heavy rock formed from thick, syrupy lava flows. The
eruptions that create basalts are less violent than those that create rhyolite but can
be equally destructive. Eruptions from the numerous Hawaiian volcanoes form
extensive basalt lava flows. The Columbia Plateau in the northwestern United
States, covered with 150,000 square miles of hardened basaltic lava, in places one
mile thick, is one of the earth's greatest volcanic constructions.
Intrusive Igneous Rocks
Intrusive igneous rocks form beneath the surface of the earth where slow-moving
magma bodies cool before they reach the surface. Intrusive magma forces its way
into or between masses of older rock and cools very slowly. As a result of this slow
cooling process, intrusive igneous rocks are coarse-grained. These rocks are later
exposed at the surface through the processes of uplift and erosion. Granite and
gabbro are two of the many varieties of intrusive igneous rocks.
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Granite is easily recognized because of its speckled appearance. Close examination
reveals that the speckling is caused by the different minerals granite is composed
of. The light gray glassy mineral is quartz; the milky white or gray mineral is
feldspar--in some samples the feldspar can be red or green (those special-colored
granites are prized for building stone and for monuments and tombstone
materials); the black shiny mineral is platy mica or blocky hornblende. Granites are
formed from magmas that are rich in silica and potassium and relatively poor in
calcium, magnesium, and iron. Masses of granite can be found in the Rocky
Mountains, the Adirondacks, the Black Hills of South Dakota, and the White
Mountains of New Hampshire. Granites are also scattered throughout Nevada.
Gabbro is also a speckled intrusive igneous rock but is noticeably darker in color
than granite. The principal minerals composing gabbro are gray feldspar and black
hornblende or mica. There is generally very little or no quartz in gabbro. Gabbros
are formed from magmas that are rich in calcium, magnesium, and iron; and poor
in silica. They can be found as isolated bodies in intrusive igneous mountain ranges
such as the Sierra Nevada, and the ranges of southern California. An interesting
outcrop of gabbro occurs east of San Diego, California, where the material weathers
into gabbro spheres from 3" to 1' across. The rock there is referred to as "orbicular
gabbro".
The differences between extrusive igneous rocks and intrusive igneous rocks result
from primarily from their mode of formation. Granite, pumice and rhyolite come
from magmas of very similar composition; gabbro and basalt come from magmas of
very similar composition.
METAMORPHIC ROCKS
In general, it may be said that when any bedrock is subjected to greatly increased
pressures or very high temperatures, or both, it may be changed in its physical and
chemical properties to become metamorphic rock. Metamorphic means "a change in
form." The pressure increase may be the result of movement of the earth's crust
which crumples and folds the bedrock. Increased pressure may also result from
deep burial of sediments as younger sediment beds are deposited over the top of
them. Increases in temperatures may result from friction created by movement or
from nearby sources of hot magma. Metamorphism can affect igneous and
sedimentary rocks, and even affect metamorphic rocks that were formed at an
earlier time. Schist is a metamorphic rock derived from sedimentary or older
metamorphic rocks that have been altered by heat and pressure from nearby
intrusive igneous bodies. Schist is identified by its platy appearance due to the
parallel orientation of sheets or grains of minerals called mica.
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Some other examples of metamorphic rocks and the rocks from which they were
derived are as follows:
Metamorphic Rock
SLATE
QUARTZITE
GNEISS
MARBLE
ANTHRACITE COAL
Derived From
Shale
Sandstone
Granite
Limestone
Bituminous Coal
Notice that the sample of gneiss has a speckled appearance like granite but that the
mineral grains are oriented in parallel bands and the bands are alternately light and
dark. The banding is a direct result of pressure and heating which actually changes
the structure of the rock. Where there has been movement, mineral grains will turn
and flow in the direction of movement.
Notice how much more durable the quartzite is compared to the sandstone. Marble
has been converted from limestone, and the result is a hard crystalline rock that is
much prized as a building stone and as a carving stone for statuary. An excellent
example of marble is found at the Crestmore quarry just outside Riverside,
California. Notable occurrences of metamorphic rocks are found along the length of
the San Andreas Fault.
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