HawaiianHawaiian

MagmaMagmaFluid

Explosive Explosive activityactivity

Fountains, low

eruption columns

Effusive Effusive activityactivity

Thin, extensive

flows

Dominant Dominant ejectaejecta

Bombs

StructuresStructuresLava

lakes and cones

StrombolianStrombolian

Moderately fluid

Weak to violent ejection

Thick, non-extensive

lava

Bombs, cinders, vitric

ash

Cinder cones

VulcanianVulcanian

Viscous

Moderate to violent ejection of

solids

Flows commonly

absent

Blocks and ash,

pumice, bombs

Ash cones

PeleeanPeleean

Viscous

Ejections of solids, nuee ardents

Domes

Lithic ejecta,

blocks and ash,

pumice

Domes, spines

PlinianPlinian

Viscous

Sudden ejection of

ash

Ash flows

Glassy ash and pumice

Ash beds

SurtseyanSurtseyan

Viscous

Violent ejections

Pillowed lava flows

Block and ash,

accretionary lapilli

Tuff rings

More Fluid --> Hotter + Easier gas release (less explosive) + Less siliceous

Eruption ProductsEruption Products

Eruption products

Lava typesLava types BasalticBasaltic

- Pahohoe- Pahohoe- A’a- A’a- Pillows- Pillows

AndesiticAndesitic- Blocky- Blocky

RhyoliticRhyolitic-Pyroclasts -Pyroclasts -Pyroclastic rocks-Pyroclastic rocks-Pyroclastic deposits-Pyroclastic deposits

In general

Molten rock (magma) that pours or oozes onto the Earth's surface is called lava. The higher a lava's silica content, the more viscous it becomes.

For example, low-silica basalt lava can form fast-moving (10-30 miles per hour), narrow lava streams or spread out in broad sheets up to several miles wide. In contrast, higher-silica andesite and dacite lava flows tend to be thick, move slowly, and travel short distances from a vent.

Basaltic lavaErupt primarily from fissure systems and shieldErupt primarily from fissure systems and shield volcanoes (eg., Hawaii). volcanoes (eg., Hawaii).

These fluid lava flows can be subdivided into two types, These fluid lava flows can be subdivided into two types, based primarily on the nature of lava flow surfaces:based primarily on the nature of lava flow surfaces: Pahoehoe LavaPahoehoe Lava -- (pa-hoy-hoy) Hawaiian for -- (pa-hoy-hoy) Hawaiian for

“smooth, ropy lava”. “smooth, ropy lava”. Aa lavaAa lava -- Hawaiian for “rocky lava”. -- Hawaiian for “rocky lava”.

The lava is identical in both pahoehoe and aa lava flows, The lava is identical in both pahoehoe and aa lava flows, the difference comes from the amount of lava erupted the difference comes from the amount of lava erupted and the speed of cooling.and the speed of cooling.

Basaltic lavaPahoehoe Lava -- Hawaiian for “smooth, ropy lava”. Surfaces are smooth, billowy, or ropy. Formed from less viscous lava that advances slowly. Forms as the outer layer of the lava cools, then is dragged and folded as the flow continues to move beneath the surface.

LobesLobes

Aa lava -- Hawaiian for “rocky lava”. Surfaces are fragmented, rough, and spiny, with a "cindery" appearance. Forms when lava initially flows rapidly resulting in heat loss and an increase in viscosity.As the flow continues to move, the surface layer is broken into jagged pieces.

Basaltic lavaBasaltic lava

Pahoehoe is common near the source of a basaltic flow, Pahoehoe is common near the source of a basaltic flow, where the lava is hottest, and aa is normally found farther where the lava is hottest, and aa is normally found farther from the source, where the lava has cooled off significantly. from the source, where the lava has cooled off significantly.

Basaltic lavaBasaltic lava

Basaltic lava

Intermediate basaltic lava flow with characters of both Aa and Pahoehoe on Etna – Sept. 2004Photograph GS -

Basaltic lavaBasaltic lava

Important note: Important note: Pahoehoe flows cannot turn into Pahoehoe flows cannot turn into aa flows, and vice versa. aa flows, and vice versa.

