Volcanic ActivityNature and Results

MagmaThe primary factor determining the nature of

volcanic eruptions is in the magmaDiffering composition, temperature, and dissolved

gas content affect activityAll three factors affect magma’s viscosity, which

in turn determines the characteristics of an eruption

Magma TemperatureViscosity is obviously affected by temperatureThe 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 contentMagma 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 flowThere 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 GraniticSilica Content Least (50%) Intermediate

(60%)Most (70%)

Typical Minerals Ca feldsparPyroxeneOlivine

Na feldsparAmphibolePyroxeneMica

K feldsparQuartzMicaAmphibole

Viscosity Least Intermediate HighestTendency to Form Lavas

Highest Intermediate Least

Tendency to Form Pyroclastics

Least Intermediate Highest

Density Highest Intermediate LowestMelting 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 above2. Hot streams of high-pressure gases expand cracks and develop passageway to surface3. Hot gases armed with rock fragments erode walls of passageway to enlarge conduit4. Magma moves upward to produce surface activity5. Volcanic pipe becomes choked with debris and must be cleared before erupting again

Volcanic Extrusion: BasaltBasaltic lavas low silica content fluidBasalt lavas flow in thin, broad sheets or tonguesTwo 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’tOceanic 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: PyroclastsGranitic and andesitic lavas don’t flow as easily

as basaltic flows, so they generally explodeAny 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 particlesProduced 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 tuffSometimes 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 formBlocks are large pyroclasts made of hardened lavaBombs are large pyroclasts made of semimolten

or incandescent lavaAs bombs get ejected, they become “streamlined”

as they fly through the airBoth 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 ventAt 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 calderaMagma 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

extrusionsShield volcanoes are formed by frequent eruptions of thin

flows of very fluid basaltic lavasAs the structure enlarges, flank eruptions occur along with

the summit eruptionsCollapse of the summit area frequently follows each

eruptive phaseLavas continually increase in viscosity over time, resulting

in thicker, shorter flowsThis explains why older volcanoes have steeper summits

than younger ones

Cinder Cone VolcanoesBuilt from ejected lava fragmentsUsually have very steep slopes, but are rather

small (300 meters high)Often form as parasitic cones on or near larger

volcanoes; also form in groupsLava 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 extrudeLava 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