Volcanism
Movement of magma inside the
earth. (ground go boom)
11/22
Volcano and earthquake quiz
Discussion
Exploring volcanism
What are volcanoes
Lava- Magma that reaches the earths
surface.
Vent- The opening that lava flows through.
Pyroclastic Material- Pyro; greek word for
fire. Clastic; small particles or fragments.
These are the hot fire fragments that are
ejected from a volcano.
Volcano- opening in the earth’s crust where
magma flows.
Anatomy of a Volcano
Volcanic activity begins in the upper mantle
of the earth also known as the
asthenosphere.
Cross-section of a Volcano
Parts of a volcano
Crater- opening at the top of a volcano.
Caldera- Large opening formed at the top of a volcano when the roof of a magma dome
collapses.
Crater Lake- lake formed in the crater of a volcano from melting snow or rain.
Pyroclastic Materials
Types of pyroclastic materials
Dust- the smallest form of pyroclastic materials erupting from a volcano.
Dust from a volcanic eruption can travel into the upper reaches of the earth’s
atmosphere. Once it reaches the upper atmosphere, miles above the earth’s surface,
atmospheric winds can move this dust around the globe.
Ash- Sand size fragments ejected from a volcano.
These fragments will normally be pushed up into the air in and around the
volcanic source. Once in the air they will eventually fall back to earth.
Cinders- Thumbnail size fragments that are ejected from a volcano.
These hot fragments fly through very close to there source. As they fly through
the air, they cool and release a small amount of gas causing tiny air holes.
Bombs- Fist size, spindle shaped fragments. These can range in size from small to
large depending on the explosiveness of the eruption.
Volcanic Eruptions
Volcanic eruptions can vary depending on the size of the volcano and the
heating source or energy available.
Volcanic Classifications
Volcanic eruptions can be classified as quiet or explosive.
Quiet eruption- lava flows freely through a vent or fissure.
Explosive eruptions- pyroclastic materials and gases shoot into the air.
Basic types of Volcanoes;
Shield Volcano- Domed shaped volcano composed of layers of solid, hardened
lava.
▪ Wide base, with gentle dome shaped slopes.
▪ Large crater or opening called a caldera.
Plate tectonics and
volcanic activity
▪ Global distribution of volcanoes
▪ Most volcanoes are located within
or near ocean basins
▪ Basaltic rocks: in oceanic and
continental settings
▪ Granitic rocks: in continental
settings
The nature of
volcanic eruptions
▪ Three physical characteristics of a magma
control its viscosity, the physical property
that ultimately determines the “violence” or
explosiveness, of the eruption:
▪ Composition
▪ Temperature
▪ Dissolved gases
The magma affects the severity
of a volcanic eruptions
▪ In summary
–Basaltic Magmas =
mild eruptions
–Rhyolitic or Andesitic
Magmas = explosive
eruptions
The nature of volcanic eruptions
▪ Viscosity is a measure of a
material’s resistance to flow
▪ Factors affecting viscosity
▪ Temperature - Hotter magmas
are less viscous
▪ Composition - Silica (SiO2)
content
The nature of volcanic eruptions
–Higher silica content = higher
viscosity (e.g., felsic lava such
as rhyolite)
–Lower silica content = lower
viscosity (e.g., mafic lava such
as basalt)
The nature of volcanic eruptions
▪ Dissolved gases
–Gas content affects magma
mobility
–Gases expand within a magma as
it nears the Earth’s surface due
to decreasing pressure
–The violence of an eruption is
related to how easily gases escape
from magma
Materials extruded
from a volcano
▪ Lava flows
▪ Basaltic lavas exhibit fluid behavior
▪ Types of basaltic flows
–Pahoehoe lava (resembles a twisted or
ropey texture)
–Aa lava (rough, jagged blocky texture)
▪ Dissolved gases
▪ 1% - 6% by weight
▪ Mainly H2O and CO2
Materials extruded
from a volcano
▪ Pyroclastic materials – “fire fragments”
• Types of pyroclastic debris
–Ash and dust - fine, glassy
fragments
–Pumice - porous rock from
“frothy” lava
–Cinders - pea-sized material
Materials extruded
from a volcano (continued)
▪ Pyroclastic materials – “fire fragments”
• Types of pyroclastic debris
–Lapilli - walnut-sized material
–Particles larger than lapilli
▪ Blocks - hardened or cooled
lava
▪ Bombs - ejected as hot lava
Volcanoes
▪ General features
▪ Opening at the summit of a volcano
–Crater - summit depression < 1 km
diameter
–Caldera - summit depression > 1 km
diameter produced by collapse following
a massive eruption
▪ Vent – surface opening connected to the
magma chamber
▪ Fumarole – emit only gases and smoke
Volcanoes
▪ Types of volcanoes
▪ Shield volcano
–Broad, slightly domed-shaped
–Generally cover large areas
–Produced by mild eruptions of
large volumes of basaltic lava
–Example: Mauna Loa on Hawaii
Volcanoes
▪ Cinder cone
–Built from ejected lava (mainly
cinder-sized) fragments
–Steep slope angle
–Small size
–Frequently occur in groups
Volcanic Classifications
Cinder Cone- Dust, ash and cinders are thrown out of the crater and settle to
form the cone.
