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Volcanoes
“And the mountains shall be molten under
Him, and the valleys shall be cleft, as wax
before the fire, and as the waters that are
poured down a steep place.”
Micah 1:4
Volcanoes
• Volcanoes are the most violent of
commonly occurring natural cataclysms.
• They give us a glimpse of the interior of
the earth.
Volcanic Structure
A volcano is an opening in the earth’s
surface though which hot gases, ash,
and molten rock are ejected from the
earth’s interior.
Volcano Characteristics
1. Vent – the central channel which ejects ash,
gases, and rock.
2. Magma – a reservoir of molten rock that may be
three or four miles beneath the surface.
3. Cone – the ejected materials accumulate to
produce a conical shaped mountain.
4. Crater – the cone is capped by a bowl shaped
depression surrounding the vent.
Volcanology
Volcanology is the study of volcanoes.
Vulcanologists are scientists who study
volcanoes.
Today vulcanologists in certain areas give
warning to eruptions and they are better
able to warn residents.
Types of Volcanoes
There are three common types of
volcanoes:
1. Cinder-cone volcanoes
2. Shield volcanoes
3. Composite volcanoes
Cinder-cone Volcanoes
• These are volcanoes that consist primarily
of erupted volcanic ash and rock
fragments.
• They are composed of loose materials
• They do not grow large and are usually
formed by a single eruption.
Shield Volcanoes
• These volcanoes pour out lava in large
quantities during gentle eruptions.
• They produce a broad, gentle sloping
volcano.
Example: Mauna Kea in Hawaii
Composite Volcanoes
• These volcanoes produce both cinders
and lava.
• They have a steeper dome.
• These are highly explosive volcanoes.
Example: Mt. Rainer and Mt. Saint Helens
Active and Dormant Volcanoes
Geologists classify volcanoes in three ways:
1. Active – a volcano that has erupted within the
past 50 years.
2. Dormant – now inactive – appears “at rest” –
Mammoth is an example and Lassen Peak
3. Extinct – will probably not erupt again – Mount
Shasta is an example
Volcanoes are unpredictable and it is difficult to tell
if a volcano is dormant or truly extinct.
http://dsc.discovery.com/convergence/supervolcano/interactive/interactive.html
Location of Volcanoes
• Almost all of the earth’s volcanoes occur in
three main belts that correspond to the
earth’s earthquake belts.
• The most important belt encircles the
Pacific Ocean Basin
called the “Ring of Fire”
Volcanic Eruptions
• Volcanoes rarely erupt without warning.
• They usually have some swelling or bulging the bulging is caused by the magma chamber
filling with magma and gases.
• Mt. Saint Helens had a bulge and it enabled
scientist to warn residents and save lives.
• Earthquakes may also precede an eruption.
Volcanic Eruptions
• The first explosion may produce a great
cloud of volcanic gases, steam, and dust
that moves into the atmosphere several
miles.
• This cloud is often accompanied by
heavier rock and lave hurled into the air.
• The eruption may last for weeks, months,
or even years.
Volcanic Eruptions
Not all volcanoes erupt
explosively…Volcanologists have discovered
the violence of an eruption depends on two
things:
1. Viscosity – the thickness of the magma – the
thicker the more violent the eruption
2. The amount of dissolved gas in the magma – If
the gases cannot escape easily (the magma is
too thick) they build up to a greater explosion.
Volcanoes and Plate Tectonics
There are several types of plate
boundaries…the places where the plates
are up against each other.
1. Divergent
2. Convergent
3. Transform
Divergent Plate Boundary
Divergent Plate Boundary
• Plates are coming apart…
• Example: Mid-Atlantic Ridge
• Example: Kenya – East African Rift
Convergent Plate Boundary
Convergent Plate Boundary
• Plates crashing into plates…
• Example: Andes Mountains
Transform Plate Boundary
Transform Plate Boundary
• Plates slide past plates…
• San Andreas
Volcanic Ejecta
There are three types of ejections:
1. Gaseous ejecta
2. Liquid ejecta
3. Solid ejeta
Gaseous Ejecta
• 70 percent of gases consist of water vapor
• Other gases include carbon dioxide,
nitrogen, sulfur dioxide, hydrogen, fluorine,
and hydrochloric acid
• Many of these gases is poisonous and can
cause sickness and death miles around.
• Sulfur dioxide makes acid rain.
Liquid Ejecta
• Includes Lava
Solid Ejecta
• Formed when molten lava is ejected into
the air.
• Particles or blocks of solid volcanic ejecta
are called pyroclasts.
• When pyroclasts are small, they are
carried by the wind in the form of ash.
• Volcanic bombs are formed when lava is
thrown high into the air in a liquid state
and hardens before it hits the ground.
Fiery Blasts
• The most dangerous part of an eruption is
the fierce blast of incandescent ash and
superheated gases that accompanies
some volcanic eruptions.
Volcanic Structures
• Calderas – the bowl shaped crater in the
ground at least two miles across.
• Tunnels – lava tunnels remain after the
lava cools – the largest known is in Hawaii
– it winds 6 miles beneath the surface.