volcanic rocks.ppt

Volcanism and volcanic rocks

rocks and sediments produced by volcanic

processes

Volcanism – plate tectonics

relationship of volcanism to movements of the earth’s plates (fig 7.1)

convergent boundaries

• seduction zones – often violent eruptions, due to high silica content

• pyroclastic sediments thrown from the volcano

divergent margins – rifting


• under oceans – • basaltic lava – pillow

lava• non violent eruptions,

– composition of gabbro,

– low silica content, – created by partial

melting of the low-temperature constituents of the mantle

• on land – • plateau basalts – • non violent eruptions of

– basalt flows from fissures (Iceland)

– mantel material


• under oceans – • basaltic lava – pillow

lava

• on land – • plateau basalts –

hot spots

• stationary heat plumes in the mantle, hot spots in the mantle produce volcanoes in a chain as the crust moves over the hot spot (Hawaiian Islands)

Eruptive Phenomena include

• lava flows• explosions• ash falls• hot-ash flows• glowing avalanches

• mudflows• fissures• earthquakes• floods• elevation changes• gas discharges

• lava flows• explosions• ash falls• hot-ash flows• glowing avalanches

• mudflows• fissures• earthquakes• floods• elevation changes• gas discharges

types of lava

• basic lava – produce non explosive eruptions or effusive eruptions, with lava fountains and lava flows, less viscous and thus does not trap gasses as much as the lava below

• intermediate and acidic lava – produce explosive eruptions – sudden release of trapped CO2 and SO2 and steam in the highly viscous lava

types of lava

• basic lava

types of lava

• intermediate / acidic lava

http://www.volcanovideo.com/Movies/littor1.avi

Warnings of an eruptionFig 7.5- 7.7 mt. st. helens

• begins with upward movement of magma from 50 km depth in the crust

• earthquake swarms – up to hundreds per day due to the rise of magma

• earthquakes at 1 km depth when the eruption is nearer at hand

• temperature rise in hot springs and steam in volcanic crater

• gas released causes asphyxiation

• snow on the volcano will melt

• bulge of the surface • explosion – casting

pyroclastic debris up into the atmosphere (known up to 80 km)

• Fall of pyroclastic debris (hot material)

• base surge outward expanding ash-laden cloud which sometimes also contains poisonous acid or toxins

On May 18, 1980, Mount St. Helens had a massive explosion that forever changed the picturesque alpine landscape, killed almost 60 people and sent ash for hundreds of miles.

The force of the eruption coated eastern Washington with a thick layer of light gray ash. When wet the ash became as dense as cement making it hard to remove from lawns, roofs and roads. The ash can still be seen along I-90 and elsewhere in the area. Parts of Idaho and Montana had deposits as the ash was caught up in the jetstream winds.The blast removed 1000 feet off the top of the mountain, leveled 200 square miles of forest to the north, moved Spirit Lake and formed new lakes. The sound of the explosion could be heard as far away as Canada. Giant mudflows raced down the mountain into local rivers destroying bridges, vehicles and houses. The sound of the explosion could be heard as far away as Canada.Mount St. Helens is one of the Cascade Volcanoes that reach from Washington to California.

http://www.kidscosmos.org/kid-stuff/kids-volcanoes-cascade

http://www.fs.fed.us/gpnf/volcanocams/msh/

events

• glowing avalanches or nueé ardente is a hot (700-1000 degree C) ash-laden gas cloud

• moves at extremely fast speed (average of 160 km/hr) down the volcano slope

• rock formed by this is called ignimbrite or welded tuff

events

• lava flows – type of flow depends on viscosity which is related to silica content

• stiff,stiff, highly viscous silica rich lava – flows in blocks and forms a blocky surface on the lava called aa textureaa texture

• fluidfluid, less viscous, lower silica lava – flows in rope like surface called pahoehoe pahoehoe texture

• Hawaiian names

events

• Volcanic mudflows (lahars) • pyroclastic material mixed with

water that flows rapidly (10 m/s)

tsunamis

• great sea waves caused by the displacement of water due to a sub oceanic volcanic eruption or earthquake

• great velocities up to 5 000 km / hr

• as they reach the shore they rise up into giant waves that flow in over the land

tsunami

Volcanic rocks

• Pyroclastic rocks - molten material is ejected and solidifies in the air

• classified as sedimentary rocks

particles in volcanic rocks

• preexisting rock particles are: blocks >64 mm, or lapilli 2-64 mm,

• molten lava which cools are: bombs > 64 mm, ash-silt size

pyroclastic rock names• Ash tuff - rock predominated by ash; sometimes simply

referred to as tuff. • Lapilli tuff - rock predominated by lapilli. • Tuff breccia - rock containing 25% to 75% blocks and/or

bombs. • Pyroclastic breccia - rock containing at least 75% blocks

and bombs. • Agglomerate - rock containing at least 75% bombs. • Agglutinate - rock composed of fused, largely

unrecognizable, basalt spatter fragments.

