Daniel Marsden
Table of Contents
Table of Contents
----------------------------------------------------page 1
What is a Rock?
----------------------------------------------------page 2
Igneous Rocks
----------------------------------------------------page 3
Sedimentary Rocks ----------------------------------------------------page 4
Metamorphic Rocks ----------------------------------------------------page 5
Rock Characteristics ----------------------------------------------------page 6
Hardness
----------------------------------------------------page 7
Color
----------------------------------------------------page 8
Streak
----------------------------------------------------page 9
Metallic Luster
--------------------------------------------------page 10
Non-Metallic Luster --------------------------------------------------page 11
Cleavage
--------------------------------------------------page 15
1
What Is a Rock?
The entire Earth is made up of
rocks and minerals. A mineral is
any substance that occurs
naturally in the ground and has
its own appearance and
chemical composition. A solid
deposit of these minerals is
called a rock. Geologists are
scientists who study rocks. They
study the Earth and the different
types of rocks that can be found
on it. The three different types
of rocks are igneous,
sedimentary, and metamorphic.
2
2
Igneous Rocks
The Earth’s crust is made up primarily of
igneous rocks. Igneous means “fire-formed,”
and these rocks are formed from magma or
lava. Some of these rocks are formed deep
inside the Earth while others are formed
when volcanoes erupt. The lava cools and
forms rocks and fertile soil. Some of the
most common igneous rocks are granite,
basalt, obsidian, dacite, and pumice.
3 3
Sedimentary Rocks
Soil, minerals, and other organic
matter called sediment coalesce to
form sedimentary rocks. Wind and
water erode rocks, especially in
rivers, creating what is known as
sediment. The dirt and other small
particles build up on the floor of the
river, and after millions of years the
pressure of all the sediment turns the
sediment into sedimentary rock.
These rocks can be found covering
the ground over the igneous rock. The
rocks in the background are called
sandstone.
4
4
Metamorphic Rocks
Metamorphic rocks are formed
when minerals change from one
type of rock to another. They
occur when a rock is moved away
from its natural location. Often,
when these rocks are exposed to
high pressures and high
temperatures, the rocks compress
together and their chemical
compositions change. Therefore,
the two rocks are combined to
form a new rock. The most
common metamorphic rocks are
marble, quartz, graphite,
amethyst, and, as seen here, coal.
5
5
Rock Characteristics
There are many characteristics that
geologists use to describe the rocks
that they find. These traits are used
to determine the identity of a
certain rock sample. Knowing what
type of rock that the sample is can
help geologists determine the
chemical make-up, age, and other
aspects. The five main
characteristics that geologists look
for are:
Hardness
Color
Streak
Luster
Cleavage
66
Hardness
The hardness of a rock
determines what types of
rocks it can scratch and
what types of rocks can
scratch it.
1
2
3
4
Geologists use the
Mohs Scale of
hardness to measure
the hardness of a
rock. On the Mohs
Scale, a rock with a
hardness of one is
very soft, such as talc;
a rock with a
hardness of ten is
very hard, such as
diamond. Just for
reference, your
fingernail has a
hardness of 2.2, and
nails have a hardness
of 5.2.
5
6
7
8
9
10
7
Color
Carnelian Quartz
The color of a rock
describes its color when it
is in its solid form.
Geologists can sometimes
determine what type of
rock a particular sample is
just by its color. Some
rocks, however, can come
in many different colors,
making it more difficult
for scientists to determine
what type of rock it is. For
example, quartz can come
in pink, brown, violet,
yellow, or even colorless.
Rock Crystal Quartz
Rose Quartz
Smokey Quartz
Adventurine Quartz
Coontail Quartz
8
Streak
Many times a mineral changes
color when it turns from a solid to
a powder. The color of the mineral
as a powder is called its streak.
Many rocks and minerals occur in
any number of different colors,
but all colors of any mineral have
the same streak. Because of this,
geologists often use streak in
determining the identity of a rock
or mineral. The way streak is
found is by using a streak plate, a
hard object that you scrape a
softer rock across to get the
powdery substance.
These two rocks are two different
types of quartz. As you can see,
one is smoky quartz and the other
is amethyst. Although they have
very different colors, they both
produce a white streak.
9
Metallic Luster
The luster of a rock is
very important in
determining what type of
rock it is. Luster is how
the rock looks in light.
There are two main
categories of luster:
metallic and non-metallic.
Metallic-looking rocks
are shiny and opaque
with black or dark
streaks.
10
Non-Metallic Luster
There are seven types
of non-metallic luster:
vitreous, resinous,
pearly, greasy, silky,
dull, and adamantine.
Rocks with a vitreous
luster appear glassy,
such as calcite and
some quartz.
Rocks with a greasy luster
appear to be covered in oil.
Some examples of rocks
with a greasy luster are
sodalite and olivine.
