Download Lab 2. Igneous Rocks and the Gems Produced from

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. Rocks
formed from cooling melted (molten) earth materials are called igneous rocks (rocks full of
fire). Gems formed in these rocks crystallize as the molten rock cools (in most cases). The
molten 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 being
formed today both above and below ground. The melts are mainly composed of silicon and
oxygen. These two elements bond together making the back bone of a group of minerals called
the 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’s
crust by weight. So in a melt, these elements usually predominate and make up most of the rock
forming minerals (dozen or so common minerals). Though they are abundant and important not
all 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. These
exotic elements, such as beryllium, boron, lithium, chromium, vanadium, and zirconium need to
be concentrated. So gem formation often requires unusual geologic circumstances.
Sometimes water-rich hot fluids generated from the cooling rock separate from the main magma
and bring with them the exotic elements that do not easily fit into the common silicate mineral
structures (that is ferromagnesian silicates, quartz, feldspar, mica, etc.). With all the water and
exotic elements excluded from the main mass of igneous magma, this very-fluid, exotic-element
rich “gem soup” forms gem pockets on the outer margin of the larger igneous rock bodies.
These very fluid magmas create pegmatites.
Figure 1 Igneous rock formation by intrusion into
layered sedimentary rocks. Note pegmatite is to the
outside of the intrusion.
B.
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Classification of Igneous rocks
Some examples:
Igneous rocks are of two main types volcanic (extrusive) rocks formed on the surface and
plutonic (intrusive) formed below the surface (see Figure 1 above).
Volcanic rocks can form from flowing lava or may be fragmental meaning that they are caused
by explosive volcanism that can have horrific consequence. The city of Pompeii was destroyed
in AD 79 by a volcanic eruption of Mt. Vesuvius.
Some gems such as diamonds and pyrope garnet are carried to the surface by volcanic
eruptions referred to as diatremes (Literally, a hole filled with breccia [broken pieces of
rock], formed by a subterranean gaseous explosion). The rock carrying diamonds to the
surface is kimberlite. It is fragmental in nature (see pyroclastic below), but no one has directly
witnessed a kimberlite eruption and the fragmental texture may in some cases even be intrusive
having never reached the surface. Other gems that are carried up with lava outpouring include
peridot (olivine) which may be carried upward from where it crystallized at a much slower rate
than diamonds in kimberlite. Diamonds and peridot carried up as solids from a deeper rock are
called, Xenoliths (which means foreign rocks). The peridot crystals have to be broken out of the
lava or glassy obsidian that oozed out with them in tow. Obsidian itself, a volcanic glass caused
by rapid cooling, is occasionally used as an ornamental gem. Snow flake obsidian is very
attractive when tumbled or polished. The snowflakes in the obsidian are crystals of feldspar that
grow in the magma or lava as it rises and are not xenoliths. They grow in the same melt and are
larger 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 the
surrounding rock creating a texture called porphyritic texture, while xenoliths did not crystalize
in 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 that
may 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 with
dissolved minerals, on cooling a precipitation of crystals into spaces in the rock may occur as the
hot gases and water rise and cool. Some opals and nodules such as thunder eggs form in this
way. Though often called geodes, many quartz and other mineral lined pockets are really
formed 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 from
igneous 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 terminology
has been developed.
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Figure 2. Pillows formed as lava erupts into
water.
Figure 3. The pillows
have hollows between
them that can be
mineralized. Similar to
geodes.
Igneous rocks are classified and named by their texture and color. These properties help us to
interpret their cooling history and the source of the magma. Knowing the history of the rock
allows 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 microscopic
crystals. Geologist call this texture aphanitic. Aphanitic rocks and glass (no
crystals/minerals) are mostly of volcanic origin as are fragmental rocks (sometimes called
pyroclastics). Pyroclasts are literally “fire fragments. ” Pyroclastics rocks are explosively
emplaced. Usually this means a volcanic eruption. Sometimes the rock material comes out of
fissure (crack) or just explodes without a prior volcano.
Fine-grained rocks may not have formed on the surface, but slightly below it (close to the
surface), 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 they
must 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 or
glass. The larger crystals called phenocrysts encased in the finer groundmass, give the rock a
special composite texture called porphyritic. The constituents of porphyritic rocks indicate two
separat
e rates of cooling in
differe
nt environments (below
the
surface for the
Figure 4. Porphyritic texture
pheno
crysts, and nearer the
is due to two rates of cooling.
surfac
e for the groundmass).
Larger crystals are called
phenocrysts that are
surrounded by ground mass.