pyroclastic rock names• pumice or scoria has numerous gas

holes• obsidian is volcanic glass which cooled

suddenly

• bentonite – pure montmorillonite clay formed from weathered ash

volcanic flow rock names

in order of increasing silica (downwards) and increasing explosiveness

1.1. basaltbasalt2. andesite

1. dacite2. latite

3. rhyolite



1. basalt2.2. andesiteandesite

1. dacite2. latite

3. rhyolite



1. basalt2. andesite

1.1. dacitedacite2. latite

3. rhyolite



1. basalt2. andesite

1. dacite2. latite

3.3. rhyoliterhyolite

Volcanic rock terms

• aphanitic – fine grains that are not visible to the eye• phenocrysts – large crystals in the aphenitic matrix• traprock – light colour aphanitic volcanic rock• felsite – dark colour aphanitic volcanic rock• vesicles – holes in the rock formed by gas bubbles • vesicular – rocks with numerous vesicles• scoriaceous – vesicular and extremely porous• amygdule – mineral that fills the vesicle• amygdaloida – a rock with numerous vesicles filled

with minerals

Volcanic rock terms

• aphanitic – fine grains that are not visible to the eye• phenocrysts – large crystals in the aphenitic matrix• traprock – light colour aphanitic volcanic rock• felsite – dark colour aphanitic volcanic rock• vesicles – holes in the rock formed by gas bubbles • vesicular – rocks with numerous vesicles• scoriaceous – vesicular and extremely porous• amygdule – mineral that fills the vesicle• amygdaloidal – a rock with numerous vesicles filled

with minerals

Volcanic rock terms

• aphanitic – fine grains that are not visible to the eye• phenocrysts – large crystals in the aphenitic matrix• traprock – light colour aphanitic volcanic rock• felsite – dark colour aphanitic volcanic rock• vesiclesvesicles – holes in the rock formed by gas bubbles • vesicularvesicular – rocks with numerous vesicles• scoriaceous scoriaceous – vesicular and extremely porous• amygdule – mineral that fills the vesicle• amygdaloidal – a rock with numerous vesicles filled

with minerals

Volcanic rock terms

• aphanitic – fine grains that are not visible to the eye• phenocrysts – large crystals in the aphenitic matrix• traprock – light colour aphanitic volcanic rock• felsite – dark colour aphanitic volcanic rock• vesicles – holes in the rock formed by gas bubbles • vesicular – rocks with numerous vesicles• scoriaceous – vesicular and extremely porous• amygdule amygdule – mineral that fills the vesicle• amygdaloidalamygdaloidal – a rock with numerous vesicles filled

with minerals

Volcanic rock-mass characteristics

• complex in composition, flows, pyroclastic debris etc. and interbeds of non volcanics

• flows follow lows in the topography• resistant to weathering – after a long

period of physical weathering the deposits which once were in the bottoms of valleys form tops of mountains, table mountains

• irregular lateral extents

fractures and permeability

Weathering products

• contrasting potential for weathering - basalt is more basic than granite and thus more inclined to decay due to chemical weathering

• on one hand the rocks are often impermeable in themselves which would deter chemical weathering

• on the other there are often numerous joint which make the rock mass on a whole very permeable, enhancing chemical weathering

• young basalt often is not weathered, but old basalt is deeply decomposed to a clay soil, expansive montmorillonite

Engineering problems with volcanism and volcanic

rocks Enormous damage potential!! ash fall risks

abrasive

clogs drains

poisonous

causes fires

•weight can damage structures (like water logged snow)


rocks Enormous damage potential!!

lava flow risks

flow relative slow

diversion possible; trenches, barriers and spraying with cold water can be used to deter the flow

•predict flow path possible


rocks Enormous damage potential!!

mudflow risks

huge quantity

high velocities

path predictable far in advance but the velocity and size of the flow makes it difficult to contain, dams are easily broken, barriers jumped

preparatory measures can be taken, lower the level of water reservoirs

Exploration and volcanic rocks

• complexity of the deposits makes it difficult to predict their vertical and lateral extent

• stratigraphy can vary greatly laterally

• marker layers are needed• need to find the extent of the

material with lower strength and high permeability

Surface excavation

• excavation often requires blasting• blocks often displaced on slopes;

float in soft materials (fig 7.18)

Underground excavations

Difficulty can be represented by two cases

1) it took 6 years to tunnel 17 km2) two years to tunnel 263 m


large water inflow is common due to the:

• open fractures and joint• highly permeable layers• permeable interbeds• folded beds can entrap water in

compartments


• hard and dense if unweathered – too hard for a tunnel machine

• large extent of jointing results in high potential for rock fall – shortcrete required

• horizontal stress zero, vertical stress = weight of the overlying rocks thus there is a high potential for the roof to collapse


• active areas – poisonous gas can occur

• young areas – non cemented rock common

• warm water flows can occur

Dams and canals

• leakage is a great problem – grouting is required

• compressibility high – bearing capacity poor

• shear strength low – slides probable

Dams and canals

• Hoover dam Fig 7.22, height 222 m • founded on volcanic breccia• grout curtain depth originally to be 40 m ended up 130

m deep and horizontally 90 m

Dam in Sardinia

• rock fill dam• concrete face• founded on a series of lava flows with columnar

jointing and tuff beds • serious differential settling• fault under dam

Engineering materials

• volcanic rocks are used in all aspects of engineering

• aggregates – concrete– asphalt

• rock fill– dams– breakwaters– coarse grade

• dimension stone

Note: there are some things to look out forNote: there are some things to look out for

Problems as engineering

material• volcanic glass reacts with alkalies in Portland cement =

cracking of the structure• amygdules that are often filled with the following

minerals: opal, zeolite, gypsum – these are not good in concrete, reactive

• pillow lava has often an unstable rind which is reactive with Portland cement

• weathering can be rapid for some rocks – X ten years = sand and gravel

• disintegration tests should be made to test the life

expectancy of the rock (top p.286)

Case studies

Protection of an Icelandic port from volcanism – barriers were erected and seawater pumped onto the flow to cool it

Case studies

Round Butte Dam, Oregon

Case studies

Round Butte Dam, Oregon • 133m high dam in lava flows with

interbeds of non-volcanics. Fig 7.25• Several layers required grouting • In all 42 km of grout holes were filled

with 4000 m3 of Portland cement grout and took two years to carryout

volcanic rocks.ppt

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