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Content Benchmark E.8.C.2
Students know rocks at Earth’s surface weather, forming sediments that are buried, then
compacted, heated and often recrystallize into new rock. E/S
Food and rocks probably have little in common unless you are thinking like a geologist. Just as a
chef uses a wide array of ingredients to create a dish, nature uses a variety minerals and
sediments to create a masterpiece we call a rock. Whether it is combining items to create a dish
like coleslaw, or baking ingredients to make a cake, rocks undergo a similar process in their
creation.
When liquid rock known as Magma cools at or near the Earth’s surface it forms igneous rocks.
On the surface of the earth, rocks are broken down by the forces of erosion. Weathering, both
chemical and mechanical, break rocks down into various sediments. These sediments are the
raw ingredients of nature. Sediments may combine into newly formed rocks just as they were
found in their original form, like a chocolate chip inside a cookie retains its original properties.
However, other sediments are buried deep inside the Earth where they may undergo extreme heat
and pressure changing form into something brand new, like flour, sugar, butter, and eggs emerge
from the oven as a cake. Just as a chef can create many dishes from the same ingredients; nature
is able to construct a multitude of different rocks from the same sediments. Therefore, let us
explore the rock cycle to further understand the geologic processes necessary to create each type
of rock.
The Rock Cycle
The Rock Cycle is the term applied to the continuous process that describes how rock material
forms, erodes, changes and is eventually recycled. The process is nonlinear, material can move
throughout the cycle through erosion, compaction, partial melting or complete melting. There is
no set pattern or sequence that must be followed nor a set duration that rocks will remain in one
form or another. The three forms of solid rock are sedimentary, metamorphic and igneous.
Figure 1. The Rock Cycle.
(From http://www.teachnet-lab.org/ps101/bglasgold/rocks/EFCycleP2.gif)
Igneous Rocks
Igneous comes from the Latin word for fire. Igneous rocks, form from the cooling of liquid rock
called magma. Igneous rocks are classified by where they originate, either inside the Earth’s
crust, intrusive igneous, or on the Earth’s surface, extrusive igneous. Magma, or molten rock,
found in the mantle below may move upward toward Earth’s surface where it is forced out of a
volcano in the form of lava and ash, solidifying into extrusive igneous rocks (rhyolite, dacite,
andesite, basalt). Or, if the magma solidifies beneath the Earth’s surface it forms intrusive
igneous rocks such as granite, diorite or gabbro. Igneous rock that forms deep below the surface
of the Earth is also known as plutonic.
Figure 2. Examples of Extrusive Igneous Rock Texture. fine-grained
Obsidian (left) and coarse-grained Pyroclastic (right).
(From http://facstaff.gpc.edu/~pgore/geology/geo101/igneous.htm)
In addition to being classified by their place of formation, igneous rocks are also classified by
their texture and chemical composition. Texture ranges from glassy, such as Obsidian, to the
chunky, such as Pyroclastic (see figure 2 above). The rate at which the molten material cools
determines the crystal size. Extrusive rocks cool extremely quickly and form very fine-grained
(aphanitic) textures, whereas, intrusive rocks which cool slowly result in coarse-grain
(phaneritic) textures.
Figure 3. Diorite – medium, course-grained intrusive igneous rock.
(From http://geologyonline.museum.state.il.us/geogallery/media/images/small/306753.jpg)
Figure 4. Granite – large, coarse-grained intrusive igneous rock.
(From http://www.beg.utexas.edu/mainweb/publications/graphics/granite-400.jpg )
Figure 5. Igneous Rock chart based on relative mineral content and environment of formation.
(From http://www.seafriends.org.nz/enviro/soil/soil50.gif )
Figure 5 above is a chart that shows the classification of common igneous rocks based on their
chemical composition, and on their location of formation. Each row on the chart represents the
equivalent of 10% composition, however you should note that each of the rock types listed not
only has specific minerals that compose them but actually have a range in the percentage of their
mineral composition. For example, Granite is composed of quartz (30%-42%), Potash Feldspar
(20% - 40%), plagioclase feldspar (5%-35%), biotite (0% - 10%). These variations in
composition explain how a given type of rock such as granite can vary greatly in color and
mineral content.
Sedimentary Rocks
The sedimentary rock gets its start on the surface of the Earth. Rocks break apart into sediments
or fine particles through the process of chemical and mechanical weathering. These sediments
are then transported (via the process of erosion) through a combination of forces including;
gravity, air and water to a new location. Once they arrive at their “final” destination, they are
deposited, settle, compacted and cemented together. It is through this process of deposition,
compaction and cementation that forms what we know to be sedimentary rock layers. If you
have ever traveled to the Grand Canyon (see figure 6 below) or looked and almost any of the
mountain ranges in Nevada, you have seen prime examples of sedimentary rocks. The vast
layers that can be observed in a deep canyon or road cuts through mountains are deposits that
formed millions of years ago, now exposed by the cutting action of the Colorado River or the
effects of crustal movement, and human activity.
Figure 6. The rock layers in the Grand Canyon.
(From
http://www.ohranger.com/files/imagecache/parkphoto_lightbox/files/parkphotos/GRCA_GrandCanyonHDR.jpg)
Sedimentary rocks can be further classified into three categories: clastic, non-clastic, and
organic. Clastic sedimentary rocks form from the weathering of existing rocks and have a
texture made up of clasts, matrix, and cement. The clasts are basically smaller rocks, grains,
gravel, or “pieces” that compose the rock. They are encased by the matrix, which is the mud or
fine grained sediment surrounding the clasts. Finally this is all bound together by the cement,
which is like glue binding all the pieces together. These clastic sedimentary rocks are
categorized along a scale by grain/clast size beginning with the finest particles (< 0.002 mm)
composing the shales, followed by siltstones (particle size 0.002 mm -0.063 mm). Sandstones
have a slightly coarser texture with grains ranging from 0.063 mm up to 2 mm in diameter.
Clastic rocks with grains between 2 mm and 263 mm are classified as either conglomerates or
breccias. Breccias (Figure 8) are composed primarily of angular clastic particles, whereas
conglomerates (Figure 9) are composed primarily of rounded clasts.
Figure 7. A diagram showing the sediments that compose the sedimentary rocks after compaction and cementation.
(From http://www.geo.umn.edu/courses/1001/1001_edwards/web_figures/WebFigures4-1.html)
Figure 8. Breccia, note the sharp edged clasts that make up the rock.
(From http://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/images/breccia3.jpg)
Figure 9. Conglomerate, note the rounded edges of the clasts that compose the sample.
(From http://www.dkimages.com/discover/previews/1051/25002102.JPG)
Non-clastic sedimentary rocks, sometimes called chemical/biochemical sedimentary rocks, are
composed of evaporites, carbonates, and siliceous rocks. Evaporites typically form from the
evaporation of sea or salt water; examples include rock salt and rock gypsum. Carbonates, such
as limestone and dolostone, are composed of the minerals calcite and dolomite and form from
both chemical and biochemical processes. Siliceous rocks are made primarily of silica. Chert
and diatomite are two examples of siliceous sedimentary rocks.
Figure 9. Limestone can be distinguished from dolostone because it will fizz
when tested with weak Hydrochorlic acid (1M HCl).
(From http://www.indiana.edu/~geol116/week2/mineral.htm)
The final category of sedimentary rocks is the organics, coal being the most common example.
Unlike the two other subgroups, organic sedimentary rocks do not contain minerals. Instead they
are composed of organic matter, typically from plants. Coals begin as peat, and include lignite,
bituminous, and anthracite varieties. Their origin is typically large prehistoric swamps and bogs.
Figure 10. Examples of two types of coal: bituminous coal (left) and anthracite (right)
(From http://geology.about.com/library/bl/images/blcoal.htm)
Metamorphic Rocks
The term metamorphosis most likely brings to mind caterpillars turning into butterflies, but if
you are a geologist you probably think of rocks. Typically we imagine the changes that insects
undergo when we think of the term; however, the term can apply to rocks too. Rocks can change
or “morph” from any type into a metamorphic rock if they are subjected to sufficient heat and
pressure. Metamorphic rocks form below the Earth’s surface. Any rock type that gets buried
deep inside the crust where it can undergo high temperatures and/or extreme pressure can be
transformed into metamorphic rocks. Unlike the complete melting that must occur in order for
igneous rock formation; metamorphic rocks are not completely melted but only partially melted,
resulting in new grain sizes and orientation of the existing minerals. You can see this in the
example of slate forming from shale, gneiss forming from granite (Figure 11), or marble forming
from limestone.
Figure 11. Granite is the “source” rock for Gneiss, it’s metamorphic offspring.
(From http://earth.rice.edu/MTPE/geo/geosphere/topics/rocks/gneiss_granite.jpg )
Metamorphic rocks can form regionally or locally. Heat, pressure, and strain are all processes
associated with regional metamorphism. Heat and pressure act together since both increase as
the depth below Earth’s surface increases. The Gneiss shown above is an example of rock
formed by this intense heat and pressure. Strain refers to any change in the shape of rocks due to
the forces of stress. As fluids form and move, new minerals grow with their grains tilted
according to the direction of pressure. When strain makes the rock stretch, these minerals form
layers. The presence of mineral layers is called foliation and is an important identifying factor
when observing metamorphic rocks.
Local metamorphism occurs on a much smaller scale than regional metamorphism and normally
takes place at shallow levels with low pressure when hot magma comes in contact with
surrounding rock and causes alterations. Unlike metamorphic rocks formed by regional
metamorphism, contact metamorphic rocks lack foliation.
Figure 12. Example of contact metamorphism. Lava flow baked mud below it into red shale.
