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Volcanoes around the World
10.1 The Nature of Volcanic Eruptions
Factors Affecting Eruptions
 Factors that determine an eruption
• Composition
• Temperature
• Dissolved gases
 Viscosity
• Viscosity is the measure of a
material's resistance to flow.
Comparison of ash plumes
Mt. St Helens was a relatively small eruption compared
to prehistoric eruptions
10.1 The Nature of Volcanic Eruptions
 Factors Affecting Viscosity
- Temperature (hotter magmas are less viscous)
- Composition (silica content)
1. More silica—high viscosity, thicker
2. Low silica—less viscous, more fluid
10.1 The Nature of Volcanic Eruptions
Factors Affecting Eruptions
- Viscous (thicker) magma produces a more
violent eruption.
10.1 The Nature of Volcanic Eruptions
 Dissolved gases
• Mainly water vapor and carbon dioxide
More trapped gas = Greater Eruption
• A vent is an opening in the surface
of Earth through which molten rock
and gases are released.
• Provide the force to extrude lava
10.1 The Nature of Volcanic Eruptions
Volcanic Material
 Lava Flows
• Basaltic lavas are more fluid.
• Types of lava
-Pahoehoe lava (resembles braids in ropes)
-Aa lava (rough, jagged blocks)
Mayon Volcano, Philippines
10.1 The Nature of Volcanic Eruptions
Volcanic Material
• Pyroclastic materials-fragments ejected during eruptions
Includes dust, ash, and rock
• Range in size from very fine dust and ash to pieces
that weigh several tons.
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Yes – write this down- names only!!!
Ash and dust — fine, glassy fragments
Pumice — frothy, air-filled lava
Lapilli — walnut-sized particles
Cinders — pea-sized particles
Blocks — hardened lava (large)
Bombs — ejected as hot lava (large)
10.1 The Nature of Volcanic Eruptions
Types of Volcanoes
 Anatomy of a Volcano
• A volcano is a mountain formed of lava and/or
pyroclastic material.
• A crater is the depression at the summit/top of a
volcano
• A conduit carries gas-rich magma to the
surface.
10.1 The Nature of Volcanic Eruptions
Types of Volcanoes – 3 main Types
 Shield Volcanoes
• Broad, gently sloping volcanoes, usually the largest
built from basaltic lavas, shaped like a shield
• Magma - Very fluid & the lava can
flow great distances – rich in Fe & Mg
• Eruptions are usually mild & can
occur several times
• Ex. Mauna Loa, HI
Cinder Cones
 Cinder Cones
- Ex. Paricutin, Mexico
• Small steep sloped volcanoes built primarily of pyroclastic
material ejected from a single vent.
-Magma: gas rich in basaltic lava, ejects lava
fragments – mostly cinders
- Eruption: typically a single eruption lasting a
few weeks to a few years
10.1 The Nature of Volcanic Eruptions
 Composite Cones
(a.k.a. Stratovolcano)
• Steep sides w/ alternating layers of ash, cinder, and lava
- Magma: Thick magma contains trapped gases, very viscous, silica rich
- Eruptions alternate between flows and explosions
(cinder & ash) & pyroclastics Most violent
Ex. Mt. Shasta, CA
Mt. St. Helens, WA
Mount St. Helens Before and
After the May 18, 1980,
Eruption
Size Comparisons of Volcanoes
Different types of volcanoes
10.1 The Nature of Volcanic Eruptions
Other Volcanic Landforms
• Calderas are large depressions in volcanoes.
• Nearly circular
• Formed by collapse of the vent
 Lava Plateaus
• Fluid basaltic lava extruded from crustal fractures
called fissures.
10.2 Intrusive Igneous Activity
Plutons
 Plutons are intrusive igneous structures
that result from the cooling and hardening
of magma beneath the surface of Earth.
10.2 Intrusive Igneous Activity
Plutons
• Sills and laccoliths are plutons that form when
magma is intruded close to the surface.
- Sills resemble buried lava
- Laccoliths are lensflows and may exhibit columnar shaped masses that arch
joints. Store magma.
overlying strata upward.
Transport magma.
Sill
10.2 Intrusive Igneous Activity
Plutons
• Batholiths are large
masses of igneous rock
that formed when magma
intruded at depth, became
crystallized, and
subsequently was exposed
by erosion. Ex) Mount Rushmore
• An intrusive igneous body
must have a surface
exposure greater than
100 square kilometers to
be considered a batholith.
10.2 Intrusive Igneous Activity
Origin of Magma
 Magma originates when rock located in the
lithosphere partially melts.
10.2 Intrusive Igneous Activity
• The geothermal gradient — Earth’s natural temperature
is not sufficient to melt rock in lithosphere
• Additional heat is generated by:
- friction in subduction zones
- rocks heated during subduction
- rising, hot mantle rocks
10.3 Plate Tectonics and Igneous Activity
Lava
Where Do Volcanoes Occur?
• Mostly occur at plate boundaries
• Convergent plate
boundaries: 75% of active
volcanoes
– Ex: Ring of Fire
• Divergent plate boundaries:
as plates move apart,
magma rises to fill the space
- Ex: Iceland, Mid-Atlantic Ridge
10.3
Plate Tectonics and Igneous Activity
Convergent Plate Boundaries
 Ocean-Ocean
• Forms volcanic island arcs in an ocean
Ex. Aleutian Islands
 Ocean-Continent
• Forms continental volcanic arcs
Ex. Andes Mountains
10.3
Plate Tectonics and Igneous Activity
Divergent Plate Boundaries
 The greatest volume of volcanic rock is
produced along the oceanic ridge system.
Ex. Mid-Atlantic ridge
• Lithosphere pulls apart.
• Partial melting occurs
• Oceanic Ridges form
10.3
Plate Tectonics and Igneous Activity
Intraplate Igneous Activity
 Intraplate volcanism occurs within a
tectonic plate away from plate boundaries.
• Occurs when hotter than normal mantle material
called a mantle plume rises to the surface.
• Forms localized volcanic regions called hot spots.
• Ex. Hawaiian
Islands (Kilauea) &
Columbia Plateau.
10 Most Active Volcanoes