Lab 2. Igneous Rocks and the Gems Produced from Them

A) Introduction to Igneous Rocks

Igneous: Gems can occur in any type of rock. In the next few exercises we will look at all three types of rock, igneous, sedimentary, and metamorphic. Each forms in a different way. Rocksformed from cooling melted (molten) earth materials are called igneous rocks (rocks full offire). Gems formed in these rocks crystallize as the molten rock cools (in most cases). Themolten material is called magma below the surface and lava when it flows on the surface.

The igneous rocks are primary and formed the crust as the earth cooled. They still are beingformed today both above and below ground. The melts are mainly composed of silicon andoxygen. These two elements bond together making the back bone of a group of minerals calledthe silicates. Along with silicon and oxygen (these two element make up 76% of the crust),aluminum, iron, calcium, sodium, magnesium, and calcium make up about 99% of the earth’scrust by weight. So in a melt, these elements usually predominate and make up most of the rockforming minerals (dozen or so common minerals). Though they are abundant and important notall that many gems are silicates, most common of these are emerald (beryl group), quartz,garnets, and tourmaline. A lot of gems, even in of silicate family, have exotic elements. Theseexotic elements, such as beryllium, boron, lithium, chromium, vanadium, and zirconium need tobe concentrated. So gem formation often requires unusual geologic circumstances.

Sometimes water-rich hot fluids generated from the cooling rock separate from the main magmaand bring with them the exotic elements that do not easily fit into the common silicate mineralstructures (that is ferromagnesian silicates, quartz, feldspar, mica, etc.). With all the water andexotic elements excluded from the main mass of igneous magma, this very-fluid, exotic-elementrich “gem soup” forms gem pockets on the outer margin of the larger igneous rock bodies. These very fluid magmas create pegmatites.

B.

Figure 1 Igneous rock formation by intrusion intolayered sedimentary rocks. Note pegmatite is to theoutside of the intrusion.

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Classification of Igneous rocks

Some examples:

Igneous rocks are of two main types volcanic (extrusive) rocks formed on the surface andplutonic (intrusive) formed below the surface (see Figure 1 above).

Volcanic rocks can form from flowing lava or may be fragmental meaning that they are causedby explosive volcanism that can have horrific consequence. The city of Pompeii was destroyedin AD 79 by a volcanic eruption of Mt. Vesuvius.

Some gems such as diamonds and pyrope garnet are carried to the surface by volcaniceruptions referred to as diatremes (Literally, a hole filled with breccia [broken pieces ofrock], formed by a subterranean gaseous explosion). The rock carrying diamonds to thesurface is kimberlite. It is fragmental in nature (see pyroclastic below), but no one has directlywitnessed a kimberlite eruption and the fragmental texture may in some cases even be intrusivehaving never reached the surface. Other gems that are carried up with lava outpouring includeperidot (olivine) which may be carried upward from where it crystallized at a much slower ratethan diamonds in kimberlite. Diamonds and peridot carried up as solids from a deeper rock arecalled, Xenoliths (which means foreign rocks). The peridot crystals have to be broken out of thelava or glassy obsidian that oozed out with them in tow. Obsidian itself, a volcanic glass causedby rapid cooling, is occasionally used as an ornamental gem. Snow flake obsidian is veryattractive when tumbled or polished. The snowflakes in the obsidian are crystals of feldspar thatgrow in the magma or lava as it rises and are not xenoliths. They grow in the same melt and arelarger than the rest of the surrounding rock. Sometimes these are called phenocrysts. Phenocrysts differ from xenoliths in that phenocrysts form from the same magma as thesurrounding rock creating a texture called porphyritic texture, while xenoliths did not crystalizein the magma they are now found in.

Some gems form after beds of ash and lava settle because the ash is soluble in the hot fluids thatmay rise from hydrothermal (hot water) vents emanating from below (see geyser in Figure 1). Hot gases also rise and can carry mineral forming elements. Because the hot water is filled withdissolved minerals, on cooling a precipitation of crystals into spaces in the rock may occur as thehot gases and water rise and cool. Some opals and nodules such as thunder eggs form in thisway. Though often called geodes, many quartz and other mineral lined pockets are reallyformed is cavities between pillow lavas. Much of the Brazilian amethyst and citrine “geodes”may have this origin. Pillow lavas form when lava flows into water (see Figure 2, 3 & 8) below.

