Rock Identification with Willsey: Intro to Igneous Rocks!
Summary
TLDRThis educational video delves into the classification of igneous rocks, focusing on texture and composition as key identifiers. The host explains that texture, determined by crystal size, shape, and arrangement, reveals cooling rates and formation environments. Composition is largely influenced by silica content, with felsic, intermediate, and mafic categories corresponding to light, medium, and dark rock colors. The video simplifies complex geological concepts, making them accessible to viewers, and hints at future detailed explorations of specific rock types.
Takeaways
- 📚 The video is an educational introduction to igneous rocks, focusing on their textures and compositions as key classification criteria.
- 🔍 Texture is defined by the size, shape, and arrangement of mineral crystals within the rock, which can indicate the cooling rate of the magma.
- 🔬 Composition is primarily determined by the mineral content of the rock, reflecting the chemistry of the original magma or lava.
- 🏔 Phaneritic texture is characterized by large, interlocking crystals, suggesting slow cooling of magma deep underground.
- 🌋 Aphanitic texture features very small crystals, typically less than a millimeter, indicating rapid cooling of lava at the Earth's surface.
- 🌈 Porphyritic texture is a combination of large and small crystals, implying different cooling rates within the magma body.
- 🌿 Pegmatitic texture is similar to phaneritic but with exceptionally large crystals, greater than a centimeter.
- 🌑 Mafic rocks are rich in dark minerals like olivine and pyroxene, and are generally darker in color due to their lower silica content.
- ⛰️ Felsic rocks are high in silica and light-colored minerals like quartz and potassium feldspar, resulting in lighter-colored rocks.
- 🌐 Intermediate rocks contain a mix of felsic and mafic minerals, often appearing as a medium gray and representing a balance in composition.
Q & A
What are the two primary criteria used to classify igneous rocks?
-The two primary criteria used to classify igneous rocks are texture and composition.
What does the texture of an igneous rock reveal about its formation?
-The texture of an igneous rock reveals information about the cooling rate of the magma and the environment in which it formed, whether it cooled slowly underground or quickly at the Earth's surface.
What is meant by the term 'phaneritic' in the context of igneous rocks?
-Phaneritic refers to an igneous rock texture characterized by large, visible crystals that are interlocking, indicating a slow cooling process.
How does the size of mineral crystals in an igneous rock relate to its cooling rate?
-Larger mineral crystals in an igneous rock suggest a slow cooling rate, allowing more time for crystal growth, while smaller crystals indicate rapid cooling.
What is the term for an igneous rock texture with exceptionally large crystals?
-The term for an igneous rock texture with exceptionally large crystals, greater than a centimeter, is pegmatitic.
What is the difference between aphanitic and porphyritic textures in igneous rocks?
-Aphanitic textures have very small crystals, generally less than a millimeter, resulting from rapid cooling, while porphyritic textures contain a mix of large and small crystals, indicating different cooling rates.
Why are vesicular textures found in some igneous rocks?
-Vesicular textures are found in igneous rocks because of the presence of gas bubbles, or vesicles, which form when gases escape from lava as it cools.
What does the composition of an igneous rock tell us about its origin?
-The composition of an igneous rock, which is largely dictated by silica content, can indicate the type of magma from which it originated and the conditions under which it formed.
How can the color of an igneous rock be used to estimate its composition?
-Light-colored igneous rocks are typically felsic, with a high silica content, while darker rocks are usually mafic, with less silica. However, color can be misleading, as in the case of obsidian, which is dark but felsic.
What is the significance of the term 'intermediate' in classifying igneous rocks?
-The term 'intermediate' refers to igneous rocks that have a composition between felsic and mafic, containing a roughly equal mixture of light and dark minerals.
