Transcript Volcanoes and Igneous Activity Earth - Chapter 4


Characteristics of a magma determine the
“violence” or explosiveness of a volcanic
eruption
 Composition
 Temperature
 Dissolved gases

The above three factors actually control the
viscosity of a given magma

Viscosity is a measure of a material’s resistance to flow
(how sticky)

Factors affecting viscosity
 Temperature - Hotter magmas are less viscous
 Composition - Silica (SiO2) content
 Higher silica content = higher viscosity
(e.g., felsic lava such as rhyolite)
Felsic = granitic composition = rock is composed almost entirely of
light colored silicates
 Lower silica content = lower viscosity
(e.g., mafic lava such as basalt)
Mafic = Basaltic = mostly dark silicates and feldspar


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Denotes compostition of silicate
minerals, magmas and rocks with
heavy elements.
Ma from magnesium and FIC from
latin word for iron.
Dark
Produced at spreading centersrifts


Examples include, olivine,
pyroxene, amphibole, biotite
mica, and plagioclase feldspars.
More silica, oxygen, aluminum,
and potassium

Fel for feldspar, Sic –from silica
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Light in color
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Denotes composition of silicate
minerals, magma, and rocks with
low amount of heavy elements.
Examples: Quartz, muscovite
mica, orthoclase feldspars,
granite
Are these mafic or felsic?
Remember the oceanic crust is made mostly of basalt
 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
 In summary
 Basaltic lavas = mild eruptions (less silica)
 Rhyolitic or andesitic lavas = explosive eruptions
(more silica)
VIOLENT

Composition
LESS VIOLENT

• More silica
• Rhyolite – Felsic


• Cannot escape easily
Gas Content
• Cannot escape easily



Temperature
Composition
Less silica
Basalt - Mafic
Gas Content
Temperature
 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
Figure 5.5 A
Figure 5.5 B
 Pyroclastic
materials – “fire fragments”
 Types of pyroclastic debris
 Ash and dust - fine, glassy fragments
 Pumice - porous rock from “frothy” lava
 Cinders - pea-sized material
 Lapilli - walnut-sized material
 Particles larger than lapilli
 Blocks - hardened or cooled lava
 Bombs - ejected as hot lava
Bomb is approximately 10 cm long
Figure 5.7
 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
 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
Figure 5.10
 Cinder cone
 Built from ejected lava (mainly cinder-sized)
fragments
 Steep slope angle
 Small size
 Frequently occur in groups
Notice the vent at
the top
Discussion question; why would
people live so close to Arenal?
 Stratovolcano
 Vent
 Basalt
Flow
 Steep Slopes
 Gases easily escape
Another stratovolcano
Figure 5.14
 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)
 Composed of interbedded lava flows and pyroclastic
debris
 Most violent type of activity (e.g., Mt. Vesuvius)
Is this a crater or a caldera?
 Crater - summit
depression < 1 km
diameter
 Caldera - summit
depression > 1 km
diameter produced
by collapse following
a massive eruption

Common in subduction zones

The magma that forms stratovolcanoes rises when water trapped
both in hydrated minerals and in the porous basalt rock of the
upper oceanic crust, is released into mantle rock of the
asthenosphere above the sinking oceanic slab.

The release of water from hydrated minerals is termed
"dewatering," and occurs at specific pressures and temperatures
for each mineral, as the plate descends to greater depths.

The water freed from the rock lowers the melting point of the
overlying mantle rock, which then undergoes partial melting and
rises due to its lighter density relative to the surrounding mantle
rock, and pools temporarily at the base of the lithosphere.
The magma then rises through the crust, incorporating silica-rich
crustal rock, leading to a final intermediate composition (see
Classification of igneous rock).
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
When the magma nears the surface, it pools in a magma chamber
under the volcano. There, the relatively low pressure allows water
and other volatiles (CO2, S2−, Cl−) dissolved in the magma to escape
from solution, as occurs when a bottle of carbonated water is
opened. Once a critical volume of magma and gas accumulates, the
obstacle provided by the volcanic cone is overcome, leading to a
sudden explosive eruption.[citation needed]
 Central
America
 Cascades of North
America
 South America –
Andes
Figure 5.9
Figure 5.12
 Nuée ardente – A deadly pyroclastic flow
 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 at speeds up to
200 km per hour
 Lahar – volcanic mudflow
 Mixture of volcanic debris and water
 Move down stream valleys and volcanic slopes, often
with destructive results
Figure 5.20
 Caldera
 Steep-walled depressions at the summit
 Generally > 1 km in diameter
 Produced by collapse
 Pyroclastic
flow
 Felsic and intermediate magmas
 Consists of ash, pumice, and other debris
 Material ejected at high velocities
 Example = Yellowstone plateau
Figure 5.22
 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
Figure 5.26
Lava dome
 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
Figure 5.27
 Most
magma is emplaced at depth in the
Earth
 Once cooled and solidified, is called a pluton
 Nature
of plutons
 Shape - tabular (sheetlike) vs. massive
 Orientation with respect to the host
(surrounding) rock
 Concordant vs. discordant
 Types
of intrusive igneous features
 Dike – a tabular, discordant pluton
 Sill – a tabular, concordant pluton (e.g.,
Palisades Sill in New York)
 Laccolith
 Similar to a sill
 Lens or mushroom-shaped mass
 Arches overlying strata upward
Figure 5.28 B
Figure 5.30
 Intrusive
igneous features continued
 Batholith
 Largest intrusive body
 Surface exposure > 100+ km2 (smaller bodies are
termed stocks)
 Frequently form the cores of mountains
Batholiths
of western
North
America
Figure 5.32
 Take
10 minutes to work with a group to
label the following pictures. You may use
your book
 Global
distribution of igneous activity is not
random
 Most volcanoes are located within or near
ocean basins
 Basaltic rocks = oceanic and continental
settings
 Granitic rocks = continental settings
Figure 5.34
 Igneous
activity at plate margins
 Spreading centers
 Greatest volume of volcanic rock is produced along
the oceanic ridge system
 Mechanism of spreading
 Decompression melting of the mantle occurs as the
lithosphere is pulled apart
 Large quantities of basaltic magma are produced
 Subduction zones
 Occur in conjunction with deep oceanic trenches
 Partially 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
 Associated with the Pacific Ocean Basin
 Region around the margin is known as the “Ring of
Fire”
 Majority of world’s explosive volcanoes
 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)
 The
basic premise
 Explosive eruptions emit huge quantities of
gases and fine-grained debris
 A portion of the incoming solar radiation is
reflected and filtered out
 Past
examples of volcanism affecting
climate
 Mount Tambora, Indonesia – 1815
 Krakatau, Indonesia – 1883
 Modern
examples
 Mount St. Helens, Washington - 1980
 El Chichón, Mexico - 1815
 Mount Pinatubo, Phillippines - 1991
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