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:

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

Laccolith : concordant

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: discordant Volcano

Dike: discordant

Ship Rock, AZ. Volcanic neck

2. Volcanic Pipe: solid lava conduit from magma chamber to surface –Mined for diamonds in northern Canada. Volcano Neck After Before Pipe Magma Chamber

Volcanic Pipe: discordant Ekati Mine, NWT

Volcanic neck

Devils Tower, WY.

3. Batholith: largest intrusive body with >100 km 2 of exposed surface area –Stock: <100 km 2 of exposed surface area –Usually granite Stock Batholith Xenolith Before After

Batholith: discordant

Chief

Stock: 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: H 2 O (50-80%), CO 2, N 2, SO 2, H 2 S (+ some CO, H 2,Cl 2 ).

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 Explosive &

Subduction: Oceanic\Cont’l

Subduction: oceanic\oceanic

Divergent

Hot Spots

Lava Flows & Pyroclastic Material fire + pieces blown out of volcano A: Lava Flows: 1.Pahoehoe: ropey, cow pie lava From thin mafic Play pahoehoe movie

Lava 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: Aa

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

Lava Flows:Columnar Jointing

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

Lava 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: Ash

Pyroclastics: Ash Fall

Pyroclastics: Ash Flow Animation

2. Lapilla: 2-64mm = pebble size

Pyroclastics: Lapilla

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

Pyroclastics: Blocks

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

Pyroclastics: 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

CO 2 being emitted with a pipe from a lake in a dormant volcano in Africa

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 Lava Ash 30’

Volcano: 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 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 Dome

3. Cinder Cones: short-lived “baby volcanoes consisting of just pyroclastics –Form from initial eruption –Up to 400 m high 33’

Volcano: 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: 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 Flood

One flow Cumulative flows in Washington are over 1 mile thick One flow

Melting Rock Mantle rock is solid, magma is only present under certain conditions Mantle rock is solid, magma is only present under certain conditions Factors that affect melting include Factors that affect melting include Temperature Temperature Pressure Pressure Volatiles (water, gas) Volatiles (water, gas)

Temperature Geothermal gradient, temperature increases 25 celsius every km depth Geothermal gradient, temperature increases 25 celsius every km depth Melting temperature of rocks at the surface is between celsius Melting temperature of rocks at the surface is between celsius Different minerals melt at lower temperatures, this produces a partial melt of mantle and crust rocks Different minerals melt at lower temperatures, this produces a partial melt of mantle and crust rocks

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

Stoping

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