Rocks



Rocks 2822
Photo by: csourav

Rocks are composed of minerals, which are natural inorganic (nonliving) substances with specific chemical compositions and structures. A rock may consist of many crystals of one mineral or combinations of many minerals. Hundreds of different kinds of minerals make up hundreds of different kinds of rocks. Geologists, scientists who study Earth and rocks, divide rocks into three main groups: igneous rocks, sedimentary rocks, and metamorphic rocks. These distinctions are made on the basis of the types of minerals in the rock, the shapes of individual mineral grains, and the overall texture of the rock. All of these properties indicate the environment, pressure, and temperature in which the rock formed.

Granite (left), an intrusive igneous rock, and obsidian (right), an extrusive igneous rock. Both samples are taken from the Sierra Nevada Mountains of California. (Reproduced by permission of Photo Researchers, Inc.)
Granite (left), an intrusive igneous rock, and obsidian (right), an extrusive igneous rock. Both samples are taken from the Sierra Nevada Mountains of California. (Reproduced by permission of
Photo Researchers, Inc.
)

Igneous rock

The first rocks on Earth were igneous rocks. Igneous rocks are formed by the cooling and hardening of molten material called magma. The word igneous comes from the Latin word ignis, meaning "fire." There are two types of igneous rocks: intrusive and extrusive. Intrusive igneous rocks form within Earth's crust: the molten material rises, filling any available crevices in the crust, and eventually hardens. These rocks are not visible until Earth above them has eroded away. A good example of intrusive igneous rock is granite. Extrusive igneous rocks form when the magma pours out onto Earth's surface or erupts at Earth's surface from a volcano. Once on the surface (where it is called lava), it begins to cool and the minerals in the rock crystallize or grow together so that the individual crystals lock together. Extrusive rocks are also called volcanic rocks. Basalt, formed from hardened lava, is the most common extrusive rock. Obsidian, a black, glassy rock, is also an extrusive rock.

Words to Know

Igneous rock: Rock formed from the cooling and hardening of magma.

Lava: Molten rock that occurs at the surface of Earth, usually through volcanic eruptions.

Magma: Molten rock found below the surface of Earth.

Metamorphic rock: Rock formed by transformation of preexisting rock through changes in temperature and pressure.

Mineral: A naturally occurring, inorganic substance with a definite chemical composition and structure.

Rock cycle: Processes through which rocks change from one type to another, typically through melting, metamorphism, uplift, weathering, burial, or other processes.

Sedimentary rock: Rock formed from compressed and solidified layers of organic or inorganic matter.

Weathering: Natural process that breaks down rocks and minerals at Earth's surface into simpler materials (sediment) by physical (mechanical) or chemical means.

Essentially, Earth's continents are slabs of granite sitting on top of molten rock. The crustal plates of Earth are continually shifting, being torn open by faults and altered by earthquakes and volcanoes. New igneous material is continually added to the crust, while old crust falls back into Earth, sometimes deep enough to be remelted. Igneous rocks are the source of many important minerals, metals, and building materials.

Sedimentary rock

Sedimentary rocks are those produced by the accumulation of sediments. These may be fine rock particles or fragments, skeletons of microscopic organisms, or minerals leached from rocks. Rock fragments and leached minerals are created through weathering, a natural process that breaks down rocks and minerals at Earth's surface into simpler materials by physical (mechanical) or chemical means.

Wind, water, ice, gravity, temperature changes, or a combination of these are all physical actions that break down preexisting rocks. Chemical weathering represents a second stage of rock disintegration in which small pieces of rock produced by physical weathering are then further

Stockbridge limestone is named after the town in Massachusetts where it is typically exposed. (Reproduced by permission of Photo Researchers, Inc.)
Stockbridge limestone is named after the town in Massachusetts where it is typically exposed. (Reproduced by permission of
Photo Researchers, Inc.
)

broken apart by chemical processes. Acid reactions are a common form of chemical weathering, and the most common such reactions occur when carbon dioxide and sulfur dioxide in the air react with water to form weak carbonic and sulfuric acid. Both of these acids have the ability to attack many kinds of rocks, changing them into other forms. For example, when carbonic acid reacts with limestone, it produces calcium bicarbonate.