Outlines
🌋 Introduction to Igneous Rocks
The speaker begins by introducing a video series on igneous rocks at the College of Southern Idaho, focusing on the geology classroom. The video aims to provide an overview of igneous rocks, emphasizing the importance of understanding their texture and composition for identification. The speaker clarifies that the class is not an advanced igneous petrology class, so the discussion will be more general, suitable for a 'Geology 101' level. The main criteria for classifying igneous rocks are texture, which refers to the size, shape, and arrangement of minerals, and composition, which is determined by the minerals present. The speaker mentions that while there are many types of igneous rocks, the video will stick to a basic classification. The goal is not just to name the rocks but also to understand their story, including the environment and conditions under which they formed.
🔍 Understanding Igneous Rock Textures
The speaker delves into the concept of texture in igneous rocks, explaining that it refers to the size, shape, and arrangement of mineral crystals within the rock. Texture is crucial as it indicates the cooling rate of the magma or lava, which in turn suggests whether the rock formed deep underground or at the Earth's surface. The speaker introduces the term 'phaneritic' to describe rocks with large, visible crystals that interlock like a mosaic, indicating a slow cooling process. 'Pegmatitic' texture is also mentioned, characterized by exceptionally large crystals, greater than a centimeter. The speaker uses visual examples to demonstrate these textures, comparing them to a pencil tip for scale and noting how the crystals' visibility and interlocking arrangement can be assessed. The discussion also touches on how the presence of water and fluids can affect texture.
🌀 Rapid Cooling and Aphanitic Textures
This section discusses how rapidly cooled lava results in small crystals, generally less than a millimeter, characteristic of 'aphanitic' textures. The speaker explains that the small size of the crystals indicates a fast cooling process, often associated with volcanic eruptions. Examples of aphanitic rocks are shown, with the speaker pointing out the fine-grained nature of these rocks and how they contrast with the larger crystals seen in phaneritic textures. The speaker also notes that the cooling rate inferred from crystal size can help determine the rock's formation environment, whether it be under the Earth's surface or at a volcanic surface.
🌈 Porphyritic and Vesicular Textures
The speaker introduces 'porphyritic' texture, which is a combination of large and small crystals, suggesting different cooling rates within the magma. This texture is common in volcanic environments and can be associated with economic geology due to the presence of ore bodies. Examples of porphyritic rocks are shown, with the speaker highlighting the presence of large 'phenocrysts' within a finer-grained matrix. 'Vesicular' texture is also discussed, characterized by the presence of gas bubbles, or vesicles, within the rock. The speaker shows examples of vesicular rocks, including a piece of pumice, and explains how the vesicles can provide insight into the volcanic origin of these rocks.
💠 Glassy and Pyroclastic Textures
The speaker describes 'glassy' textures, which are shiny and resemble glass, formed due to extremely rapid cooling where no minerals have time to form. 'Obsidian' is given as an example of a glassy rock. 'Pyroclastic' texture is also introduced, which refers to rocks made from volcanic ash and other ejected materials from a volcano. The speaker notes that pyroclastic rocks can contain fragments of various sizes, from fine ash to larger torn-up pieces of pumice. The discussion highlights the diversity of textures found in igneous rocks and how they relate to the rock's formation process.
🏔 Composition of Igneous Rocks
The focus shifts to the composition of igneous rocks, which is primarily determined by the amount of silica present. The speaker explains that felsic rocks, which are high in silica, contain light-colored minerals like quartz and feldspar, while mafic rocks, lower in silica, contain darker minerals like Olivine and pyroxene. The speaker cautions that color is not always a reliable indicator of composition, using obsidian as an example of a dark-colored rock that is actually felsic. A diagram is referenced to help viewers understand which minerals are commonly associated with each other, aiding in the determination of a rock's composition. The speaker concludes by organizing rocks by their compositional categories: felsic, intermediate, and mafic, providing a visual summary of the different rock types discussed.
📚 Wrapping Up Igneous Rock Classification
In the final part, the speaker summarizes the key points about igneous rock textures and compositions. The importance of practice in identifying textures and compositions is emphasized, with a reminder to be cautious when using color as an indicator of composition. The speaker outlines future videos that will delve into specific types of igneous rocks, promising detailed exploration and examples. The video concludes with a reinforcement of the importance of understanding both the texture and composition of igneous rocks for proper classification and storytelling of their geological history.
