Download Chapter 9 Volcanoes

CHAPTER 9 VOLCANOES
HOW AND WHERE VOLCANOES FORM
What is a volcano?
 It refers to an
opening that that
releases molten
rock, gases and
ash. It also
includes the
landform that
forms around that
opening.
HOW AND WHERE VOLCANOES FORM
Magma Formation
 Three conditions are
needed for magma
formation
A decrease in pressure
can cause the melting
point to drop allowing
melting of rock.
 Increase in temp
(DUH!). These are
called hot spots.
 Increase of water in the
asthenosphere can
lower the melt point
(subduction zones).

HOW AND WHERE VOLCANOES FORM
Subduction Boundaries
 We remember that the
products of subduction
boundaries are?
Trenches
 Volcanoes – form on
the overriding plate
due to the diving
ocean plate carrying
water and lowering
the melting points.
(Volcanic island arcs
in the ocean and
volcanoes on
continental plates)

HOW AND WHERE VOLCANOES FORM
What is a divergent
boundary?
 A boundary b/t two
lithosphere plates
that are moving
apart.
 Most exist on the
ocean floor and have
rift valleys (deep
valleys at the center
of mid ocean ridges)
 Rifts - Where molten
rock forces its way b/t
cracks on ocean floors
causing spreading
(sea floor spreading)
HOW AND WHERE VOLCANOES FORM
Hot Spots
 These are areas
where solid hot
material melts the
material in lower
pressure areas.
These are not plate
boundaries.
 As plates move, hot
spots remain
stationary.

Ex. Hawaiian
Islands
MAGMA AND ERUPTED MATERIAL
Types of Magma (The viscosity of magma is
dependent on the amount of silica present in the
magma. High SiO = high viscosity and vice versa)
Magma also contains gases like water vapor and
carbon dioxide.
 Basaltic Magma – Low SiO content which gives it
an ease of flow.
 Andesitic and Rhyolitic magma has high SiO
content, therefore it doesn’t flow easily (high
viscosity).
MAGMA AND
ERUPTED
MATERIAL
Basaltic magma
 Low silica content
 Flows easily (low
viscosity)
 Gas escapes easily
causing relatively
harmless fountains and
floods.
 Generally forms at rifts
and oceanic hotspots.
MAGMA AND ERUPTED MATERIAL
Andesitic and Rhyolitic
Magmas



High SiO content
Slower flow (high
viscosity)
Gases don’t escape as
easily causing explosions
(Mt. St. Helens)
Andesitic magmas tend to
form at subduction
boundaries.
 Rhyolitic magmas tend to
form at continental hotspots.

MAGMA AND
ERUPTED
MATERIAL
Lava Flows on land
(Lava magma that
reaches the Earth’s
surface). There are two
types of land lava flow
(both basaltic flows):
 Pahoehoe – High
temp/low
viscosity/ropelike
surfaces.
 Aa – Cooler lava that
cools quickly/ rough
jagged surfaces.
MAGMA AND ERUPTED MATERIAL
Underwater Lava
Flows:
 Pillow Lava – Has
a rounded shape
w/a crust. When
pressure
increases the
crust cracks and
more lava pours
out.
MAGMA AND ERUPTED MATERIAL
Ash and Rock Fragment
Pyroclastics Material – When magmas contain
trapped gas, the resulting action is explosive
eruptions. These eruptions throw solid fragments
called pyroclastics.
 Ash – Smallest pieces are called ashes (<2mm).
 Lapilli – The intermediate material (2-64mm)
 Blocks and Bombs – the largest pieces (>64mm)
MAGMA AND ERUPTED MATERIAL
Pyroclastic Flow –
A combination of
pyroclastic
material with
superheated
gases that flow
down hill at a
high rate of
speed. Mt.
Vesuvius buried
Pompeii with
pyroclastic flow.
VOLCANIC LANDFORMS
Shield Volcano – Basaltic lava tends to flow over great distance

before hardening. This allows for wide gently sloping bases that
can support enormous heights. They are less explosive, but the
lava flows are dangerous and can be expensive.
Mauna Loa – Starts 5000m above below sea level and rises 4170
above sea level (9170m total height)
VOLCANIC LANDFORMS
Cinder Cone
 Typically smaller
 Formed from molten
lava being thrown into
the air, breaking into
fragments and
hardening before they
hit ground. They
accumulate forming a
cone w/an oval bottom.
 Typically form in
groups and on sides of
larger volcanoes.
VOLCANIC LANDFORMS
Composite Volcano Forms when layers
are deposited from
successive explosive
eruptions around
the vent.
 Ex. Mt. St. Helen
erupted in 1980.
The hot ash mixed
w/snow and ice to
create a fast
moving mudslide
called lahar.
VOLCANIC LANDFORMS
Calderas – After a
volcano partially
empties a magma
chamber the top of
the cone collapses.
This forms a
crater (calderas).
EXTRATERRESTRIAL VULCANISM
Volcanoes occur in more places than Earth.
 Venus – Has over 1600 volcanoes although most are
inactive.
EXTRATERRESTRIAL VULCANISM
Volcanoes occur in more
places than Earth.
 Io (one of the Galilean
moons of Jupiter) One
of the most
vocanically active
places in the solar
system.
EXTRATERRESTRIAL
VOLCANISM
Volcanoes occur in more
places than Earth.

Mars – Olympus Mons is
located on Mars. It is the
tallest known volcano in
our solar system.