Gold and other minerals also may precipitate from hydrothermal solutions that emanate fromigneous activity and much prospecting for metals takes place in igneous regions as well. However, some people consider these to be metamorphic rather than igneous processes.

For geologist to have conversations about rocks special vocabulary and descriptive terminologyhas been developed.

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Igneous rocks are classified and named by their texture and color. These properties help us tointerpret their cooling history and the source of the magma. Knowing the history of the rockallows us to consider its economic potential as well.

Volcanic rock cool rapidly

The terminology can be complex. We will say that fine-grained rocks are made of microscopiccrystals. Geologist call this texture aphanitic. Aphanitic rocks and glass (nocrystals/minerals) are mostly of volcanic origin as are fragmental rocks (sometimes calledpyroclastics). Pyroclasts are literally “fire fragments. ” Pyroclastics rocks are explosivelyemplaced. Usually this means a volcanic eruption. Sometimes the rock material comes out offissure (crack) or just explodes without a prior volcano.

Fine-grained rocks may not have formed on the surface, but slightly below it (close to thesurface), they are considered volcanic because of their microscopic interlocking crystal texture.

The reason why fine-grained rocks (and volcanic glass) are considered volcanic is that theymust have cooled relatively quickly, not giving enough time for larger crystals to form.

Sometimes however, a finer-grained rock will include coarser crystals that rose with the lava orglass. The larger crystals called phenocrysts encased in the finer groundmass, give the rock aspecial composite texture called porphyritic. The constituents of porphyritic rocks indicate twoseparat e rates of cooling indiffere nt environments (belowthe surface for thepheno crysts, and nearer thesurfac e for the groundmass).

Figure 2. Pillows formed as lava erupts intowater.

Figure 3. The pillowshave hollows betweenthem that can bemineralized. Similar togeodes.

Figure 4. Porphyritic textureis due to two rates of cooling. Larger crystals are calledphenocrysts that aresurrounded by ground mass.

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Phenocrysts and xenoliths (foreign rocks) carried up can be of value, consider peridot anddiamonds. Both may exist in large quantities in the mantle but are rare on the surface and in theearth’s crust. As well, corundum, ruby and sapphire and moonstones can be carried up withvolcanic eruptions (such as alkali basalt) that flood the earth’s surface with large volumes ofvolcanic rock making gem-rich regions as in Thailand’s Chantbaburi-Trat area that includes theHill of Gems (rubies) area, the Pailin ruby and sapphire gem field in Cambodia. Peridot Mesanear San Carlos, Arizona, and Australia’s sapphire and zircon-rich Ankie and New Englanddistricts (Yellow and blue sapphires) have similar origins. The liquid magma acts like anelevator bringing these precious crystals to the surface.

Plutonic Rocks Cool Slowly

Plutonic rocks on the other hand must have cooled relatively slowly because they have large,visible crystals (This texture is called phaneritic texture = coarse-grained crystals). Thegrowth of larger crystals takes time and indicates slower cooling below the surface. Thesurrounding rocks act as a sort of thermos only allowing the heat to escape slowly. Since a hotfluid allows for more fluidity, the elements that make up crystals can travel more freely to hookup with each other and the crystals have more time to grow large. For this reason course-grainedrocks, such a pegmatite (which is a textural term meaning having crystals larger that 1 inch [2.5cm]) are more likely to have large mineral grains/crystals that potentially could yield gems ofsufficient size for cutting.

Some pegmatites apparently form from very fluid remnants of cooling magma withincompatible elements that are driven off of the main cooling plutonic rock body. Thesemineralogically complex pegmatites are especially sought after targets for gem hunters and maycontain large crystals of quartz, tourmaline, beryl, and spodumeme, and many other rare mineralsthat fascinate the mineral collector. They are also economically important for certain chemicals,such as lithium and beryllium that are mined from them.