11
Non-Metallic Luster
Rocks with a resinous
luster appear to be
made out of resin (a
translucent yellow or
brown substance
such as amber or
copal that comes from
tree sap.)
Rocks with a silky luster
appear to be made of
hundreds of fibers, such as
artinite and satin spar.
12
Non-Metallic Luster
Rocks with a pearly
luster look like
pearls. One such
rock is muscovite.
Rocks with a dull luster
appear earthy, like most
rocks you might see on the
ground. Some interesting
dull rocks are psilomelane,
chrysocolla, and illite.
13
Non-Metallic Luster
Rocks with an
adamantine luster
appear brilliant, such
as diamond, emerald,
ruby, and topaz.
14
Cleavage
Cleavage is the way a rock
breaks. Some rocks break into
perfect sheets that peel off
(one direction.) Some rocks
break into boxes with perfect
corners (two direction,) and
others break into 3-D
diamond shapes, (eight
direction.) These rocks
exhibit what is known as
perfect cleavage.
Two-Direction Cleavage
Eight-Direction Cleavage
One-Direction Cleavage
15
Cleavage
No Cleavage
Imperfect Cleavage
Some rocks do not break
into perfect slabs, cubes,
or diamonds. These
rocks, then, do not have
perfect cleavage. These
rocks can be classified as
exhibiting either good,
imperfect, poor, or no
cleavage.
The rock above and the
rock to the right display
Poor Cleavage.
16
Glossary
Cleavage – Cleavage determines the tendency for a rock to break into predictable shapes.
There are many different types of cleavage, but they have to do with the shape and
perfection with which the rock breaks.
Color – The color of the rock in solid form. The same mineral can be found in many
different colors.
Erosion – The wearing away of rock due to wind, moving water or ice, or another geological
process.
Fertile – Soil that is great to grow crops on.
Hardness – The measure of how hard a mineral is. Hardness is measured with the Mohs
Scale.
Igneous Rock – These rocks are most common in the Earth’s crust. They can be formed
either deep in the Earth’s interior by magma, or by the cooling of the lava of the
eruption of a volcano.
Lava – Magma after it erupts out of a volcano.
Luster – The appearance of a mineral in light. Luster can be either metallic or non-metallic.
Magma – Molten rock below the Earth’s surface.
Metamorphic Rock – Rocks that are formed when many different types of rock pile up over
millions of years and press together. The rocks change their shape and form,
forming completely different rocks from what they were before.
Mineral – A substance that occurs naturally in rocks and in the ground and has its own
characteristic appearance and chemical composition
Sediment – Dirt, sand, and even small microorganisms that pile up in rivers due to the
erosion of the rock in the riverbed. Sediment forms sedimentary rock.
Sedimentary Rock – Rocks that are formed when large amounts of sediment pile up after
millions of years and press together due to the large pressure that is exerted on
them.
Streak – The color of the mineral in a fine powder. No matter the color of the rock, the
streak is always the same.
17
About the Author
Daniel Marsden is a junior at the Massachusetts Academy of Math and
Science. He lives near Worcester, Massachusetts with his parents , two
brothers, and his labrador retriever. He will be attending Worcester
Polytechnic Institute for his senior year in high school and he plans on
majoring in either chemistry or biology in college. He is a huge sports fan and
much of his free time is spent playing soccer, basketball, or volleyball. He also
loves music and he enjoys playing jazz on his clarinet and listening to all
different genres of music.
18
Illustration Credits
Title Page
Rebecca Morn
http://www.rebeccamorn.com/images/gallery3/grand-canyon-sunset-4.jpg
Page 2
The Flying Kiwi – Richard Seamen
http://www.richard-seaman.com/USA/States/Nevada/ValleyOfFire/Sampler/index.html
Page 3
Alan Bauer
http://www.alanbauer.com/images/South%20Cascades/Red%20pumice%20covered%20slopes%20looking%20at%20
Racetrack%20area%20and%20Mount%20St%20Helens-Vert.jpg
Page 4
Dr. Ian West
http://www.soton.ac.uk/~imw/jpg-Budleigh-Salterton/6BS-Straight-west-sandstone.jpg
Page 5
Coal Face
http://www.cargolaw.com/2004nightmare_coal-face.html
Page 6
Erosion by Streamflow
http://www.kidsgeo.com/geology-for-kids/0078-erosion-by-streamflow.php
Page 7
Illustration of Rocks – Oxford University Museum of Natural History – The Learning Zone
http://www.oum.ox.ac.uk/thezone/minerals/detect/images/hard1.jpg
Mohs Scale Illustration – SODA Blasting Services
http://www.sodablastingservices.com/images/mohsscale.jpg
Page 8
Rock Crystal Quartz – Free Republic
http://www.freerepublic.com/focus/f-chat/1983275/posts
Smokey Quartz Illustration – Earthnet
http://earthnet-geonet.ca/images/dynamic/minerals/smokey_quartz.jpg
Coontail Quartz – Wright’s Rock Shop
www.wrightsrockshop.com/gallery/gallery5/gallery5images/2v6redquartz82807100_4255.JPG
Adventurine Quartz – The Vug
http://www.the-vug.com/vug/greenquartz01.JPG
Rose Quartz – Colorado Gem and Mineral Co.
