Geology 12 Geology 12

Presents

Chp 4 Volcanics (& Plutonics) Intrusive igneous body = pluton = when magma intrudes into and solidifies in the crust. They are later exposed due to weathering & erosion

•2 types: Concordant & Discordant

•A: Concordant: boundaries of pluton parallel to layers in country rock.

•sills & laccoliths

A: concordant features: intrusion runs parallel to

bedding• 1. sill: usually <1 m thick (but up to 300 m

thick

• Parallel to bedding structures

• Has inclusions of country rock & contact metamorphism in top & bottom

• No vessicles (bubbles)– Ex: Palisades of New York

1. Sill: 1. Sill:

• 2. Laccolith: mushroom top sill where overlying rock is pushed up “zit”– Ex: Black Hills, S.D.

Laccolith : concordantLaccolith : concordant

Laccolith : concordantLaccolith : concordant

Laccolith : concordantLaccolith : concordant

Black Hills, SD

Black Hills, SD

Black Hills, SD

• B: Discordant: boundaries of intrusion cut across layering of country rock.– Dikes, pipes, stocks/batholiths

• 1. Dikes: 1-2 m thick (but some cm to 100s of m thick

Dike: discordantDike: discordant

Volcano

Dike: Dike: discordantdiscordant

Dike: discordantDike: discordant

Ship Rock, AZ.

Volcanic neck

• 2. Volcanic Pipe: solid lava conduit from magma chamber to surface– Mined for diamonds in northern Canada.

VolcanoNeckAfterBefore

Pipe

Magma Chamber

Volcanic Pipe: discordantVolcanic Pipe: discordant

Ekati Mine, NWT

Volcanic neck

Volcanic neck

Devils Tower, WY.

• 3. Batholith: largest intrusive body with >100 km2 of exposed surface area– Stock: <100 km2 of exposed surface area– Usually granite

Stock

Stock

Batholith

XenolithBefore After

Batholith: discordantBatholith: discordant

Batholith: Batholith: discordantdiscordant

Batholith: discordantBatholith: discordant

Batholith: discordantBatholith: discordant

Chief

Stock: discordantStock: discordant

Extrusive igneous body: structures created by magma -> lava solidifing (Volcanics) on the Earth’s surface.550 active volcanoes (60% on Ring

of Fire, 20% in Mediterrean, rest mainly on divergent boundaries)

Emit many gasses: H2O (50-80%), CO2, N2, SO2, H2S (+ some CO, H2,Cl2).

• Types of Lava: Type of Lava Volcano Type Location Viscosity Colour

Felsic/silisic composite convergent thick/hi light

dome ocean\cont

intermediate cinder cone

ocean\ocean

Mafic shield divergent thin/lo dark

basalt flood

cinder cones hot spots

Exp

losi

ve

&

Subduction: Oceanic\Cont’lSubduction: Oceanic\Cont’l

Subduction: oceanic\oceanicSubduction: oceanic\oceanic

DivergentDivergent

Hot SpotsHot Spots

• Lava Flows & Pyroclastic Material

fire + pieces blown out of volcano

A: Lava Flows:

1. Pahoehoe: ropey, cow pie lavaFrom thin mafic

Play pahoehoe movie

Lava Flows: PahoehoeLava Flows: Pahoehoe

Lava Flows: PahoehoeLava Flows: Pahoehoe

Ford Tough

• 2. Aa: rough, jagged, angular blocks of lava

– As lava freezes, it is being moved and breaks into pieces like glass.

Lava Flows: AaLava Flows: Aa

Lava Flows: AaLava Flows: Aa

3. Columnar Jointing: lava 3. Columnar Jointing: lava cools, contracts and splits at cools, contracts and splits at 60’ angles into hexagonal 60’ angles into hexagonal columnscolumnsMainly mafic lavaMainly mafic lava

Lava Flows:Columnar JointingLava Flows:Columnar Jointing

• 4. Pillow lava: blobs of lava quickly freeze underwater into “pillows”

Lava Flows: Pillow LavaLava Flows: Pillow Lava

B: Pyroclastic Material = Tephra = lava ejected into air• 1. Ash: < 2mm

– Most common pyroclastic– Welds to form tuff– Can be ejected 2 ways:

• A) Ash Fall: blown high into atmosphere & may travel 1000s of km before settling

–Can cool climate for years (ice age?)• B) Ash Flow= Nuee Ardente = Pyroclastic

Flow: horizontal blast of ash and gasses–+500C at 800 km/h–Mt St Helens, Martinique

