Volcanic Activity
Nature and Results
Magma
 The primary factor determining the nature of volcanic
eruptions is in the magma
 Differing composition, temperature, and dissolved gas
content affect activity
 All three factors affect magma’s viscosity, which in turn
determines the characteristics of an eruption
Magma Temperature
 Viscosity is obviously affected by temperature
 The hotter the lava, the easier it can flow, making the
results of the cooling of lava flows different depending
on initial temperature
 As a lava flow cools and begins to congeal, its mobility
decreases and eventually the flowing halts, creating
igneous rocks of various types
 Temperature can affect texture, size, and cleavage of
igneous rocks
Magma Composition
 The major difference between igneous rocks (and
therefore their precursor magmas) is silica content
 Magma viscosity is directly related to silica content- the
higher the silica, the greater the viscosity
 Silica molecules link into long chains even before
crystallization begins, impeding flow
 There are three major types of magma: basaltic,
andesitic, and granitic
 Basaltic: 50% silica, low viscosity
 Andesitic: 60% silica, intermediate viscosity
 Granitic: 70% silica, high viscosity
Magma Gas Content
 Dissolved gases tend to increase the fluidity of magma
 Gases also have the ability to propel molten rock from a volcanic vent (more
important)
 As magma moves into a near-surface environment, confining pressure near
the top is reduced, allowing previously dissolved gases to be released
suddenly
 Low-viscosity magmas (basaltic) tend to be quiescent because they allow the
gas bubbles to pass through relatively easily
 End up making lava fountains by carrying incandescent lava hundreds of meters into
the air with escaping gas
 High-viscosity magmas (granitic) tend to be more explosive because they
impede upward migration of gases
 The gases collect as bubbles and pockets that increase in size and pressure until
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they are ejected explosively
Once upper magma is ejected, reduced pressure on molten rock below causes it to
be blown at as well, resulting in a series of explosions rather than just one
Could theoretically continue endlessly; however, since gas bubbles move slowly,
they only get enough pressure to be explosive near the top of the thermal vent
Property Variations of Magma
Due to Composition
Property
Basaltic
Andesitic
Granitic
Silica Content
Least (50%)
Intermediate (60%)
Most (70%)
Typical Minerals
Ca feldspar
Pyroxene
Olivine
Na feldspar
Amphibole
Pyroxene
Mica
K feldspar
Quartz
Mica
Amphibole
Viscosity
Least
Intermediate
Highest
Tendency to Form
Lavas
Highest
Intermediate
Least
Tendency to Form
Pyroclastics
Least
Intermediate
Highest
Density
Highest
Intermediate
Lowest
Melting Point
Highest
Intermediate
Lowest
Volcanic Extrusion: Gas
 Think of gases in magma like CO2 in soda
 As soon as pressure is reduced, gases escape
 Gases compose 1-5% of total magma weight, but can still exceed thousands of tons
of emission per day
 Composition:
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~70% water vapor
~15% CO2
~5% nitrogen compounds
~5% sulfur compounds
~5% other chlorine, hydrogen, and argon compounds
 Besides propelling magma, gases shape the conduit
1.
2.
3.
4.
5.
Intense heat from magma body cracks rock above
Hot streams of high-pressure gases expand cracks and develop passageway to surface
Hot gases armed with rock fragments erode walls of passageway to enlarge conduit
Magma moves upward to produce surface activity
Volcanic pipe becomes choked with debris and must be cleared before erupting again
Volcanic Extrusion: Basalt
 Basaltic lavas  low silica content  fluid
 Basalt lavas flow in thin, broad sheets or tongues
 Two types of basaltic lava flows:
 Pahoehoe- occurs when fluid lavas form smooth skin that
sometimes wrinkles when sub-surface lava continues to
advance
 Aa- lava has a surface of rough, jagged blocks with
dangerously sharp edges and spiny projections
 Active flows are cool and thick, resulting in jagged texture
 Escaping gases fragment the cool surface and produce voids
and spines in lava
 As molten interior advances, outer crust gets broken further
 Basically looks like a pile of rubble by the end of it
 Hawaiian lava flows are pahoehoe
Basalt cont.
