Download Igneous Rocks PPT

Rocks
• Definition of a rock:
– a group of minerals bound together
– makes up a layer of the Earth’s crust
• Lithosphere = thin layer of solid,
brittle rock
• Rocks are classified as:
– Igneous
– Sedimentary
– Metamorphic
The Rock Cycle
Cooling &
Crystallization
Igneous Rocks
• form from the
cooling and
hardening
(crystallization) of
molten rock
– magma: molten rock
existing beneath the
ground = Plutonic
– lava: molten rock
existing on the
Earth’s surface (from
volcanoes) = Volcanic
A. Intrusive (Plutonic) Rocks
• igneous rocks that form from magma that cools
slowly below the Earth’s surface
A. Intrusive (Plutonic) Igneous Rocks...
• Slow cooling at
depth allows
time for
crystals to
grow.
• Visible
interlocking
crystals
• Results in a
coarse-grained
texture
B. Extrusive (Volcanic) Igneous Rocks
• igneous rocks that form at the Earth’s surface after lava has
cooled and hardened : Lava cools quickly!
Lava flow in Hawaii…cools and hardens
quickly into extrusive igneous rock
Extrusive (Volcanic) Rocks…
• Rapid cooling of lava at or near Earth’s surface
does NOT allow time for crystals to grow
(exposure to air and water cools lava quickly)
• Rocks will be Fine-Grained texture, with no
visible crystals
• If cooling is instant – a Glassy texture will form
• No visible crystals
• Gases in lava may leave holes (vesicles) in
extrusive rocks (Scoria or Pumice) resulting in a
Vesicular texture
Extrusive igneous rocks found around this composite volcano
will have a fine-grained texture due to rapid cooling of lava.
Notice the
fine-grained
appearance of
the hardening
and cooling
igneous rock
around this
lava flow. No
visible crystals
are apparent,
and this
material is
very dark,
suggesting a
mafic
composition.
Gases bubbling out of this lava pond will cause air pockets (vesicles) to
form in the cooling igneous rocks. These rocks will then have a
vesicular texture.
II. Texture of Igneous Rocks
• Texture = size, shape,
and arrangement of the
mineral crystals that
make up a rock
Textures:
Intrusive Igneous Rock
Extrusive Igneous Rock
Textures (slow-cooling from Textures (fast-cooling from
magma)
lava)
Coarse-Grained
Very Coarse-Grained
Porphyritic
Fine-Grained
Vesicular
Glassy
1. Extrusive (volcanic) Textures
• Extrusive textures have NO visible crystals.
A. Fine-Grained
• microscopic crystals
that cannot be seen
with the unaided
eye.
• this indicates rapid
cooling at or near
Earth’s surface,
which does not
allow crystals to
grow
Fine Grained Texture…
• This sample of
basalt has a
fine-grained
texture since it
cooled quickly
from lava.
B. Glassy
• Texture with no crystals
at all due to extremely
rapid cooling
• “Pillow lavas” on ocean
floor sometimes have a
glassy texture due to cold
water instantly quenching
the lava
• Obsidian is the most
common glassy textured
Igneous rock
Obsidian results from extremely rapid
cooling at Earth’s surface; it has a
glassy texture and conchoidal fracture
C. Vesicular
• A texture that is
porous with the
holes caused by the
expanding gases
escaping from a
froth-like lava
Vesicular Texture…
• Scoria is a dark
colored igneous
rock that forms
from gases
bubbling through
the lava as it
cooled quickly
• This results in a
vesicular texture
(full of holes)
• Scoria is MAFIC
Vesicular Texture…
• Pumice is a lightcolored igneous
rock that forms
from gases
bubbling through
the lava as it
cooled quickly
• Pumice is
FELSIC
These two samples show a vesicular texture,
where gas bubbles (vesicles) remain in the
sample after cooling
Pumice is a light
colored igneous rock
and contains felsic
minerals
Scoria is a dark colored
igneous rock and
contains mafic minerals
2. Intrusive (plutonic) Textures
• Intrusive (plutonic) Textures display visible
mineral crystals due to slow cooling
D. Coarse-Grained
• large interlocking crystals that can be seen easily with
the unaided eye, from magma, 1 – 10 mm in size!
Indicates slow cooling which allows time for crystals to
grow
This sample of granite has a coarsegrained texture, with large visible crystals
E. Very Coarse-Grained
• Texture with very
large crystals due to
very slow cooling of
magma at depth!
E. Very Coarse-Grained
• extremely large
crystals at least
10mm (1 centimeter)
in size
• Some very coarse
grained igneous rocks
have mineral crystals
up to several feet
large!!!!
• Indicates very slow
cooling of a mineralrich “hydro-thermal”
fluid.
F. Porphyritic
• texture in which large
crystals (phenocrysts)
are surrounded by a
matrix of finer
material. Indicates 2stage cooling, where
large crystals form
first, and the rest cools
rapidly
This sample shows a porphyritic texture,
with very large crystals
• The geometric
shapes are called
“phenocrysts”, and
are embedded in a
brown colored
matrix or
groundmass.
Reading the Reference Tables...
Notice that igneous rocks are first classified by whether they formed
at earth’s surface (Extrusive) or deep underground (Intrusive)
Remember, the resulting texture is glassy, fine grained, or
vesicular for extrusive rocks and coarse grained or very coarsegrained for intrusive rocks
The mineral composition
of each rock can be found
by looking underneath the
rock names in the
corresponding section
below.
These rocks
Contain these
minerals
Another Example:
These rocks
Contain these
minerals
Notice that three other characteristics (color, density, and
elemental composition) appear in the center of the chart.
Light colored rocks appear on the left, dark on the right
Low density rocks appear on the left, high density on the right
Igneous Rock Quiz...
• All igneous rocks form deep underground from
the hardening of magma.
• False! Some extrusive igneous rocks form at or
near the surface from the cooling and hardening
of lava.
Igneous Rock Quiz...
• Igneous rocks that do form underground are
referred to as intrusive or plutonic igneous rocks.
• TRUE.
Igneous Rock Quiz...
• An igneous rock that forms near Earth’s surface
will display a texture where plenty of mineral
crystals can be seen.
• FALSE! Due to rapid cooling, there is no time
for mineral crystals to grow large enough to be
seen.
Igneous Rock Quiz...
• Porphyritic is the type of texture of igneous rocks
characterized by several holes caused by cooling
gaseous lava.
• FALSE! If gas bubbles are found, the sample is
referred to as vesicular.
Igneous Rock Quiz...
• What is the texture of this
igneous rock?
• Aphanitic or fine grained.
You cannot see mineral
crystals.
Igneous Rock Quiz...
• Did this rock form above
ground or under ground?
• UNDERGROUND. You
can see many mineral
crystals.
Igneous Rock Quiz...
• What is the texture of this
igneous rock?
• Coarse-grained
Igneous Rock Quiz...
• The rock below is granite.
What minerals make up a
granite?
Potassium feldspar, Quartz,
Plag. Feldspar, Biotite, some
Amphibole.
Igneous Rock Quiz...
• Once a rock becomes an igneous rock, it may
never change into a sedimentary or metamorphic
rock.
• FALSE! Rocks constantly change into other
forms of rock as part of the rock cycle.
Igneous Rock Quiz...
• The texture of igneous rocks is the size, shape
and arrangement of the minerals that make up the
rock.
• TRUE!!!