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CHAPTER 10.1
THE NATURE OF VOLCANIC ERUPTIONS
• Volcanoes give the opportunity to look at what
happens deep beneath the Earth’s surface.
• On May 18, 1980 Mount St. Helens erupted and
change the surrounding area in Yakima,
Washington. It released a lot of ash and debris
into the air and surrounding area.
FACTORS AFFECTING ERUPTIONS
• There are several factors that will determine if a
volcano erupts violently or quietly: magma
composition, magma temperature, and the
amount of dissolved gas in the magma.
VISCOSITY
• Viscosity is a substances resistance to flow.
Magma that is more viscous usually erupts more
violently than magma that is not.
• Magma that is heated is less viscous so the
ability of lava to flow is affected by temperature.
As lava cools and hardens it becomes more
viscous (less able to flow) and that will
eventually stop the lava flow.
• More silica in magma the more it’s viscosity.
DISSOLVED GASES
• The vent is the opening in a volcano to the
surface.
• Explosive eruptions trap gases in the vent. These
are gases like water vapor and carbon dioxide.
• Fluid basaltic magmas allow gases to be
released slowly and easily. These are the types
of eruptions that happen in Hawaii.
• Highly viscous lavas trap gases in and they build
up until they explosively eject from the volcano.
(like Mount St. Helens).
VOLCANIC MATERIAL
• Lava may seem like it is the main thing ejected
from a volcano but large pieces of rock, lava
bombs, fine ash, dust, and large amounts of gas
are also given off.
LAVA FLOWS
• Basaltic lavas are very fluid, they flow between
10 to 300 meters per hour. Silica rich lava is too
slow to be visible.
• Pahoehoe flows are lavas that harden and then
wrinkle up.
PYROCLASTIC MATERIALS
These are materials that are blown from the vent of
a volcano.
Fragments that come out a volcano can be very fine
dust and volcanic ash to pieces that weigh several
tons.
Smaller particles are called cinders, larger particles
are blocks.
TYPES OF VOLCANOES
• There are 3 main types of volcanoes: shield
volcanoes, cinder cone, and composite cone.
ANATOMY OF A VOLCANO
• Volcanic activity starts when a crack starts in the
crust and magma is forced toward the surface.
• Magma will move through the crack, through a
circular pipe and end up in a vent.
• As time moves on and there is more magma that
moves into the area it forms a volcano.
• At the top of many volcanoes is a steep-walled
depression called a crater.
• The type of magma usually determines the type
of volcano that is formed.
SHIELD VOLCANOES
• Shield volcanoes are broad, slightly domed
structures - most will form islands, like Hawaii.
They are formed by fluid basaltic lava.
CINDER CONES
• Lava fragments that are the size of cinders
(smaller) harden in the air and form a cinder
cone.
• They have steep sides. The volcano is usually
formed from one eruption that may last a few
weeks to a few years. Usually once that eruption
is over the volcanoes magma chamber solidifies
and it never erupts again. These are usually
smaller volcanoes.
COMPOSITE CONES
• These are the most dangerous type of volcano.
Most are found around the Pacific Ocean – an
area called the Ring of Fire. Mount St. Helens,
Mount Rainier, and Mount Garibaldi in US are
part of the Ring of Fire.
• A composite cone is a large, nearly symmetrical
structure made of layers of lava and pyroclastic
deposits. They are usually made by very viscous
lavas.
DANGERS FROM COMPOSITE CONES
• Pyroclastic flows are the most dangerous – they
have hot gases, glowing ash, and larger rock
fragments. Some move very quickly.
• Composite cones may also cause mudflows
where volcanic debris becomes saturated with
water and move quickly down the side of the
volcano.
OTHER VOLCANIC LANDFORMS
• Calderas - a large depression in a volcano. This
is cause by the collapse of the top of a
composite volcano or from the collapse of the
top of a shield volcano.
• Necks and Pipes – most volcanoes get magma
through conduits called pipes that connect a
magma chamber to the surface. A neck is when
the magma chamber withstand erosion and the
rest of the volcano erodes.
• Lava Plateaus – Sometimes a volcano isn’t
formed, instead magma comes out of fissures.
These fissures can cover a large area and have
many eruptions.