http://www.coloradogem.com/images/2962_big.jpg
Page 9
Quartz Streak Test – The Children’s Museum of Indianapolis
http://www.childrensmuseum.org/geomysteries/cube/b3.html
Streak Plate Test – The Image
http://www.theimage.com/geology/notes3/streakplate.jpg
Page 10
Gold Picture – About
http://geology.about.com/od/mineral_ident/ig/lusters/gold.-0ZX.htm
Magnetite Picture – About
http://geology.about.com/od/mineral_ident/ig/lusters/magnetitemassive.-0Zc.htm
Hemetite Picture – Merlyn Crystals – Crystal Planes
http://www.merlyncrystals.com/detail_images/di_ts_hemetite.jpg
Page 11
Rock Crystal Quartz – About
http://z.about.com/d/geology/1/0/v/A/1/qtzspears.jpg
Calcite - About
http://z.about.com/d/geology/1/0/l/C/1/calcite500.jpg
Sodalite Picture – Amethyst Galleries’ Mineral Gallery – The Mineral Sodalite
http://www.galleries.com/minerals/silicate/sodalite/sodalite.jpg
Olivine Picture – Cochise
19
http://skywalker.cochise.edu/wellerr/mineral/olivine/6olivine64.jpg
Page 12
Amber Picture – The Mineraloid Amber
http://www.galleries.com/minerals/mineralo/amber/amb-37.htm
Artinite Picture – The Mineral Artinite
http://www.galleries.com/minerals/carbonat/artinite/artinite.jpg
Selenite Picture – The Mineral Miners
http://www.mineralminers.com/images/selenite/mins/selm127.jpg
Satin Spar Picture – University of Pittsburgh
http://www.pitt.edu/~cejones/GeoImages/1Minerals/2SedimentaryMineralz/Gypsum_Halite/GypsumSatinSpar.jpg
Page 13
Muscovite Picture – About
http://z.about.com/d/geology/1/0/q/C/1/muscovite500.jpg
Pearl Picture – The Chinese Astrologer
http://www.chineseastrologer.org/images/enlarged/_gems_pearl.jpg
Dull Luster Picture – DK Images
http://www.dkimages.com/discover/previews/966/60002129.JPG
Psilomelane Picture – About
http://z.about.com/d/geology/1/0/s/C/1/psilomelane500.jpg
Chrysocolla Picture – About
http://z.about.com/d/geology/1/0/m/C/1/chrysocolla500.jpg
Page 14
Topaz Picture – The Mineral Topaz
http://www.galleries.com/minerals/silicate/topaz/topaz.jpg
Topaz Picture – Palagems
http://www.palagems.com/Images/Bancroft_Topaz/topaz_burma.jpg
Diamond Picture – Diamonds and Designs by None Other Goldsmiths
http://www.noneothergoldsmiths.com/eighternity_diamond.jpg
Emerald Picture – State Library of North Carolina
http://statelibrary.dcr.state.nc.us/nc/symbols/images/emerald.jpg
Ruby Picture – Diamond Vues
http://www.diamondvues.com/archives/Ruby%20Gemstone.jpg
Page 15
1-Direction Cleavage – Earth Science Lab – Mineral Physical Properties and Identification
facweb.bhc.edu/academics/science/harwoodr/GEOL101/Labs/Minerals/Images/min19b.jpg
Calcite Picture – Calcite Cleavage
http://skywalker.cochise.edu/wellerr/mineral/calcite/calcite6.htm
8-Direction Cleavage – Colorado State University Pueblo
http://ceeps.colostate-pueblo.edu/tep/ssc/module1/part2/smM1P2Fig5.gif
Calcite Picture – Glendale Community College – Earth Science Archive
http://www.gc.maricopa.edu/earthsci/imagearchive/calcite.htm
Page 16
Goshenit Picture – The Mineral Goshenit
http://www.galleries.com/minerals/gemstone/goshenit/goshenit.jpg
Olivine Picture – James Madison University
http://csmres.jmu.edu/geollab/Fichter/RockMin/Olivine-99.JPG
2-Direction Cleavage Picture – Barbara Smigel
http://www.bwsmigel.info/Lesson3/images.wl.3/FLU.CLEAV.jpg
Aragonite Picture – Jewels for Me – Gemstone Jewelry
http://www.jewelsforme.com/graphics/Gems-Aragonite.jpg
20