Mindmap
Keywords
💡Igneous Rocks
💡Texture
💡Phaneritic
💡Aphanitic
💡Porphyritic
💡Vesicular
💡Composition
💡Felsic
💡Mafic
💡Intermediate
Highlights
Introduction to igneous rocks and the importance of understanding their texture and composition for classification.
Emphasis on the non-detailed approach suitable for a geology 101 level, focusing on basic classification.
Explanation of the texture of igneous rocks, including the size, shape, and arrangement of minerals.
Discussion on how the texture of igneous rocks can indicate the cooling rate of magma and the environment of rock formation.
Introduction to phaneritic texture, characterized by large, interlocking crystals indicating slow cooling processes.
Description of pegmatitic texture with exceptionally large crystals, greater than a centimeter.
Aphanitic texture is highlighted, featuring very small crystals resulting from rapid cooling of lava.
Porphyritic texture is explained as a blend of large and small crystals, indicating different cooling rates.
Vesicular texture is characterized by the presence of gas bubbles, or vesicles, in the rock.
Glassy texture is described, where rocks have a shiny, glass-like appearance due to very rapid cooling.
Pyroclastic texture is introduced, relating to rocks made primarily from volcanic ash.
The composition of igneous rocks is discussed, with a focus on silica content and its relation to rock color.
Felsic rocks are described as high in silica, light in color, and typically formed from cooler magmas.
Mafic rocks are characterized as low in silica, dark in color, and derived from hotter magmas.
Intermediate rocks contain a mix of felsic and mafic minerals, often resulting in a medium gray color.
A cautionary note on using color alone to determine rock composition, with obsidian as an example of an exception.
Advice on practicing the identification of igneous rock textures and compositions for better classification.
Transcripts
all right friends we are back at the
College of Southern Idaho Evergreen
building and the geology classroom and
this is going to be our first uh video
on the igneous rocks and we're going to
start with is a overview of the igneous
rocks because to identify igneous rocks
you really need to have a good handle on
um the texture of the Rocks which we'll
look at and the composition because
that's our primary two criteria that we
use to classify igneous rocks this is
not an igneous petrology class and so we
are not going to go super detailed you
can get deep in the weeds there's
literally dozens and dozens of different
igneous rock types we're going to stick
with a more geology 101 generic
classification but I think that'll help
most of you with maybe not only
determining your rock names to some
degree but more importantly at least for
me is figuring out the story of The Rock
and figuring out how the rock came to be
what sort of environment it formed in
what were the conditions because me to
me that's much more valuable than just
knowing the rock name but nonetheless
we'll try to tackle both of those as
best we can so let's just Dive Right In
um some notes I have here yeah not
igneous petrology class that would be a
class you would take
maybe at a 300 level or maybe almost a
graduate level if you were a geology
major we're going to mainly stick with
these two things because igneous rocks
their story and their names can be
largely derived from their textures and
their compositions so when I say texture
I'm not talking about how the rock feels
in your hand i'm talking about the size
shape and arrangement of the minerals so
how big are those mineral crystals
um how are they arranged what sort of
fabric is in there and then when we look
at composition we're mainly looking at
what minerals are in the Rock themselves
and this is a reflection of the
chemistry of the original magma or the
lava that produce the igneous rock so
we're looking specifically at the
minerals that make up the Rocks but
we'll see that there's some there's some
fairly simple ways that aren't always a
slam dunk but good ways of determining
mineral composition might be things like
looking at the overall color of the rock
so what we're avoiding but you can dig
into these if you want to on the
internet so if you want a deeper dive
into igneous rocks we mainly determine
igneous rocks in terms of their mineral
composition we use these fancy diagrams
that look like pyramids called ternary
diagrams to figure out the exact name of
a rock and you'll see lots of numbers on
here these are actually percentages so
this would be a hundred percent quartz P
is for plagioclase a is for The Alkali
Phelps bars so like the potassium Felts
bars and the F's are from these other
group of mineral
um other groups of minerals like
nephilene and what are called sometimes
the the foids or the feldspathoid
minerals we're not going to dive into