The sediments created by weathering and the decay of organisms are then transported and deposited by wind, water, or ice. Over long periods of time, layer upon layer of sediments are deposited on top of each other and their own weight causes them to compress and harden into sedimentary rock. The horizontal layers of sedimentary rock are called strata. Common sedimentary rocks include shale, sandstone, and limestone.

Sedimentary rocks are the only rocks in which fossils can be preserved. The elevated temperatures and pressures needed to form both igneous and metamorphic rocks destroy fossils and organic remnants. The presence of fossils and the types of fossil organisms in a rock provide clues about the environment and age of sedimentary rocks. For example, fish fossils in sedimentary rock indicate that the sediments that make up the rock were deposited in a lake, river, or marine environment. By establishing the environment of the fossils in a rock, scientists learn more about the conditions under which the rock formed.

A sample of gneiss, a type of metamorphic rock, taken from the Sierra Nevada Mountains. (Reproduced by permission of JLM Visuals.)
A sample of gneiss, a type of metamorphic rock, taken from the Sierra Nevada Mountains. (Reproduced by permission of
JLM Visuals
.)

Metamorphic rock

Metamorphic rock is rock that has changed from one type of rock into another. The word metamorphic comes from Greek and means "of changing form." Metamorphic rock is produced from igneous, sedimentary, or even other metamorphic rocks. Most of Earth's crust is made up of metamorphic rock. Igneous and sedimentary rocks become metamorphic rock as a result of intense heat from magma and pressure from burial within Earth. Although the rock undergoes extreme heat and a great deal of pressure, it does not melt. If the rock melted, it would become igneous and not metamorphic rock. Instead, the heat and pressure combine to change the mineral makeup of the rock. Essentially, metamorphic rocks are made of the same minerals as the original rock, but the various minerals have been rearranged to make a new rock.

There are two basic types of metamorphic rock: regional and thermal. Regional metamorphic rock, found mainly in mountainous regions, is formed by pressure. Different amounts of pressure produce different types of rock. The greater the pressure, the more drastic the change (also, the deeper the rock the higher the temperature, which adds to the potential for diverse changes). For example, a pile of mud can turn into shale (a fine-grained sedimentary rock) with relatively low pressure, about 3 miles (5 kilometers) down into Earth. With more pressure and some heat, shale can transform into slate and mica. Carried even deeper, slate transforms into schist (pronounced shist) and then gneiss (pronounced nice).

Thermal metamorphic rock, also called contact metamorphic rock, is formed by considerable pressure and, more important, intense heat. When molten rock pushes up into Earth's crust, the incredible pressure behind it forces the molten rock into any empty space. The accompanying intense heat causes the surrounding rock to completely recrystallize, forming a new rock. An example of this type of thermal metamorphic rock is marble, which is actually limestone whose calcite has recrystallized. Sandstone made mostly of quartz fragments recrystallizes into quartzite. Thermal metamorphic rocks are not as common or plentiful as regional metamorphic rocks.

The rock cycle

The rock cycle depicts how the three main rock types can change from one type to another. All rocks exposed at Earth's surface undergo weathering, forming sediments that can be deposited to form sedimentary rocks. As sedimentary rocks are buried beneath more sediment, they are subjected to increases in both pressure and temperature, which can result in metamorphism and the formation of metamorphic rock. If the temperature of metamorphism is extremely high, the rock might melt completely and later recrystallize as an igneous rock. Rocks can move through the rock cycle along other paths, but uplift or burial, weathering, and changes in temperature and pressure are the primary causes of changes in rocks from one type to another.

