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Volcanoes!
Chapter 10
Origin of Magma


Magma originates when solid rock,
located in the crust and upper mantle,
melts.
Factors that influence the formation of
magma include…
Heat
2. Pressure
3. Water Content
1.
Origin of Magma
 Factors
that influence the formation of
magma
•
Heat
•
Additional heat is generated by
• Friction in subduction zones
• Crustal rocks heated during subduction
• Rising, hot mantle rocks
Origin of Magma
 Factors
that influence the formation of
magma
•
Pressure
•
•
Increase in confining pressure causes an
increase in melting temperature
Drop in confining pressure can cause
decompression melting
• Lowers the melting temperature
• Occurs when rock ascends
Origin of Magma
 Factors
that influence the formation of
magma
•
Water Content
•
Cause rock to melt at a lower temperature
• Plays an important role in subducting ocean
plates
Volcanoes and Plate Boundaries
 Global
distribution of igneous activity is
not random!
•
Most volcanoes form along divergent and
convergent plate boundaries.
• Some volcanoes form far from plate
boundaries above “hot spots”.
Locations of some of Earth’s
major volcanoes
Volcanoes and Plate Boundaries

Divergent Boundary Volcanism
•
The greatest volume of volcanic rock is
produced along the oceanic ridge system
• Lithosphere pulls apart
• Mantle rock rises to fill the gap between
the plates.
• Decompression melting occurs forming
magma.
Volcanoes and Plate Boundaries
 Convergent
•
Boundary Volcanism
Descending plate partially melts
• Magma slowly rises upward
• Rising magma can form…
• Volcanic island arcs in an ocean (Aleutian
Islands)
• Continental volcanic arcs (Andes
Mountains)
Volcanoes and Plate Boundaries
 Intraplate
Volcanism
•
Activity within a rigid plate NOT near a
plate boundary
• Plumes of hot mantle material rise
• Form localized volcanic regions called hot
spots
• Examples include…
•
•
Hawaiian Islands (middle of the Pacific Plate)
Yellowstone National Park (Middle of the N.
American Plate)
The Nature of Volcanic Eruptions

Factors Affecting Eruptions
1.
Viscosity of the magma
•
•
Viscosity is a measure of a material’s
resistance to flow
Maple syrup vs. water
The Nature of Volcanic Eruptions
 Factors Affecting
 Viscosity
•
•
Eruptions
of magma
Temperature (hotter magma is less viscous)
Composition (silica content)
• High silica content – high viscosity (e.g.,
rhyolitic lava) this creates an explosive
eruption.
• Low silica content – more fluidlike (e.g.,
basaltic lava) quiet eruption.
The Nature of Volcanic Eruptions

Factors affecting Eruptions
2. Dissolved Gases
•
Gases trapped in the magma
• Violence of an eruption is related to how
easily gases escape from magma
• Easy escape from fluid magma
• Viscous magma produces an explosive
eruption
Volcanic Material
 Lava
flows
•
Basaltic lavas are more fluid (low silica
content)
• Types of lava
•
•
Pahoehoe lava (resembles braids in ropes)
Aa lava (rough, jagged blocks)
 Gases
•
One to five percent of magma by weight
• Mainly water vapor and carbon dioxide
A pahoehoe lava flow
A typical aa flow
Volcanic Material
 Pyroclastic
•
materials “Fire fragments”
Types of pyroclastic material
Ash and dust – fine, glassy fragments
• Pumice – from “frothy” lava
• Lapilli – “walnut” size
• Cinders – “pea-sized”
• Particles larger than lapilli
• Blocks – hardened lava
• Bombs – ejected as hot lava
•
Volcanic bombs on Kilauea
volcano in Hawaii
Volcano Formation
1.
2.
Begins when a fissure or crack
develops in the crust as magma is
forced to the surface.
Repeated eruptions of lava or
pyroclastic material eventually build a
mountain called a volcano.
Three Main Types of Volcanoes
Shield Volcanoes
2. Cinder Cones
3. Composite Cones
1.
Types of Volcanoes

Shield volcano
•
•
•
•
Broad, slightly domed (like a warriors shield)
Primarily made of basaltic (fluid) lava
Generally large in size
e.g., Mauna Loa and Mauna Kea in Hawaii
Shield volcano
Types of Volcanoes

Cinder cone
•
Built from ejected lava fragments that
harden in the air
• Steep slope angle
• Small in size
A cinder cone near
Flagstaff, Arizona
Types of Volcanoes
 Composite
•
•
•
•
•
cone (or stratovolcano)
Most are adjacent to the Pacific Ocean
along the ‘Ring of Fire’ (e.g., Mt. Rainier)
Magma is viscous with a high silica content
Large size
Composed of layers of lavas and
pyroclastic material
Most dangerous and explosive volcano
Composite volcano
Mount St. Helens – a typical
composite volcano
Mount St. Helens following the
1980 eruption
A size comparison of the
three types of volcanoes
Other Volcanic Landforms
 Calderas
•
Steep-walled depression at the summit
• Formed by collapse
• Nearly circular
• Size exceeds 1 kilometer in diameter
 Lava
•
plateaus
Fluid basaltic lava erupts from crustal
fractures called fissures
• e.g., Columbia Plateau
Crater Lake,
Oregon, is a good
example of a
caldera
The Columbia Plateau
Other volcanic landforms
 Volcanic
•
pipes and necks
Volcanic necks (e.g., Ship Rock, New
Mexico) Landform made of magma that
hardened in a volcano’s pipe and later
exposed by erosion.
•
Pipes are short conduits that connect a magma
chamber to the surface
Formation of a
volcanic neck
Volcanic Hazards
 Types
•
of volcanoes
Composite cone (or stratovolcano)
•
•
Often produce
• Fiery pyroclastic flow made of hot gases
infused with ash
• Flows down sides of a volcano at speeds up
to 200 km (125 miles) per hour
May produce a lahar – volcanic mudflow
Pyroclastic flow
Mount St. Helens
A lahar along the Toutle River
near Mount St. Helens
Intrusive igneous activity


Most magma crystallizes within Earth’s crust
An underground igneous body is called a
pluton
*Pluto is Roman God of the underworld*

Types of Plutons
•
•
•
Sills - Magma flows between sedimentary layers
Laccoliths - ‘lens shaped’ Magma pushed the overlying
rock layers upward.
Dikes - Magma moves into fractures and cuts across
rock layers.
Intrusive igneous structures
exposed by erosion
A sill in the Salt River
Canyon, Arizona
Intrusive igneous activity
•
Batholith
•
Largest intrusive body
• Much larger than a pluton
• Surface exposure 100+ square
kilometers
• Frequently form the cores of mountain
ranges
A batholith exposed
by erosion