Exercise Begins Here

After a discussion of the rocks, you will be given a box of rocks and asked to identify their originand name them based on a brief description. You should learn to recognize these rocks for anexam. Your instructor will emphasize certain rocks and will help you to correctly identify the

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samples.

List of rock numbers for you to learn.

Rock numbersIgneous

Look at the texture and Composition (color).

Texture types of igneous rocks

You should locate, 1) glassy texture, 2) fine-grained (aphanitic) texture, and 3) coarse texture(phaneritic), 4) pegmatitic texture which has crystals larger than about an inch in diameter, 5) thebubbly or vesicular texture, and 6) lastly pyroclastic texture made of broken pieces of rock, suchas pumice, and ash mixed together. Use the characteristics described below.

Glassy texture–overall sheen is glassy, often transparent on the thin edges.Aphanitic texture–overall a flat or dull appearance, typically a uniform color.Phaneritic texture–may have several colors (different visible minerals), sparkles when moved

in the light.Pegmatite texture–too coarse to be in boxes, there is a side table example. Looks like granite.Vesicular texture–has openings like Swiss cheese, or cotton candy. The gray rock, pumice

(AKA: the bathroom stone) may float on water. The darker rock scoria will not float.Pyroclastic texture–broken pieces of rocks (pumice, etc.) fused together by heat. The piecesmay be different colors. Sometimes layered (usually lighter pumice and darker layers of glass). Often this rock is difficult to recognize in small specimens. List the numbers of the rocks in the table below.

Glassy Aphanitic(fine)

Phaneritic(coarse)

Pegmatite(very coarse)

Vesicular(bubbly)

Pyroclastic(fragments)

Use the “Igneous Rocks Classification Table” below to name the rocks you have.

Texture and composition

Texture

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Step 1. First recognize that texture is along each row, Use column 1 (down) to put in all texturesin this table to figure out what you have in your tray. Put the sample numbers in the firstcolumn, such as coarse, fine, glassy, etc. (note pegamatite is in with granite).

Once you have the textures from the box, check with the instructor because some samples mustbe on the side table because we do not have enough samples of them.

Composition

Step 2. Composition is a result of the minerals present and often is reflected by the color of therock as listed below. Four color groups are used, felsic (light colors), intermediate (file cabinetgray or salt and pepper), mafic (dark colors such as red, dark brown, or black), and ultramafic(having green colors).

As you will see in the chart of igneous classification on the next page’s chart, you need textureand composition to identify an igneous rock. However, composition is basically the same ascolor (composition = color)!

The amount of dark minerals is all you need to classify color (= composition)!

Remember from earlier that dark colored minerals are: Red, Brown, Dark Gray, Black, andGreen, look for the percentages of these colors in the rock. These dark silicate minerals arecalled the ferromagnesian silicates. So the ferromagnesian silicates can be used to classify thecomposition of the ordinary igneous rocks! There are a few oddball igneous rocks that have noferromagnesians. Remember that significant green is ultramafic!

We then classify the igneous rocks base on composition (color) as:

Felsic–less than 25% dark minerals; quartz is often present (usually pink and white colors).Intermediate–more than 25% and less than 45% dark minerals (usually an intermediate gray).Mafic–more than 45% and less than 85% dark mineral (usually dark gray or black, some brown). Ultramafic–more than 85% dark mineral, olivine often present (usually green with some red orbrown.)

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1) put the rock numbers in the texture box on the left (check to see if there are side table rocks)2) move the numbers over to the correct compositional box. Go over these with your instructor3) later record the number and its description on the next two pages of charts

Igneous Rocks Classification Table

Compositions

--------------->

(across)

Felsic

Light colored

Igneous rocks

mainly pink, white,

and gray less than

25% dark minerals

(see chart)

Intermediate

Gray colored Rocks

mainly gray, from

25-45% dark

minerals, quartz

uncommon

Mafic

Dark colored

igneous rocks

mainly dark green,

black, or brown

>45% dark minerals

but less than 85%

Ultramafic

Very dark colored

rocks containing

almost all iron- and

magnesium-rich

minerals, greater

than 85%

Textures (down) Plutonic rocks with coarse texture

Coarse grained

(visible minerals)

Granite if grains

are less than 2.5 cm.

