Download Aim: What are igneous rocks? Formation of Igneous Rock

 Volcanoes
erupt and eject a flow of molten
rock material, called magma.
 Magma
flows when it is hot and becomes a
solid when it cools and hardens.
 When
hot magma cools and hardens, it forms
igneous rock.
 Why
do volcanoes erupt?
 Where
does the molten material come from?
 What
comes our of erupting volcanoes?
 Temperature
and pressure in some areas
beneath the Earth’s surface allow for rocks to
melt.
 Temperature
of magma can reach
approximately 650 ˚C to 1200 ̊C.
 Because
the magma is less dense than
surrounding solid rock, it is forced upward
toward the surface.
 When
magma reaches the Earth’s surface and
flows from volcanoes, it is called lava.
 Rocks
that form from magma below the
Earth’s surface are called intrusive igneous
rocks.
 It
takes a long time for intrusive rocks to
cool.
 Intrusive
rocks are found at the surface only
after the layers of rock and soil that have
once covered them have been removed by
erosion.
 Extrusive
igneous rocks are formed as lava
cools on the surface of Earth.
 The
 The
lava is exposed to air and water.
lava will cool faster. This prevents
mineral grains from growing large.
 These
rocks are cooled so quickly that few or
no mineral grains will form.
 Gases
may become trapped in the molten
material when it cools.
 Some
of the gases may escape the rock
causing holes to form around it.
 Examples:

Pumice, Obsidian, and Scoria
 Igneous
rocks are classified depending on
how they are formed:


Intrusive
Extrusive

Basaltic Rocks – Igneous rocks that are dense,
dark colored rocks.
Rich in iron and magnesium
 Poor in silica


Granitic Rocks - Igneous rocks that are of a lower
density than basaltic rocks and are light-colored.
High in silica
 Found in regions of violent volcanic eruptions

Andesitic Rocks - Igneous rocks have a mineral
composition between that of basaltic and granite
rocks.
 Page 96: Table 1

 The
violent nature of some volcanic
eruptions is partly explained by the
composition of the magma that feeds them.