this we are not going to cover Cyanide
and quartz monzonite and monzo granite
and and everything you're seeing here
I'm not going to go to that kind of
level of detail because to determine
these percentages what you have to do is
actually figure out what percentage you
have of each mineral in the Rock and for
the most part that's hard to do with
just a simple sample you have in your
hand it takes more of an in-depth
analysis microscopes that sort of thing
but realize these things are out there
on the internet so if you want to know
what a percentage of minerals you have
in your rock and you're pretty good at
identifying minerals you might look for
a mineral composition diagram like this
that kind of shows you roughly what 20
percent of of the volume of the rock
would look like 30 for 50 and then you
can look for these ternary diagrams too
if you want to Deep dive so we'll get
rid of that because we're not going to
look at that much so what are we going
to look at we're going to start with
textures so again this is the size shape
arrangement of the minerals the textures
are important because they tell us a lot
about the cooling rate of the magma they
tell us whether the magma superficially
tells us whether the magma cooled slow
fully underground maybe miles beneath
your surface
or if that magma erupted from a volcano
so it's lava now and cooled more quickly
at the Earth's surface so the way the
minerals are kind of put together and
their sizes is very instructive and
we're going to use some fancy terms here
but these are terms that if you're real
serious about this
would be great for you to kind of get in
your Lexicon and try to learn these as
best you can ultimately though you could
you know you could pass on the fancy
names as long as you know what to look
for and what these mean so let's start
with the first one here so this is
pronounced phaneritic so a phaneritic
igneous rock texture has big crystals
when we say big you know we need to
define the size and so let's say for for
simplicity's sake that anything bigger
than a millimeter so basically if you
can see visible minerals in the rock
crystals and they're interlocking so
this is two important considerations
here they've got to be big enough to see
but they're all squished together
they're sharing boundaries together
they're squished together like a mosaic
what this means is that this is a slow
cooling process so basically is the
magma is cooling slowly to give us this
phaneritic texture
the elements have more time because of
the slow cooling to nuclear 38 and form
larger and larger crystals so slow
Cooling in magma
means that we're going to have bigger
crystals in the Rock typically and
that's an important thing to make sure
you understand because that governs a
lot of what we're doing there are some
other factors that that play a role as
well like the presence of water and
fluids but we're we're going to again
stick to the basics sometimes you see
this term and I might have used it in
one of the prass videos a pigmentite or
pigmentidic Texture would be
a fanorated texture but instead of just
being large enough you know just over a
millimeter these are exceptionally large
crystals so this would be something
where we have greater than maybe a
centimeter or so so let's look at both
of those there I've kind of laid out a
couple of these here let's start with
just a nice phaneritic texture we've
maybe seen this rock before so we could
start with this one here but we can see
you got my finger in there for scale or
what else could we use here that's kind
of Handy oh here we go this is even
better we've got a well it's hard to see
maybe through it let's just use a pencil
tip I think that will work good for sort
of a scale
um but we can see we've got big crystals
in here we've got some pink uh potassium
feldspar there's some black shiny
biotite there's some kind of uh smoky
gray somewhat translucent Quartz in here
a little bit of plagioclaves which is
the white stuff but the point is all
these crystals are big enough to see you
can see their size relative to my
mechanical pencil here they're all
greater than a millimeter and they're
all interlocking they're all sharing uh
borders and boundaries and they're all
sort of squished together so that's a
nice classic granite and we'll get to
the Rock names later in future videos
we'll go through each maybe not each
we'll bundle them up together but we'll
go through several different rock types
so there's a nice phaneritic rock here's
another one
a little scale here with the the pencils
you can see these big crystals all sort
of stuck together uh here this this one
gets a little tricky we can see these
two lighter ones have some dark minerals
and so the contrast between the minerals
and the size of the crystals is pretty
easy to ascertain but you get to a rock
like this where it's very dark and it's
hard to see the individual crystals but