Pegmatite if grains

are larger than 2.5

cm

Diorite

Usually a dark gray

with black specks

(salt & pepper look)

Gabbro

Black, but crushed

grains on broken

surfaces may appear

much lighter

Dunite

Green, usually

grains are sugary

looking.

Volcanic rocks with fine-grained texture or glassy texture

Fine grained

(microscopic

minerals)

Rhyolite– usually

pink or whitish

gray. May have

small dark minerals

Andesite–usually a

medium gray often

with splintery dark

minerals (shiny

bits)

Basalt–usually

black or dark redish

brown with even

texture

no rocks in this

category

Glassy texture Obsidian–a natural glass used in arrow

heads

uncommon in this

category

no rocks in this

category

Non-glassy, bubbly

texture

no rocks in this

category

no rocks in this

category

Scoria–dark brown,

red, or black may

have olivine grains

no rocks in this

category

Glassy, bubbly

texture

Pumice–the bathroom stone, floats on

water

no rocks in this

category

no rocks in this

category

Pyroclastics texture Tuff–a volcanic glass and layered rock.

Layers are subtle look at all sides carefully

Kimberlite–this is the rock that contains

diamonds. Fragments of many rocks

possible. May have some calcite, so may

fizz with acid!

Use the table below for your notes (next page). Mark appropriate boxes to coincide with therock names.

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Some Final Considerations of the Composition of Igneous Rocks.

As we will discuss, felsic rocks are much more common on the continents. Because they havereduced amounts of ferromagnesian (iron and magnesium) or dark minerals, they are lowdensity and float on the denser mafic and ultramafic rocks. Density differences are one reasonfor plate tectonics. A differentiation by density makes the continents behave differently thanthe ocean and underlying mantle rocks. The dark minerals (ferromagnesian minerals) havehigher melting points and thus sink, as the rocks sink the non-ferromagnesian or light mineralsmelt and the created magma rises back up. So the ferromagnesians which are still solid anddense sink back into the mantle and the lighter elements (essentially minerals) in the magmasrise.

Plate tectonics will be discussed in lecture and is important for the overturn of materials betweenthe crust and underlying mantle and explains the movement of continents (continental drift) overgeologic time. The churning caused by plate tectonics cause less denser (felsic; lighter) mineralsto rise up and form the continental crust. Thus plate tectonics does ultimately control whererocks and gem minerals will be present. It also is an encompassing theory that explains theorigins or earthquakes, mountain belts, and the chemical evolution of the earth, but this is a topic

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for physical geology and will not be discussed in detail here. However, geoscientists have thebest grasp of the reason for gem, precious metal and all other economic deposits and this coursemight be a gateway to future studies of the earth!

Figures 6A & B show two important boundaries of plate tectonics. Figure 6A shows a ridge inthe middle of the Atlantic Ocean where new igneous rock is rising to create two new platesmoving the USA away from Europe. Figure 6B shows where plates go under continents aftergrowing from a midocean ridges and sink and melt. The regions near to these sinking (orSubducting plates) experience igneous activity (rocks melt) including volcanic eruptions.

Living near a plate boundary has its negatives, such a volcanoes and earthquakes, but muchmineral wealth is generated by the processes, including gold and gemstones.

Samples for You to Study as a Group are Provided on the Side Tables

Figure 5. The plates shown here make up the earth’s outersurface and move on a plastic layer below. Plate boundaries,like the edges of broke ice on a lake, see most of the action.

Figure 6A Figure 6B

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Once you have completed your tray identifications, each table will be assigned a side tablesample. You will identify it and look up some of its characteristics and then use thesecharacteristics and terminology to present your findings to the class using guide questions. Youdo not have to use all the guide questions and you can make your own questions andanswer them instead if you so choose!