For example, pahoehoe flows cannot evolve from For example, pahoehoe flows cannot evolve from lava already erupted with an aa surface. The new lava already erupted with an aa surface. The new lava has to come from deep within the parent flow lava has to come from deep within the parent flow and has a separate crystallization history.and has a separate crystallization history.

Pillow lavaPillow lava -- when lava is erupted under water or ice. As lava is -- when lava is erupted under water or ice. As lava is extruded, the water (or ice) quickly chills the outer layer. Molten extruded, the water (or ice) quickly chills the outer layer. Molten lava beneath the chilled surface eventually breaks through the skin lava beneath the chilled surface eventually breaks through the skin and the process is repeated, resulting in a pile of lava pillowsand the process is repeated, resulting in a pile of lava pillows

Basaltic lavaBasaltic lava

Convex upward tops

Cusped bottoms filling space

Cross section of pillow basalts

Basaltic lava tubesBasaltic lava tubes

These require Pahoehoe lava which has the correct thickness to form a crust/tube from the inside of which the still-molten lava then pours out. Roof collapses frequently allow access to these caves.

Basaltic lavaBasaltic lavaSlow build up of fluid lava flows produces Slow build up of fluid lava flows produces shield shield volcanoesvolcanoes as lava pours out in all directions from a as lava pours out in all directions from a central summit vent, or group of vents, building a broad, central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape, with a profile gently sloping cone of flat, domical shape, with a profile much like that a a warrior's shield. much like that a a warrior's shield.

Achilles with shield

Basaltic lavaBasaltic lava

Mauna Loa on the island of Hawaii

Andesitic lavaAndesite generally produces Andesite generally produces blocky lavablocky lava that that advances only short advances only short distances down the flanks of a volcano. Resembles aa flows, but more distances down the flanks of a volcano. Resembles aa flows, but more regular shaped polyhedral chunks rather than jagged, vesicular, regular shaped polyhedral chunks rather than jagged, vesicular, scoriaceous deposits.scoriaceous deposits.

Bagana volcano, Bougainville Is, P.N.G. 26-April-1988

Rhyolitic lava

PyroclastsPyroclasts

Pyroclastic rocksPyroclastic rocks

PPyroclastic depositseposits

Tephra and pyroclasts

Tephra Tephra (Greek, for ash) is a generic term for (Greek, for ash) is a generic term for unconsolidatedunconsolidated, , air-fall material produced by a volcanic air-fall material produced by a volcanic eruption eruption regardlessregardless of of compositioncomposition or fragment or fragment sizesize. .

““PyroclastsPyroclasts are the individual crystals, crystal are the individual crystals, crystal fragments, glass and rock fragments fragments, glass and rock fragments generated by generated by disruption as a direct resultdisruption as a direct result of volcanic action.” (Schmid, of volcanic action.” (Schmid, 1981)1981)

AA pyroclastic rockpyroclastic rock is produced from the is produced from the consolidation of consolidation of pyroclastic accumulationspyroclastic accumulations into a coherent rock type. into a coherent rock type.

PyroclastsPyroclasts

AshAsh ---- Very fine-grained fragments ( Very fine-grained fragments (< 2 mm< 2 mm), generally dominated by ), generally dominated by broken glass shards, but with variable amounts of broken crystal and lithic broken glass shards, but with variable amounts of broken crystal and lithic (rock) fragments. (rock) fragments.

PyroclastsPyroclastsLapilliLapilli--- - Pea (cinders)- to walnut-size pyroclasts (Pea (cinders)- to walnut-size pyroclasts (2 to 64 mm2 to 64 mm). In ). In water-rich eruptions, the accretion of wet ash may form rounded water-rich eruptions, the accretion of wet ash may form rounded spheres known as aspheres known as accretionary lapilli.ccretionary lapilli.

PyroclastsPyroclasts

Blocks and BombsBlocks and Bombs -- -- Fragments Fragments >64 mm>64 mm. . BombsBombs are ejected as are ejected as incandescent lava fragments which were semi-molten when airborne, incandescent lava fragments which were semi-molten when airborne, thus inheriting streamlined, aerodynamic shapes. thus inheriting streamlined, aerodynamic shapes. BlocksBlocks are ejected as are ejected as solid fragments with angular shapes. solid fragments with angular shapes.