when I rotate this in the light you can
see some little flashes of light there's
a really nice one if you watch right
here as I rotate that in the light you
can see that little surface reflecting
the light and remember when we talked
about minerals and mineral properties
that one of the properties of a mineral
is cleavage planes so basically by
rotating this rock in the light we can
actually see the size of the crystals by
looking at the surfaces reflecting in
the light and you can see that these
surfaces here right by my finger here
are definitely more than a millimeter in
size and they're all squished together
it looks like they're interlocking so
this would also be a pheneretic texture
um much like this one but obviously the
composition is different right very
different color different mineral
distribution uh there and then we have
one here a similar sort of thing mostly
just kind of black and white but we can
see the crystals are all kind of
squished together and sharing borders
and spaces together and then for
pegmatidic we can see these crystals are
much bigger we can see there's some
flaky muscovite in here that's what's
kind of this shiny material here these
black minerals this is actually
tourmaline so we have some larger
tourmaline crystals and then big
sections of quartz in here there's a
nice muscovite kind of shining right
there as well a little muscovite Crystal
but you can see the crystals here are
much larger than in this phaneritic rock
here and so this because the crystals
are of a size greater than a centimeter
so we would call that pegmatidic let's
go to our next texture so our next
texture is well what if the crystals are
really small what if they're generally
all all less than a millimeter or so
well this is what results from very
rapid cooling of lava so this would be a
rock that erupted at the surface came
out of a volcano the lava cooled very
quickly the minerals and elements did
not have time to get together and make
bigger crystals and so we subsequently
get very rapid cooling and smaller
crystals so notice that the first thing
we've learned here is just by looking at
the size of the crystals you can infer
the relative cooling rates and therefore
the environment whether it's under the
Earth in a magma chamber cooling slowly
or erupted from a volcano
so what do our aphanitic rocks look like
well here's one here again for scale so
we can see some mineral specks in here
right we can see hopefully this will
focus for us
can I zoom in
maybe maybe not there we go that's not
bad
um so notice how much bigger the pencil
tip is now so these little black specks
and even these white ones here that'll
kind of squares thing these are crystals
but they're much much smaller if we Zoom
back out we can see some of the sizes of
these things so this would be more of an
aphonetic texture there might be a
couple crystals that exceed a millimeter
but by and large most of the crystals
that make up this rock are in the small
range
another one I have here that can kind of
prop it up so it's oriented well would
be this darker one here we might even
zoom in
yeah and so again just really really
fine-grained right you might see a few
little flashes of reflection those
crystals are much much smaller though so
those are our athanetics we have
fanoritic and athenitic let's look at
some variations now so our next one is
what we call porphyritic or sometimes
you've maybe seen rocks called
porpherease These are actually pretty
common uh in some Mining and economic
geology applications because they
oftentimes have ore bodies in them so
porphyritic is actually going to kind of
be a blend of these two we get big and
little crystals and what that implies is
that there's different cooling rates
happening so remember our big crystals
are slow Cooling and our small crystals
are fast cooling so what we might expect
then in a porphyritic rock is a magma
body that starts to cool slowly creating
these big crystals but then ultimately
it gets pushed towards the surface the
magma erupts the the big crystals that
have already formed get carried with the
magma up to the surface where the rest
of the molten material cools quickly and
gives us these small crystals so here's
some examples of some perforated rocks
um
and these are quite common in a lot of
volcanic environments so we can see this
has big crystals these big white
crystals are quite large
but they are let's see if it'll focus
here
um they are sort of embedded in this
much finer grained material so
everything's sort of surrounding it is
much more fine-grained
a little bit over here
there we go okay so this would be a
porphyritic rock here I've got another
one right here
again we've got some of these big white
crystals but there's also some you know
they're not nearly as big but there's
also some of these black crystals in
here as well so these are both
porphyritic here's a little bit
different one here where the black
crystals are
um the big crystals in here we sometimes
call these big crystals pheno crisps so
we need a fun word for the day you could
do that one p-h-e-n-o
c-r-y-s-t so this has big phenocris of
amphibole these big black crystals but
everything else in here if you look at
the area between the crystals it's very
fine grained so it's not phaneritic
because we don't have all big crystals
squished together we have big crystals
floating around in this more small or
finely crystalline Matrix and here's one
last one here
where we have these uh
sorry working on the camera here here we
go uh we have these clusters of white
crystals that are much larger and then
all the black stuff's incredibly
fine-grained we can't even see how small
some of this is we also have some
bubbles in here which we'll talk to
let's talk about so that's our
porphyritic texture
um talking about uh vesicular so the gas
bubbles we see in igneous rocks are
called vesicles
um and so a rock that has a lot of these
vesicles is it has a texture that we
might call vesicular so a couple
examples of that might be this rock here
kind of your typical what most people
would call lava rock kind of dark
colored but it has a lot of these holes
in it another example of a rock I would
consider to be vesicular has a lot of
holes in it it also has
um other another texture in here so it's
possible to have more than one texture
we can see some really nice green
Olivine crystals in here so you might
call this porphyritic or slightly
porphyritic because it does have some
bigger crystals floating around in this
dark gray material here and just so we
can see that they the Rocks do not have
to be dark or basaltic here is a highly
vesicular rock this is a piece of pumice
which we'll spend some more time with if
we kind of turn it on end we can kind of
zoom in here and see all the holes in
this rock the holes are a little bit
different though they're actually kind
of stretched out it's almost more like
meringue or froth if you will and when
you pick this rock up it's incredibly
light but you can see a lot of the kind
of stretched holes and bubbles or
vesicles in this rock as well so a
couple different vesicular rocks
uh one texture we have is glassy
um what can I say about glassy it looks
like glass it's shiny glassy rocks tend
to form primarily due to very very rapid
cooling and our true glassy rocks that
will spend some time with are the
obsidians and the obsidian rocks
um basically cool so quickly that no
minerals actually form they're actually
unordered elements we'll spend we'll
spend time more on these later but this
is an example of a glassy texture and
then our last texture here is
pyroclastic if you tear apart this word
pyro means fire like a pyromaniac or a
pyrotechnic show clasp means Rock So
this literally translates to fire rock
so one of the things that uh the
volcanoes produce oftentimes is when the
lava gets exploded into shrapnel tiny
tiny shards of torn lava and crystals we
call that Ash the size of the material
Ash so if we have an igneous rock that's
pretty much mostly made out of Ash then
the textural term we use there is
pyroclastic and a couple of pyroclastic
rocks so if we kind of look at this one
here
we can see there's some like fragments
of rocks in it but as I kind of hold
this and move it between my hands you
can see it's kind of and they don't
always do this this one's actually
leaving a little powdery residue on my
fingers because that is all fine Ash
volcanic ash
another one here you might see a few
crystals in there there's a nice Crystal
right there so crystals can be in these
pyroclastic rocks
um there's some actually some pieces of
torn up pumice in here as well and
here's another one here but mostly what
this ash is comprised of is Ash so
that's a good example of a pyroclastic
texture okay so a lot to digest there
but maybe practice your textural terms
with the Rocks you have you could even
practice this with uh with Foods right
if I go to Dairy Queen and get a
blizzard that's a great example well the
blizzard I get is an Oreo blizzard so
I've got my big Oreo chunks in my
chocolate ice cream that's a great
example of a perforated texture the big
Oreo chunks are like the large crystals
and the finely the fine ice cream
materials kind of like the small
crystals so you could maybe apply these
to other things so some rocks might give
us more than one texture and that's okay
but we want to classify or try to name
the Rock based on the most dominant one
so as much as you can look at your rock
figure out which one of these six terms
I suppose best fits the rock overall and
then stick with that as you go to try to
classify it okay so that's texture
now let's look at composition so
composition of igneous rocks is largely
dictated by how much silica is in the
