9.2 Magma and Erupted
Materials
Purpose: After completing this
lesson we learn what materials
erupt from volcanoes.
Types of Magma
Magma is molten rock. The main “ingredient” in
rocks is a mineral named silica (SiO2); besides
silica, there are many other minerals.
 Magma also contains significant amounts of
fluids dissolved in the molten rock: water vapors
and carbon dioxide are most common
 Silica, however, is the one that determines the
type of magma in every igneous activity.
 Silica determines the viscosity (resistance to
flow) of magma as well.

Types of Magma
 There
are three types of magma,
depending on the content of silica:
1. Basaltic
2. Andesitic
3. Rhyolitic
Basaltic Magma
Some characteristics of basaltic magma are:
 it has about 50% silica
 has a small amount of gases dissolved
 flows easily (least viscous)
 the eruption is rarely explosive
 it has the highest melting point (>1300 C
degrees)
Basaltic Eruption
 Lava
flows easily, due to reduced viscosity
Andesitic Magma
 contains
about 60% silica
 Has a moderate amount of gasses
 Sometimes it generates explosive
eruptions
 The melting temperature is around 1000 C
degrees
Andesitic Eruption
 This
is an explosive eruption
Rhyolithic Magma
 Contains
most silica ( over 70%)
 Contains a significant amount of dissolved
gasses
 It has a high viscosity (flows very slowly)
 The eruptions are usually explosive and
create tremendous damage.
 The melting point is below 900C degrees.
Rhyolithic Eruption
 This
type is highly explosive
Location of Each Type of Magma
Each type pf magma form in certain areas:
 Basaltic magma is found at rifts and hot
spots
 Andesitic magma is found at subduction
boundaries
 Rhyolitic magma is found at continental
hot spots.
Magma versus Lava


Magma is the molten material that rises from the deep. It
contains a significant amount of volatile compounds;
water vapors and CO2 are most common. As the
magma reaches the surface of the earth, it loses very
quickly the volatiles, because of the difference in
pressure. (Shake a can of soda and open it quickly; the
fizz is nothing else but the CO2 which is released from
the liquid)
Lava is magma which has lost most of its volatile
components. Lava has temperatures between 700 and
1200 C degrees.
Other Volcanic Products



Lava is the main product that is released from a volcano.
Pyroclastic materials (“pyro” means hot and “clast”
means fragment in Greek). These are hot, solid
materials of different sizes. Sometimes the lava cools
down on the pipe, creating a plug; the magma is climbing
up the pipe at great pressures, and breaks apart this
plug, creating fragments of different sizes.
The most common pyroclastic materials are:
-volcanic ashes (the particles have less than 2 mm in
diameter)
-lapilli- pebbles with diameters between 2-64 mm
- bombs or blocks, are fragments larger than 64 mm
Types of Lava Flow

Pahoehoe is lava with smooth, ropelike
surfaces.
Pillow Lava
 Lava
that cools under water has a
rounded, pillow like form.
Aa
 Aa
is lava that cools quickly with a rough,
jagged surface.