Your sidetable samples

Kimberlite Pillow lava Pegmatite Volcanicrock

Vein quartzwith gold

1) Kimberlite Several samples are given. They appear coarse grained, but careful inspectionshows that the samples are composed of fragments of both older rocks and broken minerals. Kimberlite is the rock in which most primary diamonds are found. The rock is created by anultramafic magma that is extremely explosive due to carbon dioxide and other volatile elementsthat cause it to explosively rise from depths around 100 miles (160 km) below the crust. That isthe magma has its source in the mantle. The magma drags diamonds up with it from this depth. The diamonds are already solid when they are taken up by the kimberlite magma, but as themagma rises it takes many fragments of other solid rocks up with it till it reaches the earth’ssurface. Do not expect to see a diamond in your sample, as the concentration of diamonds inkimberlite is typically 0.1 grams per ton.

Use the Internet to find some illustrations and answers tothe following questions.

A) Kimberlite–where is it named from?

B) What is the texture of kimberlite? What is the color?

C) Is kimberlite volcanic?

D) What does the word, “Xenolith” have to do withdiamonds in kimberlite?

E) Where did the diamonds form?

F) Are any diamonds really formed from “Coal?” Coal is plant material that has fossilized to

Figure 7. Kimberlite formation.

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make a fossil hydrocarbon fuel. How could the pressure get strongenough to do this?

2) Pillow lava and related“geodes” (There are sedimentarygeodes; these are not!) is a rockcreated when lavas, typicallybasaltic lavas, flow into a lake or theocean. Small pillows or blocks oflava pile up (Figures 2 & 3) , butthere are typically spaces betweenthe pillows that allow latermineralization to occur in betweenthe pillows. These spaces can befilled with many minerals, but a

typical infilling is quartz, such as amethyst and zeolite minerals such as natrolite and stilbite. The figure (Figure 8) above is from Paterson, New Jersey, less than 25 miles from New YorkCity (Sinkankas, 1974).

Amethyst is the purple form of quartz, usually forming good crystals. These grow into a hollowbetween the pillows. Pillow lavas have such “openings” as do “geodes.” The term pillow has todo with the shape of the lava, not the holes or openings between them, but other lavas flow likesheets and don’t have openings of this size for minerals to grow in.

Consider the following in making your report.

A) Pillow lava–why this name?

B) What rock type is the lava?

C) Can you find a “Youtube” video on pillow lava.

D) Name or describe some locations and say a little bit about them (hint: Hawaii, New Jersey,Brazil). In Brazil, amethyst cathedrals really are the result of pillow lava flows, let’s find somepictures and discuss the evidence.

E) Some of the minerals associated with pillow lavas include, quartz, zeolite minerals, andcalcite. Let’s see if we can find something out about these minerals and their local availability,such as in New Jersey.

Figure 8.

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3) Quartz Veins Be Careful! Warning, the valuable mineral on the quartz veins is verysoft and could be scraped off with your fingernail! So what is it?

Well before you answer this question, there are things toconsider. People have gone prospecting for this valuablemineral for thousand of years, and though it mainly is foundin streams/rivers, there are primary deposits of this metalfound in some igneous provinces. This precious mineral hasan affinity for the watery solutions found circulating above anigneous intrusion. Some people would say it is not trulyigneous since it is not from a direct magma, but preciousmetal still has an igneous association.

A) Your samples was part of a larger vein of quartz. Describe a vein of quartz and find a picture on the Internetthat we can project.

B) Where in the USA might this valuable mineral be found? Mention a place or two (not astream deposit, but a vein area).

C) What are some properties of the materials involved? Valuable versus what is called thegangue mineral? Notice that in the illustration above dark areas represent open space. What might minerals that grow into that space look like compared to if all the space was filled?

D) What is a nugget compared to crystallized material from primary deposits?

E) Borrow a microscope and set it up so that people can look at your specimens. Could this be afake? Give some opinions after you have looked at it with the microscope.

4) Pegmatite First look up or find a description of pegmatite. It is a lot like granite. Yoursamples have blue/blue-green minerals in it that are sometime valuable. These are not as clear assome gem material. Yours may have fractures.