Rock and silica is basically quartz or
chemically it's the same as quartz it's
the most common
compound in the Earth's crust but we're
going to be able to figure out how much
silica The Rock has somewhat by the
mineral composition you can also crudely
use the color of the rock to indicate
the composition as well so the terms
we're going to use here to describe
composition also apply to magma type so
you've probably seen some of these
before they actually also apply to some
minerals as well but the three big
groups we have are felsic mafic and
intermediate so felsic magmas
are magmas that um have a lot of silica
greater than 65 percent just know that
it's a Hyatt it's a high silica magma it
tends to have mostly the felsic minerals
so it's going to have quartz potassium
Fells bar plagioclase feldspar muscovite
all of these minerals are fairly light
in color so if you have a magma that
dominantly has these minerals in it when
it cools and crystallizes and these
minerals are all dominantly light in
color then the subsequent Rock should be
fairly light in color so this rock
granite is mostly made out of felsic
minerals are there mafic minerals in
here dark minerals yeah but they
represent a much smaller percentage or
portion compared to the felsic minerals
that make up most of the rock okay if we
look at mafic which is at the opposite
end of the spectrum these are fairly low
in silica and they contain mostly mafic
minerals so the dark minerals and if you
need a review of these minerals remember
I did a video on all all of these eight
minerals so if you are looking at this
and it's a little confusing I'd
encourage you to go back back and look
at my mineral Series where we cover
these in much greater detail so Olivine
which is kind of that olive green color
pyroxine amphobiotite these are all
blackish or dark green minerals and so
what will a mafic rock look like that
has mostly mafic minerals that came from
a mafic magma will be quite a bit darker
might look like this might look
something like this
might even look like this or that these
are all mafic rocks and so generally you
can use the color to determine the
composition but but color is not perfect
and a good example of that is our good
friend obsidian which is as black as
they come in terms of color so one would
tend to think right off the bat that
this is a mafic rock when in fact this
is actually a felsic rock this comes
from felsic magmas High silica magmas
and volcanoes and it's only black
because of the little bit of iron and
period impurities that it contains
within it and so it tends to be this
darker color but this is actually a
felsic rock by composition by by
chemically even though the color doesn't
match up but generally as you kind of
scan these rocks you can generally use
the color to some degree to figure it
out another handy way to figure out
composition and you can find something
like this on the internet as well or
I'll put these handouts I'll put a link
to these handouts on this YouTube page
as well
as you can look at this graph of mineral
types and get a rough sense of what it
likes to hang out with so for example if
you know your rock has quartz and you
undeniably are sure that quartz is in
your rock well you might use a diagram
like this to figure out what other
minerals are in it by looking to see who
quartz likes to hang out with what who
are his buddies or her buddies what are
the other minerals that associate
themselves with quartz and what we see
is orthoclase which is potassium
feldspar plagioclase and plagioclase
actually hangs out with all sorts of
minerals the micas like muscovite and
biotite and then amphibole but notice
that quartz and pyroxene
or Olivine do not associate together you
do not see these minerals in the same
Rock generally
um and so this is somewhat helpful so if
you can identify one mineral that might
help you figure out this whole felsic
intermediate and mayfit game we're not
going to deal with ultramafic rocks
which are much more rare in general the
felsic rocks are derived from magmas or
lavas that are lower in temperature so
they are not as hot as the mafic ones
which are much higher in temperature
and then I love this little this shaded
bar up here notice that there's a
gradation here from White through
different shades of light gray to kind
of medium gray to kind of dark to Black
this is kind of a a rough good guide
that will generally help you so if you
see that your rock is kind of a a light
gray color that's suggestive that it's a
felsic rock but you would want to make
sure and back up that interpretation by
coming back to maybe a diagram like this
and seeing if you can identify by some
minerals that are in a felsic rock right
so if I think that rock Scott is felsic
and I see Quartz in it then I feel even
better about it right but if I have a
a dark colored Rock and I'm sure it has
