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Formation and characteristics
The term igneous is from the Latin
word “ignis" which means FIRE.
When most people think about igneous
rocks they imagine a volcano erupting
ash and lava. Igneous rocks are
produced this way but most igneous
rocks are produced deep underground
by the cooling and hardening of magma.
Magma is molten (melted) rock under
the surface of the Earth. It is produced
in the upper parts of the mantle or in the
lowest areas of the crust usually at a
depth of 50 to 200 kilometres.
This diagram
shows where
magma is
produced at
a subduction
MAGMA is less dense than the surrounding rock
which causes it to rise.
When magma reaches the surface it is then called
LAVA and the eruptions of lava and ash produce
When lava reaches the surface of the Earth through
volcanoes or through great fissures the rocks that are
formed from the lava cooling and hardening are called
Some of the more common types of extrusive igneous
rocks are lava rocks, cinders, pumice, obsidian, and
volcanic ash and dust.
This diagram shows a
large intrusive igneous
body called a batholith.
A batholith is the largest
of the intrusive bodies.
They are larger than 100
square kilometres and
usually form granite
• Millions and even billions of years ago molten rock
was cooling and thus hardening into igneous rocks
deep under the surface of the Earth.
• These rocks are now visible because mountain
building has thrust them upward and erosion has
removed the softer rocks exposing the much
harder igneous rocks.
• These are called INTRUSIVE or PLUTONIC
IGNEOUS ROCKS because the magma has intruded
into pre-existing rock layers. Types of intrusive
igneous rocks are granite and basalt.
The composition of igneous rocks falls into
categories determined by the amount of
silica (SiO2) that the rocks contain – the less
silica, the darker the rock. The four main
categories are acidic, intermediate, basic
and ultrabasic.
Acidic rocks have a high silica content (65% or
more) along with a relatively high amount of
sodium and potassium.
These rocks are composed of the minerals
quartz and feldspar.
RHYOLITE and GRANITE are the two most
common types of acidic rock.
The composition of igneous rocks
falls into categories determined by
the amount of silica (SiO2) that the
rocks contain. The four main
categories are acidic, intermediate,
basic and ultrabasic.
• Intermediate rocks contain between 53% and 65%
• They also contain potassium and feldspar with a small
amount of quartz.
• DIORITE and ANDESITE are the two most common
types of intermediate rock.
The composition of igneous rocks
falls into categories determined by
the amount of silica (SiO2) that the
rocks contain. The four main
categories are acidic, intermediate,
basic and ultrabasic.
• Basic rocks are composed of less than 52% silica and
a large amount of feldspar and very rarely quartz.
• The two most common types of basic rocks are
The composition of igneous rocks falls into
categories determined by the amount of
silica (SiO2) that the rocks contain. The
four main categories are acidic,
intermediate, basic and ultrabasic.
• Ultrabasic rocks are composed of less than 45% silica and
contain no quartz or feldspar.
• They are composed mainly of a dense iron and magnesia
mineral called olivine and the mineral pyroxene.
• The most common ultrabasic rock is PERIDOTITE. It is a dark
green, coarse-grained igneous rock that many scientists
believe is the main rock of the mantle.
The most
igneous rock
Basalts are dark coloured, fine-grained extrusive rock.
The mineral grains are so fine that they are
impossible to distinguish with the naked eye or even a
magnifying glass.
They are the most widespread of all the igneous
Most basalts are volcanic in origin and were formed
by the rapid cooling and hardening of the lava flows.
Some basalts are intrusive having cooled inside the
Earth's interior.
This is a vertical columnar basalt
When basaltic lava cools it often forms
hexagonal (six sided) columns.
Some famous examples of columnar basalt
formations are the Giant's Causeway in
Northern Ireland (bottom left) and the
Devil’s Postpile National Monument in
California (bottom right).
Giant’s Causeway
Scoria is a type of basalt that's full of bubble
holes. The bubbles formed as the lava was blasted
out of a volcano, and were trapped as the lava cooled
and hardened.
The bubble holes are often uniform in size and shape.
Despite all the holes, scoria doesn't float in water.
Scoria can be black, dark gray, or red.
the rock
that floats
PUMICE is a very light coloured, frothy volcanic rock.
Pumice is formed from lava that is full of gas. The
lava is ejected and shot through the air during an
eruption. As the lava hurtles through the air it cools
and the gases escape leaving the rock full of holes.
Pumice is so light that it actually floats on water.
Huge pumice blocks have been seen floating on the
ocean after large eruptions. Some lava blocks are
large enough to carry small animals.
Pumice is ground up and used today in soaps,
abrasive cleaners, and also in polishes.
• RHYOLITE is very closely related to granite. The
difference is rhyolite has much finer crystals.
• These crystals are so small that they can not be seen
by the naked eye.
• Rhyolite is an extrusive igneous rock having cooled
much more rapidly than granite giving it a glassy
• The minerals that make up rhyolite are quartz,
feldspar, mica, and hornblende.
Granite is one of the most common igneous
rocks. Many headstones are made of granite.
You can't scratch granite with a nail or knife.
Some broken surfaces have flat surfaces that
shine in sunlight.
Granite is made mostly of the minerals
feldspar and quartz. (Reddish feldspars give
this granite its colour and break to form flat
surfaces. The quartz crystals may be a semiclear greyish or purplish colour.)
Many granites also contain small crystals of
mica or darker minerals.
• GABBROS are dark-coloured, coarse-grained intrusive
igneous rocks.
• They are very similar to basalts in their mineral
• They are composed mostly of the mineral feldspar
with smaller amounts of pyroxene and olivine.
• OBSIDIAN is a very shiny natural volcanic glass.
When obsidian breaks it fractures with a distinct
smooth shell-like curved fracture. Look at the
fractures in the photo above.
• Obsidian is produced when lava cools very quickly.
• When people make glass they melt silica rocks like
sand and quartz then cool it rapidly by placing it in
water. Obsidian is produced in nature in a similar way.
It is the result of volcanic lava coming in contact with
water - often the lava pours into a lake or ocean and
is cooled so quickly that no crystals can form,
producing a glassy texture in the resulting rock.
• OBSIDIAN is usually black or a very dark green
caused by the iron and magnesium in the rock, but it
can also be found in an almost clear form and can
also contain white 'snowflake' crystal patterns of the
mineral Cristobalite as shown in the photo above.
• Ancient people throughout the world have used
obsidian for arrowheads, knives, spearheads, and
cutting tools of all kinds. Today obsidian is used as a
scalpel by doctors in very sensitive eye operations.
- review
IGNEOUS rocks are formed from the
cooling and consolidation of magma
• PLUTONIC (intrusive) – cooled below the
• VOLCANIC (extrusive) – cooled on the
- review
IGNEOUS ROCK textures are formed by the
rate of cooling and the chemical composition
of the magma
• GLASSY – no minerals present
e.g. obsidian
• CRYSTALLINE – rocks made of mineral grains
e.g. granite
• VESICULAR – with bubble holes
e.g. pumice, scoria
• PORPHYRITIC – mixture of coarse and fine grains
e.g. basalt
Subduction-related igneous rocks