4 Igneous Bodies Chp 40

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 100 km2 of exposed surface area – Stock: 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:

Felsic/silisic & intermediate

Mafic

Volcano Type Location

Viscosity Colour

composite convergent thick/hi dome ocean\cont cinder cone ocean\ocean

light

Explosive

Type of Lava

shield basalt flood cinder cones

divergent hot spots

thin/lo

dark

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

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

Lava Flows: Aa



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

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.

solid molten

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

CO2 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

Dome

Old volcano

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

One flow

•Cumulative flows in Washington are over 1 mile thick

Melting Rock Mantle rock is solid, magma is only present under certain conditions  Factors that affect melting include 

 Temperature  Pressure  Volatiles

(water, gas)

Temperature Geothermal gradient, temperature increases 25 celsius every km depth  Melting temperature of rocks at the surface is between 750-1000celsius  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

• 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 WS 4.1  Do Chps 3-4 Review WS 