Rock Identification with Willsey: Intro to Igneous Rocks!

Summary

This 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

00:00

🌋 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.

05:01

🔍 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.

10:02

🌀 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.

15:03

🌈 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.

20:04

💠 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.

25:05

🏔 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.

Keywords

💡Igneous Rocks

Igneous rocks are formed from the solidification of magma or lava. They are one of the three main rock types, along with sedimentary and metamorphic rocks. In the video, the speaker focuses on igneous rocks, discussing their textures and compositions, which are key to identifying and classifying them. The script mentions that understanding igneous rocks involves looking at their textures and compositions to tell the 'story of the rock' and determine the conditions under which they formed.

💡Texture

In geology, texture refers to the size, shape, and arrangement of the mineral grains within a rock. It is a critical criterion for classifying igneous rocks. The script explains that textures can indicate the cooling rate of magma and the environment in which the rock formed. For instance, phaneritic texture, characterized by large, interlocking crystals, suggests slow cooling, while aphanitic texture, with small crystals, indicates rapid cooling.

💡Phaneritic

Phaneritic is a texture of igneous rocks where the mineral crystals are large enough to be seen with the naked eye (greater than a millimeter) and are interlocking, like a mosaic. This texture results from slow cooling of magma, allowing minerals to grow into larger crystals. The script uses granite as an example of a phaneritic rock, highlighting the visible pink potassium feldspar, black biotite, and smoky gray quartz.

💡Aphanitic

Aphanitic texture is characterized by very fine-grained crystals that are generally less than a millimeter in size, making them difficult to see without magnification. This texture is indicative of rapid cooling of lava, such as when it erupts from a volcano. The script contrasts aphanitic rocks with phaneritic ones, noting the smaller crystal size and the inference of a faster cooling process.

💡Porphyritic

Porphyritic texture is a combination of large and small crystals within the same rock. This texture suggests a complex cooling history, where an initial slow cooling phase forms the large crystals, followed by a faster cooling phase that forms the smaller ones. The script mentions that porphyritic rocks are common in volcanic environments and can be associated with ore bodies, making them of interest in economic geology.

💡Vesicular

Vesicular texture refers to the presence of gas bubbles, or vesicles, within an igneous rock. This texture is typical of rocks that have cooled quickly at the Earth's surface, often after volcanic eruptions. The script gives examples of vesicular rocks, such as certain types of 'lava rock' with visible holes, and pumice, which is very light due to its vesicular nature.

💡Composition

The composition of igneous rocks is determined by the types of minerals present, which in turn reflect the chemistry of the original magma or lava. The script emphasizes that composition, along with texture, is a primary criterion for classifying igneous rocks. The speaker discusses how the mineral composition can be inferred from the rock's color and the presence of certain minerals like quartz, feldspar, and micas.

💡Felsic

Felsic rocks are high in silica content (greater than 65%) and are rich in light-colored minerals such as quartz and feldspar. The script explains that felsic rocks tend to be lighter in color due to the dominance of these minerals. An example given is granite, which is composed mainly of felsic minerals, despite containing some mafic minerals like biotite.

💡Mafic

Mafic rocks are low in silica and high in iron and magnesium content, containing dark-colored minerals like olivine, pyroxene, and amphibole. The script describes mafic rocks as generally darker in color due to the presence of these minerals. However, it also cautions that color is not always a reliable indicator of composition, as exemplified by obsidian, which is dark but felsic.

💡Intermediate

Intermediate rocks have a composition that falls between felsic and mafic, containing roughly equal parts of both light and dark minerals. The script mentions that these rocks are typically medium gray in color and can exhibit a variety of textures. An example provided is an intermediate rock with a porphyritic texture, displaying both large and small crystals.

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