Download Igneous Rocks - Red Hook Central Schools

Rocks
are made of a
mixture of minerals
Monomineralic describes a rock
made of only one type of mineral
There are 3 types of rocks
Igneous, Sedimentary, and Metamorphic
How do you think we assign rocks to each group?
Mineral composition?
Depth found in the earth?
How the rock formed?
Other?
Igneous Rocks
Igneous means fire
Molten (liquid) rock beneath the Earth’s surface is
called MAGMA
When magma erupts onto the surface of the
Earth, it is called LAVA.
Igneous rocks form when magma or lava cools
and solidfies (or crystallizes)
Intrusive/Plutonic
Igneous Rocks
form underground when MAGMA cools and
solidifies
Intrusive rocks cool slowly, which gives the
mineral crystals time to grow LARGE
This texture
is called
Coarse Grained
(plug)
Rock that solidifies
underground, has large
crystals
Granite forms in
this way and makes
up most of the
Earth’s crust
Granite has a coarse
crystal texture
Thin section of granite
Igneous rock is composed of randomly interlocking crystals
The slower the magma cools, the larger the crystals
When the magma solidifies
at the earth’s surface, the
igneous rocks are called
extrusive
or volcanic
Lava that cools at the
Earth’s surface usually
has very tiny crystals,
Why?
Extrusive igneous
rock has no time
for large crystals
to grow
Basalt is an Extrusive rock
This texture is
called
Fine Grained
Most of Earth’s
oceanic crust is basalt
Here, lava is flowing into
the sea, where is solidifies
instantly
The igneous rock obsidian
forms in this way and has
no crystals at all
Obsidian is an extrusive rock with
glassy texture
Similarly rock shot
into the air cools
quickly and also
has no crystals
Pumice is a rock
that forms in this
way
Pumice has a
porous,
or vesicular
texture
Extrusive: cool rapidly, have tiny or no crystals
Texture = glassy, vesicular, fine grained
Intrusive: cool slowly, have large crystals
Texture = coarse grained
Composition of Igneous Rocks
In addition to texture, igneous rocks are
classified by their mineral composition.
Rocks with lots of Aluminum and Silicon are
called “felsic” and tend to have light colors
Rocks with lots of Iron and Magnesium are
called “mafic” and tend to have dark colors
Felsic Rock
Granite is
light colored
= FELSIC
Mafic Rock
Basalt is
dark colored
= MAFIC
How to tell if your rock is IGNEOUS
1. Look for fine or coarse randomly
interlocking mineral crystals
2. Look for glassy texture
3. Look for vesicular texture
Any vertical line
drawn through
the flow chart
is a possible rock
The more of the
mineral the line
cuts through, the
more of that mineral
is in the rock
Which is the
most abundant
mineral in this
rock?
The minerals that
the red line cuts
through make up
the rock
To get the exact name
of the rock we must
look at the size of
It’s crystals (texture)`
If we extend the line
into the upper regions
of the table we can
determine other
properties of the rock
If the rock had a
coarse grain
texture, what
would it be called?
Would this rock be
high or low density?
The name of the rock
is either Pegmatite,
Granite, Ryolite,
Pumice or obsidian
Let’s try another
example
If this rock is fine
grained without
gas pockets,
what is it’s
name?
What minerals are
in this rock?
Would this rock be
MAFIC or FELSIC?
What are the
approximate
percentage of each
mineral?