PyroclastsPyroclasts

Pyroclastic texturesPyroclastic textures

Within this size classification, specific Within this size classification, specific types of pyroclasts can be further defined types of pyroclasts can be further defined by physical attributesby physical attributes

A little controversialA little controversial

Pyroclastic texturesPyroclastic texturesLapilli-size fragments of basaltic lava may cool quickly while Lapilli-size fragments of basaltic lava may cool quickly while airborne, to form glassy teardrop-shaped lapilli called airborne, to form glassy teardrop-shaped lapilli called Pele's tearsPele's tears..

During strong winds (in fire fountains), these molten fragments can During strong winds (in fire fountains), these molten fragments can be drawn out into fine filaments called be drawn out into fine filaments called Pele's hairPele's hair..

(Pele is the Hawaiian fire goddess of volcanoes).(Pele is the Hawaiian fire goddess of volcanoes).

Pyroclastic texturesPyroclastic texturesNon-explosive eruptions often produce lapilli- to bomb-size Non-explosive eruptions often produce lapilli- to bomb-size fragments, called fragments, called spatterspatter which remain airborne for only a short which remain airborne for only a short amount of time so that are still liquid when they hit the ground amount of time so that are still liquid when they hit the ground surface.surface.

Pyroclastic texturesPyroclastic texturesVigorous gas escape in felsic lavas produces Vigorous gas escape in felsic lavas produces pumicepumice, a very light , a very light colored, frothy volcanic rock colored, frothy volcanic rock texturetexture made of glass. made of glass.

The lava is ejected and shot through the air during an eruption. As The lava is ejected and shot through the air during an eruption. As the lava hurtles through the air it cools and the gases escape the lava hurtles through the air it cools and the gases escape leaving the rock full of holes. leaving the rock full of holes.

Pumice is so light that it actually floats on water. Pumice is so light that it actually floats on water.

Pyroclastic texturesPyroclastic textures

Similar gas escape in mafic lavas produces Similar gas escape in mafic lavas produces reticulite.reticulite.

Reticulite has a lower density than pumice, with vesicles occupying up to Reticulite has a lower density than pumice, with vesicles occupying up to 98% of the total volume. Unlike pumice, however, most of the bubble 98% of the total volume. Unlike pumice, however, most of the bubble walls in reticulite are broken so that it sinks in the presence of water.walls in reticulite are broken so that it sinks in the presence of water.

Pyroclastic texturesPyroclastic texturesReticulite, however, is not as common as Reticulite, however, is not as common as scoriascoria, a denser mafic rock , a denser mafic rock texturetexture containing a smaller abundance of relatively large vesicles. containing a smaller abundance of relatively large vesicles.BrittleBrittleVesicularity: 70-85% Crystallinity: 2-50%Vesicularity: 70-85% Crystallinity: 2-50%

Pyroclastic RocksPyroclastic Rocks

Classification of the pyroclastic rocks. a. Based on type of material. b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.

Pyroclastic depositsPyroclastic depositsPyroclastic FALL depositsPyroclastic FALL depositsRelate to magma chamber geometry, discharge rate etc.Relate to magma chamber geometry, discharge rate etc.Derived from Derived from Plinian eruptionsPlinian eruptions where they fall out of the eruption plume where they fall out of the eruption plumeThe deposit is composed of The deposit is composed of highly vesiculated dacitic to rhyolitic highly vesiculated dacitic to rhyolitic pyroclastspyroclasts which can be distributed for hundreds of square kilometers which can be distributed for hundreds of square kilometers away from the vent.away from the vent.Larger bombs deposited near the vent, smaller ash and lapilli and pumice Larger bombs deposited near the vent, smaller ash and lapilli and pumice further away.further away.

Pyroclastic depositsPyroclastic depositsPyroclastic SURGE depositsPyroclastic SURGE depositsDilute mixture of gas and ash (similar to pyroclastic flows) but are Dilute mixture of gas and ash (similar to pyroclastic flows) but are mostly gasmostly gas..Turbulent suspension results in wavy-, lenticular- or low angle cross bedding .Turbulent suspension results in wavy-, lenticular- or low angle cross bedding .Richer in crystals and lithics than pyroclastic flows and are better sorted.Richer in crystals and lithics than pyroclastic flows and are better sorted.

