Volcanoes and Other
Igneous Activity
Volcano Defined
• A volcano is an opening in a planet's crust,
which allows hot magma, ash, rock and
gases to escape from below the surface.
Volcano Word Origin
• The word volcano is derived from the name of
Vulcano island off Sicily which in turn, was named
after Vulcan, the Roman god of FIRE.
Introduction to Volcanoes + Top Five Volcano Webcams
Volcano Origins
Volcanoes are found on top of magma chambers
and have large central vents through which the
tephra and gases erupt. At the top of the
central vent is a crater.
Tephra, from the Greek meaning
ash, are rock fragments and particles
ejected by the volcanic eruption.
Tephra = Pyroclastic materials
10.1 The Nature of Volcanic Eruptions
Factors Affecting Eruptions
 Factors that determine the violence of an
eruption
• Composition of the magma
• Temperature of the magma
• Dissolved gases in the magma
 Viscosity
• Viscosity is the measure of a material's
resistance to flow.
10.1 What is viscosity?
What is more viscous?
 Viscosity: The state of being thick, sticky,
and semifluid in consistency.
 Increasing Viscosity:
Water
Orange Juice
Canola Oil
Maple Syrup
Motor Oil
Jams
Peanut butter
Grease
Cement
Glass
Viscosity of two liquids
10.1 The Nature of Volcanic Eruptions
Factors Affecting Eruptions
 Viscosity
• Factors affecting viscosity
- Temperature (hotter magmas are less viscous)
- Composition (silica content)
1. High silica—high viscosity
(e.g., rhyolitic lava stream stone)
2. Low silica—more fluid (e.g., basaltic lava)
Magma Composition
Magma Composition (Raise the BAR)
Magma Definitions:
 Basaltic: (Touchstone) Fine-grained black
volcanic rock.
 Andesitic: (Andes Mountains) Finegrained
brownish-gray volcanic rock
of intermediate composition between
basaltic and rhyolitic.
 Rhyolitic: (Lava-stream stone) Finegrained pale volcanic rock of granite
composition.
10.1The Nature of Volcanic Eruptions
Volcanic Material
 Lava Flows
• Basaltic lavas are more fluid because lower
silicates.
• Types of lava
- Pahoehoe lava (resembles braids in ropes)
- Aa lava (rough, jagged blocks)
 Gases
• One to 5 percent of magma by weight
• Mainly water vapor and carbon dioxide
Silica Levels
• The magma that flows from volcanoes
contains rock/minerals including silica.
• If the magma is high in silica, the higher the
viscosity and the less the lava flows.
• The lower the silica level, the lower the
viscosity and the more easily the lava flows.
Magma Flow Conclusions:
• Sooooooo…
Silicates slow lava flow
High Silicates = High Viscosity
Low Silicates = Low Viscosity
Pahoehoe (Ropy) Lava Flow
Pahoehoe lava flow video
Slow-Moving A’a Flow
A'a Lava flow
Which are they?
10.1 The Nature of Volcanic Eruptions
Factors Affecting Eruptions
 Dissolved gases
• Mainly water vapor (H2O) and carbon dioxide
(CO2), but also include, sulfur dioxide (SO2),
hydrogen chloride (HCl), hydrogen fluoride (HF)
• Gases expand near the surface
• 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
Factors Affecting Eruptions
 Dissolved gases
Many different gases can be ejected from a volcano
into the atmosphere. The gases provide the force to
eject the tephra from the central vent.
Dissolved Gases
• The more viscous (thicker) the
magma, the slower the gases
can rise through the central
vent and, therefore, the more
pressure that builds and the
more explosive the eruption.
Dissolved Gases
Sooooo…….
Thicker lava (more viscous) =
more pressure building up =
more explosive eruption
10.1 The Nature of Volcanic Eruptions
Volcanic Material
 Pyroclastic Materials
• Pyroclastic materials is the name given to
particles produced in volcanic eruptions.
• The fragments ejected during eruptions range in
size from very fine duct and volcanic ash (less
than 2 millimeters) to pieces that weigh several
tons.
• Sizes from 2 mm to 2 tons!
Tephra = Pyroclastic materials
10.1 The Nature of Volcanic Eruptions
Volcanic Material
 Pyroclastic Materials (Lava Solids!)
(pyro (fire) + clast (broken in pieces)
• Types of pyroclastic material
- Ash and dust—fine, glassy fragments
- Pumice—frothy, air-filled lava
- Lapilli—walnut-sized particles
- Cinders—pea-sized particles
• Particles larger than lapilli
- Blocks—hardened lava
- Bombs—ejected as hot lava
10.1 The Nature of Volcanic Eruptions
Volcanic Material
 Pyroclastic Clouds (Flows)
• These fast moving clouds of hot gas and rock
are the Real Killers!
- Normally hug the ground and flow downhill
- Up to 700 km/hr (450 mph), cannot outrun
them.
- People cooked in the 1000 °C (1,800 °F)
heat.
- Body instantly reduced to charcoal.
Pyroclastic Clouds: The Real Killers
Pyroclastic Flows
10.1 The Nature of Volcanic Eruptions
Types of Volcanoes
 The four main volcanic types are: 1) shield
volcanoes, 2) cinder cones, 3) composite cones,
and 4) lava domes.
 Anatomy of a Volcano
• A volcano is a mountain formed of lava and/or
pyroclastic material.
• A crater is the depression at the summit of a
volcano or that which is produced by a meteorite
impact.
• A conduit, or pipe, carries gas-rich magma to
the surface.