Pyroclastics: AshPyroclastics: Ash

Pyroclastics: AshPyroclastics: Ash

Pyroclastics: Ash FallPyroclastics: Ash Fall

Pyroclastics: Ash FlowPyroclastics: Ash Flow

Animation

• 2. Lapilla: 2-64mm = pebble size

Pyroclastics: LapillaPyroclastics: Lapilla

• 3. Blocks: solid chunk of igneous rock >64mm blown out of volcano

Pyroclastics: Pyroclastics: BlocksBlocks

• 4. Bombs: blob of hot molten lava >64mm that is ejected, and partially freezes in flight.

molten

solid

Pyroclastics: BombsPyroclastics: Bombs

• Volcano: conical mountain formed around a vent

• Crater: depression near summit of volcano

• Caldera: huge depression when most of volcano falls back into magma chamber

COCO22 being being emitted with emitted with a pipe from a a pipe from a

lake in a lake in a dormant dormant

volcano in volcano in AfricaAfrica

• Types of Volcanoes

• 1. Composite/Strato: composed of layers of pyroclastics and lava flows– Andesitic rock usually (intermediate lava)– Ex. Mt. Baker, Mt. Vesuvius, Mt Fuji

LavaAsh

30’

Volcano: Composite/StratoVolcano: Composite/Strato

Mt. Rainier

Mt. St. Helens

Fuego, Mexico

Mt. Jefferson

Mt. Washinton

Mt. Shasta

Three Sisters & Black Butte

• Lahar: mudflow of ash & water

– Kill 1000s

– Tacoma is built on old lahars!

– *Lahars and Pyroclastic flows pose the greatest human danger from volcanoes

Volcano: lahar mud flowVolcano: lahar mud flow

• 2. Lava Dome: very, thick felsic/silisic lava moves up slowly under immense pressure– Ex: Lassen Peak, CA, inside of Mt. St. Helens

Old volcano

Dome

Lassen Dome, CA

Volcano: Lava DomeVolcano: Lava Dome

• 3. Cinder Cones: short-lived “baby volcanoes consisting of just pyroclastics– Form from initial eruption– Up to 400 m high

33’

Volcano: Cinder ConeVolcano: Cinder Cone

• 4. Shield: shallow sloped consisting of many low-viscosity mafic lava flows– Largest volcanoes on Earth– Ex: Hawaii (10 km high = BIG)

10’

Volcano: ShieldVolcano: Shield

• 5. Basalt Flood/Basalt Plateau: large flows of low-viscosity mafic lava that flow from long fissures (crack), rather than a single vent.– Result of divergent boundaries

Volcano: Basalt FloodVolcano: Basalt Flood

One flow

•Cumulative flows in Washington are over 1 mile thick

One flow

Melting RockMelting Rock

Mantle rock is solid, magma is only Mantle rock is solid, magma is only present under certain conditionspresent under certain conditions

Factors that affect melting includeFactors that affect melting includeTemperatureTemperaturePressurePressureVolatiles (water, gas)Volatiles (water, gas)

TemperatureTemperature

Geothermal gradient, temperature Geothermal gradient, temperature increases 25 celsius every km depthincreases 25 celsius every km depth

Melting temperature of rocks at the Melting temperature of rocks at the surface is between 750-1000celsiussurface is between 750-1000celsius

Different minerals melt at lower Different minerals melt at lower temperatures, this produces a partial temperatures, this produces a partial melt of mantle and crust rocksmelt of mantle and crust rocks

PressurePressure

Pressure increases with depth, Pressure increases with depth, causes a higher melting temperaturecauses a higher melting temperature

• Why Magma Rises• 1. Forceful Ejection: magma moves

from high pressure at depth to low pressure at the surface

• 2. Stoping: displacement of magma by country rock (i.e. xenoliths) pushes magma up (like placing ice cubes in a glass of water)

StopingStoping

• Explosiveness of a volcano is dependent on:

1. viscosity of magma: high viscosity slows the escape of gases which expand greatly near the surface

2. “stickiness” of magma = “corking effect”. The stickier, the more pressure it requires to erupt so when it does erupt…

• Higher silica increases both viscosity & stickiness

Do Lab 4-1 Igneous Rock IDDo Lab 4-1 Igneous Rock IDDo WS 4.1Do WS 4.1Do Chps 3-4 Review WSDo Chps 3-4 Review WS