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Igneous Rock
TAG Earth Science
Introduction Video to “The Rock
Cycle”
Brainpop: Rock Cycle
Igneous Rock
 An
Igneous Rock is an intrusive or
extrusive rock formed when hot
magma cools and hardens.
– Igneous rocks are called fire rocks
and are formed either underground
or above ground.
Where do Igneous Rocks begin???


Igneous Rocks begin as magma.
There are three ways that magma
can form:
1. When rocks are heated
2. When pressure is released
3. When rock changes composition
 When magma cools enough, it
solidifies to form igneous rock.
Magma
 In
certain places within Earth, the
temperature and pressure are just
right for rocks to melt and form
magma.
 Most magmas come from deep below
Earth’s surface.
 When magma reaches Earth’s
surface and flows from volcanoes, it
is called lava.
Intrusive or Extrusive
A
type of Igneous Rock that
generally contains large crystals and
forms when magma cools slowly
beneath the Earth’s Surface is called
an intrusive rock.
 A fine-grained Igneous Rock that
forms when magma cools quickly at
or near Earth’s surface is called an
extrusive rock.
Intrusive vs Extrusive

Intrusive
– Form from magma
– Occurs underneath
ground
– Found at the surface
only after the layers of
rock and soil have been
removed by erosion.
– Slowly cooled magma
produces mineral grains
that are large enough
to be observed by the
eye.

Extrusive
– When lava flows on the
surface it is exposed to
air and water
– Lava then cools very
quickly
– This quick cooling rate
keeps mineral grains
from growing large
– Therefore, extrusive
igneous rocks are finegrained.
How to Classify Igneous Rocks


Igneous Rocks are intrusive or extrusive
depending on how they are formed
An igneous rock can either be mafic
rocks or felsic rocks.
1. Felsic rocks: Light-colored, less dense,
rich in elements such as aluminum,
potassium, silicon, and sodium.
2. Mafic rocks: Dark-colored, more
dense, rich in elements such as
calcium, iron, magnesium, and poor in
silicon.
Felsic Igneous Rocks
 Granite-
rock
Intrusive, felsic igneous
Mafic Igneous Rocks
 Gabbro-
mafic, coarse-grained
igneous rock
Awesome Rock Website
 http://geology.com/rocks/igneous-
rocks.shtml
Classification Continued….
 The
type of magma that cools to
form an igneous rock determines
important chemical and physical
properties of that rock.
 These include mineral composition,
density, color, and melting
temperature.
Fine-Grained Crystals vs. Coursegrained

Fine-Grained:
– Less time for magma to cool
– Less time for crystals to grow
– Contains very small crystals, or if the cooling is
very rapid, it contains no crystals.
Course-Grained:
 Longer time for magma to cool
 More time for crystals to grow
 Contains very large crystals
Igneous Rock
 What
is the most abundant igneous
rock?
 Granite- it is an intrusive igneous
rock