Quartz in it well something's off there
right because quartz is oftentimes not
found in these mafic or very dark
colored rocks okay so you could look for
a diagram like this or I'll put this one
up I didn't make this one I just kind of
grab that off the internet so
um
okay so let's uh quickly go through the
I'm going to reorganize these rocks into
compositional equivalents so let's look
at all the dark colored rocks we've
organized them by texture but now I'm
going to put all the felsic rocks
together and all the e and all the mafic
Rocks together and all the intermediate
rocks together which I probably need to
explain a little bit better so let's see
that's pretty good there I'm Gonna Leave
obsidian out
sorry Chuck just for now so all of these
rocks here are felsic I've got some that
are pyroclastic made out of Ash some
that have lots of bubbles in them but
some crystals as well this pumice some
that are aphonitic some that are
phaneritic pegmatidic there's another
one that's pyroclastic but
compositionally these are all the same
if you grind up these rocks and present
them to a chemist they basically have
the same chemistry these are all felsic
Rocks coming from a magma body either
underground or erupted from a volcano
the only difference here is the the
texture right this how the crystals are
arranged or the size of the crystals
okay if we swing over to here we can see
a diversity of mafic rocks we've got a
couple that are vesicular they have the
holes in them we've got one here with
bigger crystals that's phaneritic we've
got one here that actually has two
Textures in it we can see the big
crystals in it sorry just trying to help
get it there we go we can see the big
white crystals in it but you can also
see the gas bubbles the little holes in
it so this would be vesicular and
porphyritic
um but nonetheless it's the same Rock
and then we have one here that's that's
a phonetic right very small uh
fine-grained uh crystals throughout
so briefly on the intermediate rocks and
then we'll we'll we'll call it a day so
the intermediate rocks or the
intermediate compositions are going to
be in between in terms of silica content
that should make sense and more or less
they contain about a 50 50 mixture of
felsic and mafic minerals so they kind
of run right down the right down the
middle here and this is a crude
classification right we just classify
them into broadly three categories so
amphibol is a dominant mineral
plagioclase which is white is dominant
but you don't get much potassium
feldspar you oftentimes don't get much
quartz you don't get at all any Olivine
so you can kind of see the distribution
of and you can kind of see that overall
it might give you kind of a medium gray
it says purple here but gray is probably
the the more common color here overall
kind of medium gray shade there and so
these are all intermediate rocks this
one is beautiful because it shows us a
fanoritic texture
but also know oats he's cooperate I'll
just stick you in this little seam there
there we go also note that this rock has
kind of a nice
um diversity of white and black crystals
it looks like salt and pepper right like
there there's just as many felsic or
light-colored minerals as there is dark
mafic minerals so this is a great
example of an intermediate texture these
rocks here which are all porphyritic
um
are kind of like a medium gray the shade
of gray changes a little bit but if you
sort of take all these big crystals out
of them they're kind of in the same
ballpark in terms of the the shade of
gray that they are so these would be
intermediate rocks
um so hopefully that's helpful a lot to
digest hopefully you took notes there
may or may not be a quiz later and then
moving forward these are the Rocks I
plan to cover I don't know that I'm
gonna do I probably will not do a video
on each one
some like Basalt I will give them their
own video because they've got so much
diversity I may package a few together
but in the future look for these rocks
to be covered in some detail showing
lots of examples explaining some of the
variations we see therein so hopefully
this was helpful texture composition
those are your those are the buzzwords
for today that's what I want you guys to
focus on is practice identifying
your igneous rock textures
um and then try to also determine
composition composition can be based on
color to a degree but we want to be a
little bit careful because we know some
rocks like obsidian can burn us a little
bit if we're if we're too casual with
using color as a guide we want to use
mineral composition more than color if
we can so for now hopefully that's
helpful that's another video this one on
igneous rocks sort of your primer on
igneous rocks and their textures and how
to distinguish one from another hope
these are helpful and we'll see you next
time thanks friends
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