[ See also Coal ; Minerals ]



Also read article about Rocks from Wikipedia

User Contributions:

1
mjnnjhbg
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May 16, 2006 @ 8:08 am
you should mention somewhere the answer a person needs to know
2
Samantha Tennant
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May 21, 2006 @ 11:23 pm
We think that your website thingy is OK because it provides an exellent deal of information but you should provide more specific information on igneous, metamorphic and sedimentary rock formations and what these words mean.
3
jelly
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Nov 27, 2006 @ 9:21 pm
I loved your website about all of these rocks!!!
I found a lot of information i needed to know on how the rocks change into other rocks for my science report!!!!
THANK YOU SO MUCH FOR MAKING THIS WEBSITE!!!!
4
i
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Dec 14, 2006 @ 6:18 pm
thanks for all of the info i got a A+ on my science report :)
5
Armani
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Dec 17, 2006 @ 5:17 pm
your site helped me so much in understanding the different rocks and the rock cycle for my geology final. :)
6
Ron
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Mar 24, 2007 @ 1:13 pm
Yo, thanks for making this dude(s) Really helpful. I got an A+ because of you guys. Thanks.
7
laisnajkh
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Oct 10, 2007 @ 7:19 pm
answer what people ask to know the info this info is good but we need more
8
Latonya
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Oct 16, 2007 @ 12:00 am
Thank you so much for this website it is wonderful it is such in detail about rocks. My oldest daughter came home with this project about the three main rocks and I truly believe this will tremendously help her in getting a good grade Thank you :)
9
kristina
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Oct 28, 2007 @ 9:21 pm
this website is good for info but needs to say how they are useful and some types of that rock but otherwise ok.
10
Brittany Pugh
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Nov 5, 2007 @ 8:20 pm
thanks so much for the information it was very helpful for my reprot i really apreciate it !!!!!!!!!!!thanks!!!!!!!!!!!!!
11
cam
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Nov 14, 2007 @ 3:03 am
this has helped me get into year 9 cause im failing but its help heaps
12
Ann Leslie R. Bangcaya
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Nov 17, 2007 @ 11:23 pm
I REALLY LOVE TO READ YOUR ARTICLE ABOUT ROCKS. I LEARNED SO MUCH ABOUT ROCKS, AND IT CAN HELP ME IN MY SCIENCE LESSONS AND MY SCIENCE REPORT. I REALLY THANKED YOU FOR THIS ARTICLE. I LOVED IT VERY MUCH.
13
J.L.Bird
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Nov 18, 2007 @ 2:02 am
wow, thank you, i have been looking for this information for a while.

keep on dancin'
14
brianna
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Jan 24, 2008 @ 6:18 pm
this really helped me with my sience project!!!!!!
thanks!!,
briannna
15
Fernanda Barrientos
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Feb 4, 2008 @ 9:21 pm
thanxz this site really helped me wit my homework!!!!!!!
16
shristi
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Feb 11, 2008 @ 11:11 am
thanz for the infos + site, it really helped me wit my homework!!
i really love it sooooooo much...
THANKZS!!
17
Rizza
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Mar 8, 2008 @ 1:01 am
i love this informations. Because it really helped me... THANK YOU.
18
lia
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Mar 26, 2008 @ 12:12 pm
I loved this site, it got me and A* on my science project about rocks and weathering! It is truly amazing!
19
lilica_repilica
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Apr 26, 2008 @ 5:05 am
thanks folks!!! this site really helped! my students love it! :)
20
chase
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May 14, 2008 @ 4:16 pm
this website helped alot for my A on my final science project
21
fred
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Aug 3, 2008 @ 3:03 am
thanks!!!!!!!!!!!
i used this information for my science project
22
EMMANUEL MWEMA
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Sep 9, 2008 @ 4:04 am
Thanks much for introducing this kind of information materials,it has helped me to solve some questions relating rocks. Thanks in advance.
23
Christian
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Sep 22, 2008 @ 6:18 pm
Thanks sooosooo much I have a project due tomorrow and your site helped make it the very best!!
24
kate
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Oct 12, 2008 @ 8:08 am
i need more i need what types are in the igneoius rock and same with sedimentary and metamorphic!!! i no all i need to o about the types of rocks, but i just need it to go deeper!!! if you can do that great, but if not well then ill just be at a different website!!!

thanks
25
Emilyy
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Nov 12, 2008 @ 3:03 am
this info has really helped me with our dep study!!!



thankss =) nice website!!


good information
26
joyce
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Nov 22, 2008 @ 12:00 am
i love your website this can will help student to her projects and assignments
27
shaine
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Dec 8, 2008 @ 3:03 am
I LOVE YOUR WEBSITE.I SEE THE ANSWER TO MY ASSIGNMENT/HOMEWORK,AND I ALSO LEARN ABOUT THE DIFFERENT KIND OF ROCKS,THEIR MEANING AND THEIR TYPES.AGAIN THANKS A LOT.
28
edward cullen
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Dec 12, 2008 @ 11:11 am
Good site, I love it!! More info please!


Awesome!
29
aldbert clark
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Dec 13, 2008 @ 7:07 am
I realy love this article because many of i need in school is in here and thank you very much who make this article
30
Shiki Senri
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Feb 16, 2009 @ 4:04 am
thanks....:) for the information but pls. include that rocks is a volcanic materials....

shiki_rima03
31
sarah babes
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Feb 17, 2009 @ 12:00 am
thank you for giving a lot of information.............
for giving me the answers in my assignment............
32
sarah babes benobo
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Feb 17, 2009 @ 12:00 am
comtinue for helping other students like me......
that's all..
thank's again..
33
Haylee
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May 26, 2009 @ 7:19 pm
i loved this website.i had some homework and i forgot my science notes. this web site really helped me!!
34
Zoe W
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Jun 8, 2009 @ 3:15 pm
It is really helpful and can be eaasily understood but some diagrams would also be really helpful
35
sudharsan
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Jul 9, 2009 @ 1:13 pm
this informs with pictures gives a detailed account on rocks.
36
shezam
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Aug 1, 2009 @ 12:12 pm
what a nice website its helps a lot, keep up the good site.
37
elainehateshernewproject!
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Aug 6, 2009 @ 5:05 am
this is great thank-you soo much before i went to your website i only had like 2 sentences for each rock on my report but now i have heaps more THANK-YOU
38
flordeluna
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Aug 16, 2009 @ 8:08 am
thank you its very helpful to us.......
i saw it clearly the types of rocks.......
and i hope there is so many improvements to your website...
39
vidush singhal
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Sep 16, 2009 @ 8:08 am
pumice is also a igneaus rock which is formed by the air traps in lava.coal is a sedementry rock.taj mahal is made from sandstone.Limestone contains calcite mineral.Gniss rock shows alternative pattern.
40
mohamad
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Oct 7, 2009 @ 1:01 am
Thanks guys
saved me lot of time
you got everything i needed
41
QUANNEA
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Oct 8, 2009 @ 7:19 pm
CLASTIC SEDIMENTARY ROCKS ARE MADE FROM WEATHERING BITS OF ROCKS AND MINERALS MANT DIFFERENT TYPES OFMINERALS ARE FOUND IN CLASTIC ROCKS
42
jaruise
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Oct 22, 2009 @ 6:18 pm
this is a weird web site but cool, i'm doinga project on quartzite 'm only in the 6th grade but it has given little information so bye for know im going to finish my project due on Mondy 26,2009 hope i get a A+ on it wish me luck!!.BYE BYE
43
jlegendsrock
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Nov 4, 2009 @ 8:08 am
ty scientists i am goin to get a good grade when the semester ends which is on friday this week ty. i owe u one
44
girl
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Nov 10, 2009 @ 3:15 pm
YAY! thanks for the help, I think I'll ace my test today!
45
yuki
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Nov 13, 2009 @ 8:20 pm
Thanks. iluv this website!it provides me with so much info for my homework and science class. i had to write this paper for science and i got an A+. thanks again. :D
46
mary joy
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Nov 24, 2009 @ 4:04 am
thank in your this website i learned so much in my cience project and im sure my science project is higher grade
47
shayshay
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Dec 2, 2009 @ 1:13 pm
it rocked i loved alot i got an 100 4 it i hope in the future it will help me some more and get me even better grades lol ;)
48
trisha
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Jan 9, 2010 @ 12:00 am
i really love your website... i can really learn may things about rocks