Anatomy of a “Typical” Volcano
Types of Volcanoes
There are three major types of
volcanoes (and one minor):
• Shield volcanoes
• Cinder cones
• Composite/Stratovolcano
• Lava domes (minor)
10.1 The Nature of Volcanic Eruptions
Types of Volcanoes
 Shield Volcanoes (made by lava liquid)
• Shield volcanoes are broad, gently sloping
volcanoes built from fluid basaltic lavas.
• Liquid, fluid lava flows versus pyrocastic solid
lava.
Shield Volcanoes (from liquid)
• Shield volcanoes have shallow-sloping
sides (looking like a shield from the
side or from the air).
• These volcanoes form from fluid lava
that can travel for long, long
distances before hardening. This
leaves a large volcano with a small
slope.
Shield Volcanoes
Hawaii Big Island: Shield flow volcano
Examples of Shield Volcanoes
• Kilauea
• Lo`ihi
• Mauna Loa
• Hualalai
• Mauna Kea
• Kohala
10.1 The Nature of Volcanic Eruptions
Types of Volcanoes
 Cinder Cones (made by lava solids)
• Cinder cones are small volcanoes built primarily
of pyroclastic material ejected from a single vent
and then cool quickly in the air. These accumulate
to make the sides of the volcano.
- Steep slope angle
- Rather small in height and size
- Frequently occur in groups
The cinders
up-close!
Cinder Cones
Cinder Cone: Pericutin, MX
Paricutin, MX: Cinder Cone Volcano
Cinder Cone: Sunset Crater,AZ
10.1 The Nature of Volcanic Eruptions
Types of Volcanoes
 Composite Cones (a.k.a. stratovolcanoes)
• Composite cones are volcanoes composed of
both lava flows and pyroclastic material.
- Most are adjacent to the Pacific Ocean
(e.g., Mt. Rainier).
- Large size and high in height
- Interbedded lavas and pyroclastics
- Most violent type of activity
Mount Vesubius/Pompeii: Composite Cone
Composite/stratovolcanoes
• Stratovolcanoes are tall and steep.
They have periodic, explosive
eruptions.
• These volcanoes are tall and steep
because they erupt viscous (thick) lava
that cools and hardens before is
spreads very far.
Composite/stratovolcanoes
• Composite means BOTH…both what?
• Made by BOTH lava solids (pyroclastic
materials) and by lava liquid (lava
flows).
Composite Cones
Stratovolcanoes
 Mammoth Mountain, CA (dormant)
 Mt. Fuji, Japan
 Mt. Pinatubo, Phillipines
 Mt. Etna, Sicily
 Mt. Shasta, CA (dormant)
 Mt. Unzen, Japan
 Mt. Rainier, WA
 Mt. St. Helens, WA
 Krakatoa, Indonesia
 Mt. Vesuvius, Italy
Mount St. Helens Before and
After the May 18, 1980, Eruption
Before
After
Krakatoa (1883) decreased global
temperatures by 4⁰ F for 5 years
Lava Domes
• Lava domes are a type of volcano that
are formed by lava that is too viscous
to flow any great distance.
• The thick lava extrudes from the
central vent, cools down and hardens
around the vent. Lava domes can
occur by themselves or on the flanks of
large composite volcanoes.
Lava Domes
Lava Domes at Mount St.
Helens
Lava Domes
• Lava domes are often very,
very dangerous. Thinking
about the viscosity of the
lava, why do you think they
are dangerous?
Lava Domes
Lava
Dome
Profiles of Volcanic Landforms
10.1 The Nature of Volcanic Eruptions
Other Volcanic Landforms
 Calderas
• Calderas are large depressions in volcanoes.
• Nearly circular
• Formed by collapse
• Size exceeds one kilometer in diameter
10.1 The Nature of Volcanic Eruptions
Other Volcanic Landforms
 Volcanic Neck
• A volcanic neck is a cylindrical-shaped landform
standing above the surface created by magma
solidifying in the vent of a volcano. Erosion of the
sides of the volcano exposes the neck.
10.1 The Nature of Volcanic Eruptions
A Volcanic Neck:
Devil’s Tower, WY
10.1 The Nature of Volcanic Eruptions
Build Your Own Volcano!
Click here to: Build Your Own Volcano!
10.1 The Nature of Volcanic Eruptions
Other Volcanic Landforms
 Lava Plateaus
• Wide areas formed when fluid basaltic lava
extrudes from crustal fractures called fissures.
 Complete Study Guide 10.1
Supervolcanoes!
• A supervolcano is a volcano that
produces an eruption that ejects more
than 1,000 km3 of tephra (240 mi3).
• This amount of ejecta is thousands of
times larger than most historic volcanic
eruptions.
• Supervolcanoes usually occur
when magma in the Earth rises into
the crust from a hotspot but is
unable to break through the crust.
Pressure builds in a large and
growing magma pool until the crust
is unable to contain the
pressure….
…. and BAM!!!!!!!!!!!!!!!!!!!!!!!!!
The world is changed!
Six, Pretty-Famous
Supervolcanoes!
1)
2)
3)
4)
Yellowstone, WY
Long Valley, CA (Mammoth Mt.)
Valles Caldera, NM.
Lake Toba, Indonesia (Largest eruption in
the past 25 million years! It erupted 74,000
years ago.)
5) Taupo Volcano, New Zealand;
6) Aira Caldera, Japan.
Cross-Section through Mammoth
Supervolcano
Hotspot Locations for Yellowstone
Supervolcano (million of years ago)
• An eruption of a
supervolcano can cause a
global climatic change
(typically a mini-Ice Age,
“volcanic winter”) and major
species extinctions!