Download What Controls Volcanic Eruptions?

What Controls Volcanic
Eruptions?
Ch 12.2
Some eruptions are quiet…
Kilauea, Hawaii
Some eruptions are EXPLOSIVE!
Mt. Pinatubo
Montserrat
Mt. St. Helens
What Controls Volcanic Eruptions?
1. How much water vapor (a gas) and other
gases are trapped in the erupting magma
2. How much silica is contained in the
magma
Trapped Gases
• water vapor and carbon dioxide are
dissolved in magma
– When magma rises to surface, gases escape
because they are under less pressure
– Some magma is thin and fluid- this lets gases
escape easily- resulting in quiet eruption
– Other magma is thick and stiff- when gases
erupt it results in explosive eruption
Water Vapor
Water may be added to magma when
ocean crust rock is melted at
convergent plate boundaries
Composition of magma
• Silica-rich magma is
called
granitic/rhyolitic
magma
• Results in explosive
eruptions
• Ex. Soufriere Hills in
Montserrat
• Characteristic of
convergent plates
• Silica-poor magma is
called basaltic
magma
• Results in quiet
eruptions
• Ex. Iceland
• Characteristic of
divergent plates
Basaltic magma may result in
different three lava types:
• Pahoehoe
• Aa
• Pillow (cools
the fastest)
Pahoehoe lava:



Hot, thin, fast flowing
harden with a
relatively smooth
surface
Often has a ropy or
wrinkled appearance
Pahoehoe lava
Aa lava:
• Cooler, thicker,
slow moving
• Hardens with a
rough, jagged,
sharp edge
surface
Pillow Lava:
 Lava cools
underwater
 resembles
stuffed pillows
Explosive Magma/Lava
• Granitic/Rhyolitic
Magma
– high in silica
– high amount of
trapped gases
– very viscous
– lead to explosive type
eruptions
Soufriere Hills volcano-active since 1995
At what type of boundary would granitic/rhyolitic magma form?
convergent boundaries
Composition of magma
• Andesitic magma – more silica than
basaltic magma, but less than granitic
magma
– Common at convergent boundaries
– Named after the Andes Mountains in South
America
– Also violent eruptions
– Ex. Krakatau- one of the most violent
eruptions in history
Andes
Mtns
Krakatau 1883
Sound heard 3,000 mi away!
13,000 x more powerful than nuclear blast at Hiroshima in WWII
Anak Krakatau (Child of Krakatau)
Forms of Volcanoes
• Depends on:
– Type of magma it was formed from
– Type of eruption- quiet or explosive
• 3 basic types of volcanoes
1. Shield
2. Cinder cone
3. Composite
1. Shield volcanoes
• Result of quiet eruptions of basaltic lava
flows (no ash)
• May form volcanic cones or fissures- long
cracks that result if flood basalts (Ragged
Mtn, Lamentation Mtn)
• Ex. Hawaiian Islands and the Columbia
Plateau of Northwest US
Shield volcanoes
Mauna Kea, Hawaii
Shield volcanoes
Mauna Loa, Hawaii
Shield volcanoes
5 Shield volcanoes make up the
island of Hawaii
Fissure eruption
Fissure eruptions
• Common at divergent
plate boundaries,
mid-ocean ridges
Columbia Plateau
2. Cinder Cones
• Small base, steep-sided, loosely
consolidated
• Up to 1000 feet tall
• Life span of a few years
• Commonly built from solidified rock thrown
through the air called tephra
• Has violent eruptions, dangerous when
close
2.
Cinder Cone Volcanoes
Paricutin, Mexico (1943-1952, newest volcano in Western Hemisphere, 1000 killed)
Cinder Cone Volcanoes
Paricutin
Cinder Cone Volcanoes
Pu'u ka Pele on flank of Mauna Kea volcano, Hawaii
Cinder cone example: Paricutín, Mexico
1943
Cinder Cone Volcanoes
Composite Volcano
(aka stratovolcano)
•most beautiful, but most deadly
•characteristic shape
•made of alternating layers of lava and tephra- periods
of quiet, then explosive eruptions
Mt. Fuji, Japan- one
of the most
photographed
volcanoes in the
world because of its
near perfect
symmetry
Ex. Mt. St. Helens
Composite Volcanoes
What are the three types of magma
in order from least viscous to most
viscous?
• a. Basaltic
• b. Andesitic
• c. Granitic
Tephra
• Names are based on size:
• Ash
• Lapilli/Cinders
• Bombs/Blocks
Volcanoes
• Volcanoes eject
different types of
material. This material
is known as tephra or
pyroclastic material.
• There are 3 main types:
– Ash-less than 2 mm
in diameter
– Lapilli/cinders2mm-64 mm in
diameter
– Blocks or Bombslarger than 64mm
Ash
Lapilli/Cinders
Mt Pinatubo,
Philippines 1991
Bombs
Blocks