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Phenocrysts and xenoliths (foreign rocks) carried up can be of value, consider peridot and
diamonds. Both may exist in large quantities in the mantle but are rare on the surface and in the
earth’s crust. As well, corundum, ruby and sapphire and moonstones can be carried up with
volcanic eruptions (such as alkali basalt) that flood the earth’s surface with large volumes of
volcanic rock making gem-rich regions as in Thailand’s Chantbaburi-Trat area that includes the
Hill of Gems (rubies) area, the Pailin ruby and sapphire gem field in Cambodia. Peridot Mesa
near San Carlos, Arizona, and Australia’s sapphire and zircon-rich Ankie and New England
districts (Yellow and blue sapphires) have similar origins. The liquid magma acts like an
elevator 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). The
growth of larger crystals takes time and indicates slower cooling below the surface. The
surrounding rocks act as a sort of thermos only allowing the heat to escape slowly. Since a hot
fluid allows for more fluidity, the elements that make up crystals can travel more freely to hook
up with each other and the crystals have more time to grow large. For this reason course-grained
rocks, such a pegmatite (which is a textural term meaning having crystals larger that 1 inch [2.5
cm]) are more likely to have large mineral grains/crystals that potentially could yield gems of
sufficient size for cutting.
Some pegmatites apparently form from very fluid remnants of cooling magma with
incompatible elements that are driven off of the main cooling plutonic rock body. These
mineralogically complex pegmatites are especially sought after targets for gem hunters and may
contain large crystals of quartz, tourmaline, beryl, and spodumeme, and many other rare minerals
that 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 origin
and name them based on a brief description. You should learn to recognize these rocks for an
exam. 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 numbers
Igneous
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) the
bubbly or vesicular texture, and 6) lastly pyroclastic texture made of broken pieces of rock, such
as 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 pieces
may 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 textures
in this table to figure out what you have in your tray. Put the sample numbers in the first
column, 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 must
be 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 the
rock as listed below. Four color groups are used, felsic (light colors), intermediate (file cabinet
gray 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 texture
and composition to identify an igneous rock. However, composition is basically the same as
color (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, and
Green, look for the percentages of these colors in the rock. These dark silicate minerals are
called the ferromagnesian silicates. So the ferromagnesian silicates can be used to classify the
composition of the ordinary igneous rocks! There are a few oddball igneous rocks that have no
ferromagnesians. 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 or
brown.)
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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 instructor
3) 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)
Textures (down)
Coarse grained
(visible minerals)
Intermediate
Gray colored Rocks
mainly gray, from
25-45% dark
minerals, quartz
uncommon
M afic
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%
Plutonic rocks with coarse texture
Granite if grains
are less than 2.5 cm.
Pegmatite if grains
are larger than 2.5
cm
Diorite
Gabbro
Dunite
Usually a dark gray
with black specks
(salt & pepper look)
Black, but crushed
grains on broken
surfaces may appear
much lighter
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
Glassy texture
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
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
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!
no rocks in this
category
Use the table below for your notes (next page). Mark appropriate boxes to coincide with the
rock 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 have
reduced amounts of ferromagnesian (iron and magnesium) or dark minerals, they are low
density and float on the denser mafic and ultramafic rocks. Density differences are one reason
for plate tectonics. A differentiation by density makes the continents behave differently than
the ocean and underlying mantle rocks. The dark minerals (ferromagnesian minerals) have
higher melting points and thus sink, as the rocks sink the non-ferromagnesian or light minerals
melt and the created magma rises back up. So the ferromagnesians which are still solid and
dense sink back into the mantle and the lighter elements (essentially minerals) in the magmas
rise.
Plate tectonics will be discussed in lecture and is important for the overturn of materials between
the crust and underlying mantle and explains the movement of continents (continental drift) over
geologic time. The churning caused by plate tectonics cause less denser (felsic; lighter) minerals
to rise up and form the continental crust. Thus plate tectonics does ultimately control where
rocks and gem minerals will be present. It also is an encompassing theory that explains the
origins 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 the
best grasp of the reason for gem, precious metal and all other economic deposits and this course
might be a gateway to future studies of the earth!
Figure 5. The plates shown here make up the earth’s outer
surface 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
Figures 6A & B show two important boundaries of plate tectonics. Figure 6A shows a ridge in
the middle of the Atlantic Ocean where new igneous rock is rising to create two new plates
moving the USA away from Europe. Figure 6B shows where plates go under continents after
growing from a midocean ridges and sink and melt. The regions near to these sinking (or
Subducting plates) experience igneous activity (rocks melt) including volcanic eruptions.
Living near a plate boundary has its negatives, such a volcanoes and earthquakes, but much
mineral wealth is generated by the processes, including gold and gemstones.
Samples for You to Study as a Group are Provided on the Side Tables
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Once you have completed your tray identifications, each table will be assigned a side table
sample. You will identify it and look up some of its characteristics and then use these
characteristics and terminology to present your findings to the class using guide questions. You
do not have to use all the guide questions and you can make your own questions and
answer them instead if you so choose!
Your side
table samples
Kimberlite
Pillow lava
Pegmatite
Volcanic
rock
Vein quartz
with gold
1) Kimberlite Several samples are given. They appear coarse grained, but careful inspection
shows 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 an
ultramafic magma that is extremely explosive due to carbon dioxide and other volatile elements
that cause it to explosively rise from depths around 100 miles (160 km) below the crust. That is
the 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 the
magma rises it takes many fragments of other solid rocks up with it till it reaches the earth’s
surface. Do not expect to see a diamond in your sample, as the concentration of diamonds in
kimberlite is typically 0.1 grams per ton.