 As lava flows harden, tunnels form that were once used to
transport molten lava
 As lava flows occur, the outer lava congeals faster than interior
lava
 The rocks around the interior lava also insulate it
 Therefore, interior lava cools much more slowly, allowing it to
travel much further before congealing
 As it flows, it leaves behind tunnels where the outer lava
cooled, but the inner didn’t
 Oceanic Flows
 When lava flows enter the ocean, the outer zones quickly
congeal, but the lava within usually moves forward and breaks
the hard surface
 This occurs repeatedly, creating pillow lava
Volcanic Extrusion: Pyroclasts
 Granitic and andesitic lavas don’t flow as easily as
basaltic flows, so they generally explode
 Any particle produced by the processes of superheated
gases blowing pulverized rock and lava from the
volcanic vent is called a pyroclastic material
 Vary in size as ash and dust, lapilli and cinders, and
blocks and bombs
Ash and Dust
 Finest size of all particles
 Produced when extruded lava contains so many gas bubbles
that it resembles froth
 Think froth from a bottle of champagne
 As hot gases expand explosively, lava disseminates into very
fine fragments
 When it falls, glassy shard often fuse to form welded tuff
 Sometimes froth-like lava is ejected in larger pieces as
pumice
Lapilli and Cinders
 Cinders
 Pea-sized
 Form when blobs of lava get pulverized by escaping gas
 Contain numerous voids
 Lapilli
 Walnut-sized
 Simply a size classification; any particle within size range
is a lapilli
Blocks and Bombs
 Both are considered any particle larger than lapilli, but
they differ in extrusive form
 Blocks are large pyroclasts made of hardened lava
 Bombs are large pyroclasts made of semimolten or
incandescent lava
 As bombs get ejected, they become “streamlined” as
they fly through the air
 Both usually end up on the slope of the volcano, but
they can sometimes be ejected like rockets by
escaping gases
Volcanoes
 A volcano is a mountainous accumulation of material
formed from successive eruptions from a central vent
 At the summit of many volcanoes is a crater which is
connected to a magma chamber by a pipe-like conduit
 If a volcano has a summit depression (crater) that is larger
than 1 kilometer, it is known as a caldera
 Magma doesn’t always flow out the central vent; if easier, it
may push through fissures located on flanks of volcano
 Known as a parasitic cone
 If they only extrude gases, the secondary vents are called
fumaroles
Shield Volcanoes
 Broad, slightly domed structure formed from fluid lava
extrusions
 Shield volcanoes are formed by frequent eruptions of thin
flows of very fluid basaltic lavas
 As the structure enlarges, flank eruptions occur along with
the summit eruptions
 Collapse of the summit area frequently follows each eruptive
phase
 Lavas continually increase in viscosity over time, resulting in
thicker, shorter flows
 This explains why older volcanoes have steeper summits
than younger ones
Cinder Cone Volcanoes
 Built from ejected lava fragments
 Usually have very steep slopes, but are rather small
(300 meters high)
 Often form as parasitic cones on or near larger
volcanoes; also form in groups
 Lava rarely issues from the top except as a fountain
because the walls are generally too weak to support
the pressurized magma moving upward through the
conduit
Stratovolcanoes
 A.k.a. composite cones
 Produced when relatively viscous lavas of andesitic composition are
extruded
 Composite cone extrudes viscous lava for a long time, then suddenly
violently ejects pyroclastic material and deposits it near the summit
 Creates alternating lava/pyroclast layering
 Vesuvius was a stratovolcano
 Nuee Ardente
 Occurs when hot gases infused with incandescent ash are ejected
 These “glowing avalanches” are black in daylight and red at night
 Although very dense, they are supported by expanding gas emitted from
hot lava particles, and thus flow nearly frictionlessly down slopes
 Most devastating type of volcanic eruption
Stratovolcanoes cont.
 Lava Domes
 When highly viscous lava is extruded by a volcano, it can
sometimes form a lava dome
 This occurs when the thick lava is squeezed out of the
vent and create a bulbous mass at the opening of the
vent
 They’re like giant volcano buttplugs
Volcanic Necks and Pipes
 Volcanoes are eroded by natural erosion forces
 Cinder cones are the easiest to erode because they are
composed of unconsolidated materials
 Rock occupying the vent is often more resistant and remains
standing above the terrain after the cone has vanished
 Called volcanic necks
 Most volcanic conduits extend to the upper mantle; others
can reach the asthenosphere
 Ferromagnesian-rich pipes extend a distance of 200
kilometers into the asthenosphere, allowing observation of
the largely unknown characteristics of the layer
Fissure Eruptions
 Fissures are cracks or fractures in the crust through
which large amounts of volcanic material extrude
 Lava is usually extruded from several vents along
fissures, resulting in a wide distribution of volcanic
material
 Fissure eruptions can extrude very fluid basaltic lava,
creating a lava plain
 The general name for this type of flow is flood basalts
 This is most common in the ocean near the mid-ocean
ridge
Pyroclastic Flows
 Almost identical to fissure eruptions, but involves highsilica magmas rather than basaltic magmas
 Ash and pumice fragments are the most common
constituents of pyroclastic flows
 When ejected, they move away from the vent at high
speeds and blanket extensive areas before coming to
rest
 After deposition, they closely resemble lava flows