volcanic activity nature and results. magma the primary factor determining the nature of volcanic...
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Volcanic ActivityNature and Results
MagmaThe primary factor determining the nature of
volcanic eruptions is in the magmaDiffering 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 TemperatureViscosity 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 CompositionThe 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 viscositySilica molecules link into long chains even before
crystallization begins, impeding flow
There are three major types of magma: basaltic, andesitic, and graniticBasaltic: 50% silica, low viscosityAndesitic: 60% silica, intermediate viscosityGranitic: 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
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 feldsparPyroxeneOlivine
Na feldsparAmphibolePyroxeneMica
K feldsparQuartzMicaAmphibole
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: ~70% water vapor ~15% CO2
~5% nitrogen compounds ~5% sulfur compounds ~5% other chlorine, hydrogen, and argon compounds
Besides propelling magma, gases shape the conduit1. Intense heat from magma body cracks rock above
2. Hot streams of high-pressure gases expand cracks and develop passageway to surface
3. Hot gases armed with rock fragments erode walls of passageway to enlarge conduit
4. Magma moves upward to produce surface activity
5. Volcanic pipe becomes choked with debris and must be cleared before erupting again
Volcanic Extrusion: BasaltBasaltic 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 projectionsActive flows are cool and thick, resulting in jagged textureEscaping gases fragment the cool surface and produce voids
and spines in lavaAs molten interior advances, outer crust gets broken furtherBasically 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 lavaAs lava flows occur, the outer lava congeals faster than
interior lavaThe rocks around the interior lava also insulate itTherefore, interior lava cools much more slowly, allowing
it to travel much further before congealingAs it flows, it leaves behind tunnels where the outer lava
cooled, but the inner didn’t
Oceanic FlowsWhen 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: PyroclastsGranitic 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 DustFinest size of all particles
Produced when extruded lava contains so many gas bubbles that it resembles frothThink 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 CindersCinders
Pea-sizedForm when blobs of lava get pulverized by
escaping gasContain numerous voids
LapilliWalnut-sizedSimply a size classification; any particle within size
range is a lapilli
Blocks and BombsBoth 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
VolcanoesA 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 VolcanoesBroad, 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 VolcanoesBuilt 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 PipesVolcanoes 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 EruptionsFissures 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 plainThe general name for this type of flow is flood
basalts
This is most common in the ocean near the mid-ocean ridge
Pyroclastic FlowsAlmost identical to fissure eruptions, but
involves high-silica 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