Pyroclastic depositsPyroclastic depositsPyroclastic FLOW depositsPyroclastic FLOW depositsNuee Ardents (Peleean activity) -- these contain dense lava fragments derived from the Nuee Ardents (Peleean activity) -- these contain dense lava fragments derived from the collapse of a growing lava dome or dome flowcollapse of a growing lava dome or dome flow

Non vesiculated block fragments suspended in ash (block and ash flows)Non vesiculated block fragments suspended in ash (block and ash flows)Pumice flows (ignimbrites) -- these contain vesiculated, low-density pumice derived from the Pumice flows (ignimbrites) -- these contain vesiculated, low-density pumice derived from the collapse of an eruption column.collapse of an eruption column.

Vesiculated, pumice-dominated with subordinate ashVesiculated, pumice-dominated with subordinate ash

Pyroclastic depositsPyroclastic depositsStratovolcanoesStratovolcanoes show inter-layering of lava flows and pyroclastic show inter-layering of lava flows and pyroclastic material, which is why they are sometimes called composite material, which is why they are sometimes called composite volcanoes. Pyroclastic material can make up over 50% of the volcanoes. Pyroclastic material can make up over 50% of the volume of a stratovolcano.volume of a stratovolcano.Have steeper slopes than shield volcanoes, with slopes of 6 to 10 Have steeper slopes than shield volcanoes, with slopes of 6 to 10 degrees low on the flanks to 30 degrees near the top.degrees low on the flanks to 30 degrees near the top.

Pyroclastic depositsPyroclastic deposits

Mount Ngauruhoe, New Zealand 61 eruptions since 1839 Mount Doom, Mordor

Pyroclastic depositsPyroclastic depositsTephra (or cinder) conesTephra (or cinder) cones are small volume cones consisting are small volume cones consisting predominantly of tephra that result from strombolian eruptions. predominantly of tephra that result from strombolian eruptions. They usually consist of basaltic to andesitic material and are They usually consist of basaltic to andesitic material and are actually fall deposits that are built surrounding the eruptive vent.actually fall deposits that are built surrounding the eruptive vent.Slopes of the cones are controlled by the angle of repose (angle of Slopes of the cones are controlled by the angle of repose (angle of stable slope for loose unconsolidated material) and are usually stable slope for loose unconsolidated material) and are usually between about 25 and 35 degrees.between about 25 and 35 degrees.

Pyroclastic depositsPyroclastic deposits

SP Crater, in the San Francisco Volcanic Field

Pyroclastic depositsPyroclastic deposits

Volcanic domesVolcanic domes are masses of solid rock that are formed when are masses of solid rock that are formed when viscous lava is erupted slowly from a vent.viscous lava is erupted slowly from a vent.Most domes are short-lived features, because they are commonly Most domes are short-lived features, because they are commonly destroyed by collapse partly due to volcanic explosions and due to destroyed by collapse partly due to volcanic explosions and due to strains set during cooling. The speed of dome growth varies strains set during cooling. The speed of dome growth varies considerably, but some rise by as much as 25 m/day.considerably, but some rise by as much as 25 m/day.

Pyroclastic depositsPyroclastic deposits

Mt St. Helens

Pyroclastic depositsPyroclastic deposits

Plinian fall deposit (1m)

Lithic clast

Ignimbrite (10-100m)

Pumice clast

Ash fall deposit

Lava flow

Eruption productsEruption products

Basaltic lava flows and depositsBasaltic lava flows and deposits

Andesitic flowsAndesitic flows

Rhyolitic pyroclasts, pyroclastic rocks and Rhyolitic pyroclasts, pyroclastic rocks and pyroclastic depositspyroclastic deposits

Used to interpret volcanic history of a Used to interpret volcanic history of a region.region.

Lecture ppt file:

my web page:www.zolfo78.com/lectures or

http://people.stfx.ca/gsolferi/

Hornito structure – Etna Sept. 2004

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