Earth Science
10-2
Factors Affecting Eruptions
1. The Primary factors that determine whether a
volcano erupts explosively or quietly include
characteristics of the magma and the amount
of dissolved gases in the magma.
Mount Saint Helens before and after eruption
Viscosity
• Viscosity
o Is a substance’s resistance to flow
o Viscosity is strongly affected by temp.
o If lava cools and begins to harden, its
Viscosity increases
Continued…
• Chemical composition
– Chemical composition on magma has a more
important effect on the type of eruption
– Because of high silica content, rhyolitic lavas
are very viscous and erupt explosively
– Less silica has a calmer eruption
Dissolved Gases
• Vent
– An opening to the surface
– During explosive eruptions, the gases trapped
in magma provide the force to propel molten
rock out of the vent.
– These gases are mostly water vapor and
carbon dioxide
Volcanic Material
1. Lava Flows
a) Silica content and the
temperature affect
the characteristics of
lava flows.
b) Temperature
differences produce 2
types of basaltic lava:
pahoehoe and aa
2. Pahoehoe
a) Is hotter, fast moving
basaltic lava
b) Pahoehoe forms relatively
smooth skin that wrinkles
as the still-molten lava
continues to flow
3. Aa
a) Is cooler, slower-moving
basaltic lava
b) Aa forms a surface of
rough, jagged blocks with
sharp, spiny projections
4. Gases
a) Magmas contain
varied amounts of
dissolved gases held
under pressure in the
molten rock
b) The gaseous portion
of the magma is a
small portion about 1
to 6%
5. Pyroclastic Materials
a) Particles produced in
volcanic eruptions
are called pyroclastic
materials
b) The fragments
ejected range in size
from very fine dust
and volcanic ash to
pieces that weigh
several tons
Types of volcanoes
1. Anatomy of a volcano
a) Volcanic activity usually
begins when a fissure, or
crack, develops in the
crust
b) Volcano- repeated
eruptions of lava or
pyroclastic material
eventually build up a
mountain
c) Crater- steep-walled
depression
• The three main
volcanic types of
volcanoes are shield
volcanoes, cinder
cones, and composite
cones.
Volcano eruption
types
Shield volcano
Cinder cones
Composite
cones
1. Shield volcanoes
a) Are produced by the
accumulation of fluid
basaltic lavas.
b) Shield volcanoes
have a shape of a
broad, slightly domed
structure that
resembles a warrior’s
shield.
2. Cinder cones
a) Ejected lava
fragments that
harden in the air build
a cinder cone
b) The shape of a
cinder cone is
determined by the
steep-sided slope
that forms as loose
pyroclastic material
builds up around the
vent
3. Composite cones
a) Is a large, nearly
symmetrical volcano
mountain composed
of layers of both lava
and pyroclastic
deposits
b) Most composite
cones are located in
a relatively narrow
zone that rims the
Pacific Ocean, the
Ring of Fire
How many of these Volcanoes are
located in the U.S.
600
500
400
300
200
100
0
composite
cinder
shield
• Volcanic landforms include calderas,
volcanic necks, and lava plateaus
• Long after eruptions have ended, these
landforms can provide evidence of
volcanic activity
1. Calderas
a) A caldera is a depression in a volcanic
mountain.
b) Most calderas form in two ways:
1) By the collapse of the top of a composite volcano
after an explosive eruption
2) From the collapse of the top of a shield volcano
after the magma chamber drained
2. Volcanic necks
a) Is a landform made of magma that hardened
in a volcanoes pipe and later was exposed
by erosion
b) Weathering and erosion work constantly to
wear away the volcanoes
c) When a volcanoes activity ends, magma
remaining in the pipe hardens to form
igneous rock
3. Lave plateaus
a)
b)
c)
Is a volcanic landform
produced by repeated
eruptions of very fluid,
basaltic lava
Instead of building a
cone the lava spreads
out over a wide area
The lava that forms the
lava plateaus erupts
through long cracks
called fissures
Volcanic Hazards
• Volcano hazards include lava flows, volcanic ash,
pyroclasitc flows, and mudflows.
• Lahar
– Occurs when water-soaked volcanic ash and rock slide rapidly
downhill
• Composite volcanoes are among the most dangerous.
They can cause a lahar
• An explosive eruption can also create a pyroclastic flow
• Volcanoes usually give some warnings that an eruption
is near
• Usually most people settle near a volcano because of
the fertile land from the ash