▪ Steep sides - (34% angle of repose)
▪ Narrow base (not very tall)
▪ Explosive eruption
Volcanoes
▪ Composite cone (stratovolcano)
–Most are located adjacent to
the Pacific Ocean (e.g.,
Fujiyama, Mt. St. Helens)
–Large, classic-shaped volcano
(1000’s of ft. high and several
miles wide at base)
Mt. St. Helens – prior
to the 1980 eruption
Mt. St. Helens (after
the 1980 eruption)
Volcanoes
▪ Composite cone (stratovolcano) continued
–Composed of interbedded lava
flows and pyroclastic debris
–Most violent type of activity
(e.g., Mt. Vesuvius)
Stopped 2nd block
Volcanic Classifications
Composite Volcano (Strato)- Volcano made of alternating layers of pyroclastic
dust, ash and hardened lava.
▪ Quiet and explosive eruptions
▪ Wide base (miles across)
▪ Very Tall (Thousands of feet tall)
Volcanic Hazards
Nuée ardente –
▪ A Fiery pyroclastic flow made of
hot gases infused with ash and
other debris
▪ Also known as “glowing
avalanches”
Move down the slopes of a volcano with
velocities approaching 200 km/hour
Volcanic Hazards
Lahar – volcanic landslide or
mudflow
Mixture of volcanic debris and
water
Move down slopes of volcano and
stream valleys with velocities of 30
to 60 miles/hour
volcanic hazards
▪ Pyroclastic flow
▪ Felsic and intermediate magmas
▪ Consists of ash, pumice, and other
debris
▪ Material ejected at high velocities
▪ Example: Yellowstone plateau
Other volcanic landforms
▪ Caldera
▪ Steep-walled depressions at the
summit
▪ Generally > 1 km in diameter
▪ Produced by collapse
▪ Example: Crater Lake, Oregon
Other volcanic landforms
▪ Fissure eruptions and lava plateaus
▪ Fluid basaltic lava extruded from
crustal fractures called fissures
▪ Example: Columbia River Plateau
▪ Lava domes
▪ Bulbous mass of congealed lava
▪ Associated with explosive eruptions of
gas-rich magma
Other volcanic landforms
▪ Volcanic pipes and necks
▪ Pipes - short conduits that
connect a magma chamber to the
surface
▪ Volcanic necks (e.g., Ship Rock,
New Mexico) - resistant vents
left standing after erosion has
removed the volcanic cone
Shiprock, New Mexico
Intrusive igneous activity
▪ Most magma is emplaced at
depth in the Earth
▪ Once cooled and solidified, it
is called a pluton
Intrusive igneous activity
▪ Nature of plutons
▪ Shape - tabular (sheetlike) vs.
massive
▪ Orientation with respect to
the host (surrounding) rock
–Concordant vs. discordant
Intrusive igneous activity
▪ Types of intrusive igneous features
▪ Dike – a tabular, discordant
pluton
▪ Sill – a tabular, concordant
pluton (e.g., Palisades Sill in
New York)
Intrusive igneous activity
▪ Types of intrusive igneous features
▪ Lacolith
–Similar to a sill
–Lens or mushroom-shaped mass
–Arches overlying strata upward
Intrusive igneous activity
▪ Intrusive igneous features continued
▪ Batholith
–Largest intrusive body
–Surface exposure > 100+ km2
(smaller bodies are termed
stocks)
–Frequently form the cores of
mountains
Plate tectonics and
igneous activity
▪ Global distribution of igneous activity
▪ Most volcanoes are located within
or near ocean basins
▪ Basaltic rocks: oceanic and
continental settings
▪ Granitic rocks: continental settings
Plate tectonics and
igneous activity
▪ Igneous activity at plate margins
▪ Spreading centers
–Greatest volume of volcanic
rock is produced along the
oceanic ridge system
Plate tectonics and
igneous activity
▪ Igneous activity at plate margins
▪ Mechanics of spreading
▪ Decompression melting of the
mantle occurs as the lithosphere
is pulled apart
▪ Large quantities of basaltic
magma are produced
Plate tectonics and igneous activity
▪ Subduction zones
–Occur in conjunction with deep
oceanic trenches
–Location of partial melting of
descending plate and upper mantle
–Rising magma can form either
▪ An island arc if in the ocean
▪ A volcanic arc if on a continental
margin
Plate tectonics and
igneous activity
▪ Subduction zones are associated
with the Pacific Ocean Basin
▪ The region representing the
Pacific Rim is known as the
“Ring of Fire”
▪ Location of majority of
world’s explosive volcanoes
Plate tectonics and igneous activity
▪ Intraplate volcanism
▪ Occurs within a tectonic plate
▪ Associated with mantle
plumes
▪ Localized volcanic regions in the overriding
plate are called a hot spot
–Produces basaltic magma sources in
oceanic crust (e.g., Hawaii and Iceland)
–Produces granitic magma sources in
continental crust (e.g., Yellowstone Park)
Volcanoes and climate
▪ The basic premise
▪ Explosive eruptions emit huge
quantities of gases and finegrained debris
▪ A portion of the incoming solar
radiation is reflected and
filtered out
Volcanoes and climate
▪ Past examples of volcanism
affecting climate
▪ Mount Tambora, Indonesia –
1815
▪ Krakatau, Indonesia – 1883
Volcanoes Can Impact Regional
And Global Climate
▪ Modern examples
▪ Mount St. Helens, Washington 1980
▪ El Chichón, Mexico - 1982
▪ Mount Pinatubo, Philippines - 1991