Use the Internet to find some illustrations and answers to
the 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 with
diamonds in kimberlite?
E) Where did the diamonds form?
Figure 7. Kimberlite formation.
F) Are any diamonds really formed from “Coal?” Coal is plant material that has fossilized to
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make a fossil hydrocarbon fuel.
How could the pressure get strong
enough to do this?
2) Pillow lava and related
“geodes” (There are sedimentary
geodes; these are not!) is a rock
created when lavas, typically
basaltic lavas, flow into a lake or the
ocean. Small pillows or blocks of
lava pile up (Figures 2 & 3) , but
there are typically spaces between
the pillows that allow later
mineralization to occur in between
Figure 8.
the pillows. These spaces can be
filled 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 York
City (Sinkankas, 1974).
Amethyst is the purple form of quartz, usually forming good crystals. These grow into a hollow
between the pillows. Pillow lavas have such “openings” as do “geodes.” The term pillow has to
do with the shape of the lava, not the holes or openings between them, but other lavas flow like
sheets 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 some
pictures and discuss the evidence.
E) Some of the minerals associated with pillow lavas include, quartz, zeolite minerals, and
calcite. Let’s see if we can find something out about these minerals and their local availability,
such as in New Jersey.
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3) Quartz Veins Be Careful! Warning, the valuable mineral on the quartz veins is very
soft and could be scraped off with your fingernail! So what is it?
Well before you answer this question, there are things to
consider. People have gone prospecting for this valuable
mineral for thousand of years, and though it mainly is found
in streams/rivers, there are primary deposits of this metal
found in some igneous provinces. This precious mineral has
an affinity for the watery solutions found circulating above an
igneous intrusion. Some people would say it is not truly
igneous since it is not from a direct magma, but precious
metal still has an igneous association.
Figure 9. Quartz vein.
A) Your samples was part of a larger vein of quartz.
Describe a vein of quartz and find a picture on the Internet
that we can project.
B) Where in the USA might this valuable mineral be found? Mention a place or two (not a
stream deposit, but a vein area).
C) What are some properties of the materials involved? Valuable versus what is called the
gangue 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 a
fake? 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. Your
samples have blue/blue-green minerals in it that are sometime valuable. These are not as clear as
some gem material. Yours may have fractures.
A) Can you recognize some minerals in it? Look up pictures and try and figure out what the
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blue mineral is. What the
pink mineral is. What the
gray mineral with no cleavage
is. What the
platy mineral with one
cleavage is. The blue/bluegreen mineral was not in your
mineral box, but the others
were.
B) Ask your instructor for last
weeks tray of minerals and
identify the gray mineral,
flaky mineral, and pink
mineral in your tray samples.
These same mineral are in the
pegmatite.
C) Discuss with the group
your definition of pegmatite
and what kind of gems you
might find in a pegmatite.
D) Can we find pictures or
film of pegmatite on
“Youtube,” etc? (May not be
easy!).
E) What is a gem pocket?
Figure 10. Pegmatite pocket (gem pocket).
5) Peridot from Arizona and Hawaii Peridot is a green gem quality olivine (as a mineral it is
called 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 pressure
structural variants constitute over 50% of the Earth's upper mantle, and thus olivine is one of the
Earth's most common minerals. The rise of these deep (mantle formed) minerals is unusual and
so 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
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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 one
link http://www.gemsociety.org/info/igem17.htm Look for “Gems formed in the mantle on this
page.
C) Your igneous rock sample is one that we studied today. What is it called? Why is it volcanic
or plutonic (is there evidence in the rock that supports your statement)? What is its classification
in terms of texture? What is its classification in terms composition (but remember the peridot is
not a phenocryst, but a xenotlith)? Being that the peridot is a xenolith, is this rock truly
ultramafic? 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, 2nd edition, New York, Van
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 the
classification 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 negative
for finding gems in A) some instances, B) all instances, C) let me research that--What gems are
found in lavas?
) What is the general origin of obsidian? Can you find two present/past uses of obsidian that
make 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 producing
pegmatite in New York, Connecticut, or Pennsylvania? (Hint try: “Gemstone Occurrences in
Connecticut”; 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 pegmatite
than out of a granite or basalt?
) A very important mineral in igneous rock classification is a green mineral. As we learned last
time 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 the
classification chart of igneous rocks does it tend to be on? (Felsic or Ultramafic)
) Pillow lavas form when lava__________________________ and tend to
have_______________________ that minerals can grow in. See if you can fill this in.
) Plate tectonics has moved things around. Continents have drifted. Continents have been torn
asunder (apart by rifting). Think about it. Could gems prospecting and an understanding of
plate tectonics have a potential to help/explain each other.
) What about volcanoes and plate tectonics? Where is “The Ring of Fire?” Could volcanoes
help create gemstones?
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Look at Figure 6.
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