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
Laccolith : concordant
Laccolith : 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: discordant
Volcano
Dike:
discordant
Dike: discordant
Volcanic neck
Ship Rock, AZ.
• 2. Volcanic Pipe: solid lava conduit from
magma chamber to surface
– Mined for diamonds in northern Canada.
Before
Volcano
After
Pipe
Magma Chamber
Neck
Volcanic 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
Before
Xenolith
After
Stock
Batholith
Stock
Batholith: discordant
Batholith:
discordant
Batholith: discordant
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: 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