49
someone
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Jan 21, 2010 @ 12:12 pm
how is the igneous rock formed and is it found in neveda or some where else in the world and how do people discover it
50
nickales
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Feb 2, 2010 @ 8:08 am
the website was very help full i am going to get and a on my project
51
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Feb 3, 2010 @ 4:16 pm
thanks,this website helped me ALOT with my science report:)lol
52
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Jun 3, 2010 @ 2:14 pm
i lovve this website i got an A+ in my projet and it has pictures of sedimentary, igneous but not metamorphic I LOVE THIS WEBSITE!!!
53
Mary
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Sep 22, 2010 @ 5:05 am
This site was a big help has all thw imfo i needed!
54
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Dec 2, 2010 @ 7:19 pm
wow i love this site i didn't find every thing but i found some things.
55
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Jan 3, 2011 @ 9:21 pm
that's good info.! i need that for my science project!
56
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Jan 5, 2011 @ 5:17 pm
thank you so much cause i need this to help me with my science homework!! :D
57
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Jan 12, 2011 @ 6:06 am
thanks, now I know about rocks and good information I love to read your information thank you verry much :)
58
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Jan 14, 2011 @ 4:16 pm
than you so so so so so much this helped me a huge amount for my science mid-term project
59
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Jan 17, 2011 @ 2:02 am
if sedimentary rocks change to igneous rocks what do you call that?
60
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Feb 3, 2011 @ 4:16 pm
Thanks for the informations.
in science im learning rocks and menerals and these are good infromations
61
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Feb 14, 2011 @ 3:15 pm
um i have a quiz tomarrow and i want to know some more information thank you bye.
62
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Mar 12, 2011 @ 5:05 am
This website has helped me, i love this website its really good
63
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Apr 5, 2011 @ 5:17 pm
Can you explaine what all three of those rocks have in common?
64
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Oct 12, 2011 @ 10:10 am
Hi im in 6th grade and love sciens so i copyed your summery about igenous rocks and yes gave u credit
65
philip
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Jan 5, 2012 @ 9:09 am
this web site is so good because i gust used it for a school project and got 100% good job
66
Kyle Howard
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Jan 9, 2012 @ 6:18 pm
This was very helpful and I thank you for helping me.
67
joey
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May 10, 2012 @ 3:03 am
very well written information. A pat on the back and 10 out of ten. The descriptions were solid and at the same time simple to understand. Well done all you guys from science clarified I am very happy for you and I am sm sure everyone else with their comments would agree. Thank you for your help. :D
68
Dean Soto
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Dec 16, 2012 @ 12:12 pm
I have a very strange natural glass rock 3" diam molten clear with closer look fine ripples in the surface. Also light brown stoney patches full of dark sparkles of a metalic or dark pyrite look. Ive had this item for 18 years and always wondered what it is but never pursued it because it is red to me do to when adndel how it came to be reveal
69
kai
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Oct 29, 2014 @ 11:23 pm
can you put carbon on or don't put it on google if it does'nt mention carbon
70
Robert
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Mar 12, 2015 @ 5:05 am
Thanks..I've got a lot of usefull information for my geoloy assignment.
71
JESSAMIN BARRANTES
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Jun 10, 2015 @ 7:07 am
OH MY GOD..THANK U VERY MUCH ..I LEARNED MORE INFORMATIONS ABOUT ROCK..
72
allycat
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Nov 12, 2015 @ 6:18 pm
wow this really helped me study for science class and it doesn't confuse me
73
Jolee M M
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Jan 7, 2016 @ 3:15 pm
This did not help I need to know what rock is the result of igneous and sedimentary rock being changed by extreme heat and pressure.But thanks
74
Andrea
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Feb 14, 2016 @ 7:19 pm
OMG THANK YOU SO MUCH!!! THANKS ALL FLR YOUR HELP I GOT AN A+!

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