A) Can you recognize some minerals in it? Look up pictures and try and figure out what the

Figure 9. Quartz vein.

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blue mineral is. What thepink mineral is. What thegray mineral with no cleavageis. What theplaty mineral with onecleavage is. The blue/blue-green mineral was not in yourmineral box, but the otherswere.

B) Ask your instructor for lastweeks tray of minerals andidentify the gray mineral,flaky mineral, and pinkmineral in your tray samples. These same mineral are in thepegmatite.

C) Discuss with the groupyour definition of pegmatiteand what kind of gems youmight find in a pegmatite.

D) Can we find pictures orfilm of pegmatite on“Youtube,” etc? (May not beeasy!).

E) What is a gem pocket?

5) Peridot from Arizona and Hawaii Peridot is a green gem quality olivine (as a mineral it iscalled olivine) that does not form on the earth’s surface or crust, but comes from the mantle,sometimes starting as deep as 75 Km below the earth’s surface. Olivine and high pressurestructural variants constitute over 50% of the Earth's upper mantle, and thus olivine is one of theEarth's most common minerals. The rise of these deep (mantle formed) minerals is unusual andso peridot is a rare gem. Both diamond and peridot come from the mantle.

A) Explain, in simple terms, what the mantle is? Give or find an illustration of the earth’s

Figure 10. Pegmatite pocket (gem pocket).

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concentric layers and the mantle should be a focus of your picture. We may project this.

B) Find something on the internet about mantle sources for minerals like peridot. Here is onelink http://www.gemsociety.org/info/igem17.htm Look for “Gems formed in the mantle on thispage.

C) Your igneous rock sample is one that we studied today. What is it called? Why is it volcanicor plutonic (is there evidence in the rock that supports your statement)? What is its classificationin terms of texture? What is its classification in terms composition (but remember the peridot isnot a phenocryst, but a xenotlith)? Being that the peridot is a xenolith, is this rock trulyultramafic? Just consider the matrix or ground mass and not the xenoliths.

D) What is a flood basalt? Find some gems that come from these? Name a few locations.

References

Sinkankas, J., 1974, Prospecting for Gemstones and Minerals, 2 edition, New York, Vannd

Nostrand Reinhold, 350 P.

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Evaluation Questions for Igneous Rocks

) Would a denser rock (one that has denser minerals) generally lie toward one side of theclassification of rocks (Igneous Rocks Classification Table)? Think about composition.

) Could the phenocrysts in a rock be potential gemstones? Explain.

) Would finding a particular igneous rock type get you excited about potentially becoming rich? Explain.

) Lavas tend to have what type of grain size?___________________. Is the grain size a negativefor finding gems in A) some instances, B) all instances, C) let me research that--What gems arefound in lavas?

) What is the general origin of obsidian? Can you find two present/past uses of obsidian thatmake it valuable?

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) What attribute sets pegmatite aside from other rocks? What gems might one hope to find in it?

) Locally pegmatites have produced gems. Can you find a local example of a gem producingpegmatite in New York, Connecticut, or Pennsylvania? (Hint try: “Gemstone Occurrences inConnecticut”; see if that includes pegmatites). What was produced?

) Why might it be easier to get a gemstone crystal (specimen that looks nice) out of a pegmatitethan out of a granite or basalt?

) A very important mineral in igneous rock classification is a green mineral. As we learned lasttime this mineral is considered a dark-colored, or ferromagnesian silicate. What is this mineral? Towards which side of the chart is it found (left or right)?

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) Quartz is almost as important as the green mineral mentioned above. What side of theclassification chart of igneous rocks does it tend to be on? (Felsic or Ultramafic)

) Pillow lavas form when lava__________________________ and tend tohave_______________________ that minerals can grow in. See if you can fill this in.

) Plate tectonics has moved things around. Continents have drifted. Continents have been tornasunder (apart by rifting). Think about it. Could gems prospecting and an understanding ofplate tectonics have a potential to help/explain each other.

) What about volcanoes and plate tectonics? Where is “The Ring of Fire?” Could volcanoeshelp create gemstones?

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Look at Figure 6.

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