Chapter 6:
What are igneous rocks?
• Igneous rocks gets its name from the Latin word
for “fire”, because it is formed from hot molten
rock (magma) inside the Earth. When the
molten rock solidifies, it forms solid igneous
rock.
• The way the magma cools determines the kind
of igneous rock that is formed.
• Magma is molten rock, while lava is magma on
the Earth’s surface.
• Igneous rocks may be either extrusive if they
form at the surface (ex. basalt) or intrusive if
magma solidifies underground (ex. granite).
Watch Video
From Minerals to Rocks…
Igneous Rock Textures
• Texture refers to a rock’s appearance with respect to
the size, shape and arrangement of its grains.
• Extrusive rocks are typically fine-grained (grains smaller
than 1mm). The grains are small because the magma
cooled rapidly at the surface, and large crystals did not
have time to form. Some intrusive rocks are fine
grained, but these would have formed near the surface
in “cold rock”.
Extrusive rock
called obsidian.
This is a glasslike rock that
forms at the
surface
Igneous Rock Textures
• Intrusive rocks usually form at considerable depth (more
than a few km) and are called plutonic rocks (after Pluto,
the Roman god of the underworld).
• Intrusive rocks are typically coarse-grained (bigger than
1mm) because of the slow cooling and solidification of
magma
How are igneous rocks classified?
• Igneous rocks names are based on texture
(basically grain-size) and mineral
composition.
• So, each type of igneous rock will have:
1. a specific combination of minerals.
2. coarse or fine grains.
Plutonic rocks are easier to identify
because of the larger crystals.
Common Igneous Rocks
Group 1
Granite
Rhyolite
•Formed inside Earth
•Formed at surface
•Coarse-grained
•Fine-grained
•Quartz crystals visible
•Crystals too small
•Feldspars
•Feldspars
Common Igneous Rocks
Group 2
Diorite
Andesite
•Formed inside Earth
•Formed at surface
•Coarse-grained
•Fine-grained
•No quartz
•Crystals too small to see
•Feldspars; 30-50%
ferromagnesian minerals
•Feldspars; 30-50%
ferromagnesian minerals
Common Igneous Rocks
Group 3
Gabbro
Basalt
•Formed inside Earth
•Formed at surface
•Coarse-grained
•Fine-grained
•Mostly ferromagnesian minerals
•Mostly ferromagnesian minerals
• Some feldspars
• Some feldspars
Identification of Igneous Rocks
Coarse –
Grained
(slow cool)
Fine
Grained
(fast cool)
Mineral
Content
Granite
Diorite
Gabbro
Peridotite
Rhyolite
Andesite
Basalt
--
Quartz &
feldspars
Feldspars &
Mostly
Entirely
ferromagnesian
ferromagnesian ferromagnesian
minerals; no quartz minerals
minerals
Colour
LightMedium grey or
coloured med.-green
(white,
light grey
or pink)
Dark grey
to black
Green to black
Occurrences of Igneous Rock
Rock Type
Occurence
Occurrence
Basalt & gabbro Oceanic crust at
divergent boundaries
Andesite &
diorite
Granite &
rhyolite
Convergent plate
boundaries (ex.
Volcanoes)
Continental crust at
convergent
boundaries & over
mantle plumes
What happens when magma
cools underground?
• Cooling magma may form intrusive
structures, or bodies of magma that
crystallize/harden after flooding
surrounding rock layers
Examples of Intrusive
Structures
1.Volcanic neck (magma is solidified in the
neck of a volcano)
2.Dike & Sills (magma solidifies into a wall
after being squeezed through two rock
layers)
Ex. Ship Rock in New Mexico, USA
Shiprock, New Mexico
Volcanic neck
Shiprock Video (4:20 mark)
Dike
Dikes & Sills
Examples of Intrusive
Structures (cont’d)
• Plutons are blobs of magma that
crystallize within the crust. If these plutons
make it to the surface, large areas of rock
(usually granite) form huge outcrops of
rock at the surface. Batholiths are areas
of plutons greater than 100km2. Batholiths
are present in the Sierra Nevada
Mountains of California and in the
Appalachian Mountains
Halifax Pluton, South Mountain
Batholith, Nova Scotia
Batholith Formation
Examples of Extrusive
Structures
• The most obvious structures made of
extrusive igneous rock (or igneous rock at
the surface) are: