Download Section 6.1 Volcanic eruptions


Volcanoes
 Areas of Earth’s surface through which
magma & volcanic gas passes
 Creative Forces
 forming fertile farmland & large mountains
 Destructive Forces
 Turning mountains into clouds of ash &
rock, destroying forests & homes
Structure of a
typical volcano.
Notice the
alternating
layers of ash
and hardened
lava.
Why does this
occur?

Nonexplosive Eruptions
Most common type of eruption
 Release large amounts of lava
 Calm lava flow
 Pacific NW of USA covered from
these eruptions


Explosive Eruptions


Rare & incredibly destructive
Debris/molten rock blown out of volcano
 Large debris falls close to volcano
 Smaller, dust-sized particles (ash) can travel miles

Inside a Volcano

Magma
 Body of molten rock (underground) that feeds a
volcano

Vents
 Opening through which
volcanic material passes

Magma


Molten rock
Composition affects how explosive a volcano is
 Silica (SiO2), Water, Gases (CO2, water vapor)
 When pressure decreases as magma rises, the CO2 and
water vapor expand rapidly
 Silica Rich (Felsic) Magma
Traps Gases
 Thicker, viscous
 Difficult for gases to escape,
so pressure builds up

What Erupts from a Volcano?

Explosive Volcano
 Mainly pyroclastic material
 Magma that is blasted into the air and hardens

Nonexplosive Volcano
 Mainly lava
Eruptions can (and do) alternate between lava &
pyroclastic eruptions (even within the same eruption)

Types of Lava




Aa
jagged surface; brittle crust
is torn by lave flow underneath
Pahoehoe
flows slowly; glassy
surface has rounded
wrinkles
Pillow Lava rounded lumps formed
under water
B locky Lava does not travel far; heaps
of sharp-edged chunks

Types of Pyroclastic Material

Formed when magma explodes and solidifies in the
air
 Volcanic Bombs
 Large blobs that cool & harden
in the air
 Lapilli
 Smaller, pebble-like stones that
harden before hitting the ground
 Volcanic Ash
 Most of the material in an eruption
 Walls of gas bubbles explode into
tiny, glass-like slivers

Pyroclastic Flows
Produced when enormous amounts of hot ash, dust,
and gases are ejected from a volcano
 Speeds up to 200 km/h
 Temperatures over 700 C


Review
1)Which of the following factors influences whether a
volcano erupts explosively?
A. The concentration of bombs in the magma
B. The concentration of phosphorous in the magma
C. The concentration of aa in the magma
D. The concentration of water in the magma
Review
2) How is lava classified? How are pyroclastic
materials classified?
3) Explain how the presence of silica (felsic) and
water in magma increases the chances of an
explosive eruption.


Answers
1) D
2) Lava is classified by its surface texture; the way it flows
may also be used to help classify lava.
Pyroclastic material is classified by its size and how it
forms.
3) As the magma moves towards the surface, the water
changes to water vapor and expands rapidly; silica rich
(felsic) magma is viscous and tends to trap volcanic
gases and plug vents, the results in high pressures
forming