(From http://geology.about.com/library/bl/images/blcontactmet.htm)
Conservation of Matter
The rock cycle is nature’s way of recycling to solid portion of our planet. The rocks found by
today’s geologists, students, hikers, and other nature lovers are composed of the same materials
that have always existed since Earth’s beginning. Over time, rocks are recycled back into the
mantle where they are melted and possibly re-solidified to become solid rocks again. This is a
continuous process and rocks tend to move through the cycle relatively quickly, geologically
speaking. Although, there are a few examples of rocks that are over two billion years old, but
most are much younger than that.
Content Benchmark E.8.C.2
Students know rocks at Earth’s surface weather, forming sediments that are buried, then
compacted, heated and often recrystallize into new rock. E/S
Common misconceptions associated with this benchmark.
1. Students have difficulty understanding the definition of a rock.
Geologists use the term “rock” to define a category or large mass. When referencing rock,
scientists think on a large scale. Whereas, what most people think of a rock, pebble or stone,
geologists call a “clast.” Students often believe rocks and minerals are the same thing. They
also believe humans can fabricate rocks and minerals.
For more information about these misconceptions and for strategies to address them, visit the
Common Misconceptions about Rocks and Minerals website from Beyond Penguins and
Polar Bears at
http://beyondpenguins.nsdl.org/issue/column.php?date=September2008&departmentid=profe
ssional&columnid=professional!misconceptions
What Is It? Interactive activity can be used to assess prior knowledge,
http://www.contentclips.com/services/getPresenterHtml?uri=:cli:79
What Is It? Directions and answer key:
http://onramp.nsdl.org/eserv/onramp:1227/what_is_it_answer_key.pdf
2. Students do not comprehend the geologic scales of space and time.
Humans view the world through human scales of space and time. It is challenging for
students to comprehend the vast nature of geologic time. Although most can quote the age of
the Earth, students do not realize the amount of time it takes for the geologic processes
responsible for the formation, destruction, and processes in the rock cycle to occur.
For strategies to address this misconception, visit Understanding Geologic Time, an
informational tour in which students gain a basic understanding of geologic time, and the
significance of the Geologic Time Scale.
See the Teacher’s Guide for an overview and links to lesson plans and interactive student
pages at
http://www.ucmp.berkeley.edu/education/explorations/tours/geotime/guide/index.html
Content Benchmark E.8.C.2
Students know rocks at Earth’s surface weather, forming sediments that are buried, then
compacted, heated and often recrystallize into new rock. E/S
Sample Test Questions
Content Benchmark E.8.C.2
Students know rocks at Earth’s surface weather, forming sediments that are buried, then
compacted, heated and often recrystallize into new rock. E/S
Answers to Sample Test Questions
Content Benchmark E.8.C.2
Students know rocks at Earth’s surface weather, forming sediments that are buried, then
compacted, heated and often recrystallize into new rock. E/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.
1. Schoolyard Geology, Lesson 2: Rock Stories
This site was developed by the United States Geologic Survey (USGS). In this lesson,
students learn that a rock's properties tell stories about where it came from and where it has
been. The lesson illustrates how to make geologic observations and gives background about
the important properties of rocks to observe. Students then use those observation skills to
describe rocks they find on their own Schoolyard. This web site describes specific "geologic"
features found on playgrounds (with photos and rocks from example schools).
To access lesson plans go to
http://education.usgs.gov/schoolyard/RockActivity.html
2. Mineralogy 4 Kids, The Rock Cycle
Developed by the Mineralogical Society of America and selected by the SciLinks program, a
service of the National Science Teachers Association. This site includes interactive links to
show students the types of rocks found in the rock cycle, provides pictures of rock samples,
and a rock identification chart.
To access this activity go to
http://www.minsocam.org/MSA/K12/rkcycle/rkcycleindex.html
3. The Rock Cycle (Beyond Books - Earth Science: Part 2)
The Rock Cycle is from Beyond Books.com, a highly interactive site with readings, games,
experiments, and literacy strategies.
To access readings and activities go to
http://www.beyondbooks.com/ear82/7.asp
4. Rock and the Rock Cycle
From Windows to the Universe website the activity “Interactive Rock Cycle” allows students
to click on various parts of the cycle to find out about the different rock types. Also provides
information on geologic time and plate tectonics. Each page can be viewed in English or
Spanish with beginner, intermediate, and advanced levels for each.
To access the website go to
http://www.windows.ucar.edu/tour/link=/earth/geology/rocks_intro.html&edu=elem
5. Rock Cycle: Cycling
This is a free offer from the National Science Teacher’s Association (NSTA). This activity
explores the variables that contribute to rock transformation and the continuous processes of
rock formation that constitute the rock cycle. It uses recycling as a context for students to
comprehend observation of matter in relation to the rock cycle.
To download the free SciPack use the following link:
http://www.nsta.org/store/product_detail.aspx?id=
10.2505%2f7%2fSCB-RK.3.1