Volcanism
Any activity that includes the movement of
magma towards or onto Earth’s surface
http://dsc.discovery.com/tvshows/discoverypresents/videos/understandingvolcanoes-lava-flow.htm
Volcano:
A vent or fissure in Earth’s surface through
which magma and gases are expelled
Volcanoes are formed by:
- Subduction - Rifting - Hotspots
Pacific Ring of Fire
An area around the Pacific Ocean characterized by
volcanoes and seismic activity
Hotspot
volcanoes
Magma
Liquid rock produced
under the Earth’s
surface
Lava
Magma that flows onto
the Earth’s surface
Magma
• What is magma?
– Magma is a mixture of molten rock,
mineral grains, and dissolved gases
formed deep beneath the earth’s surface
at high temperatures
• Where does it come from?
– From the upper part of the mantle
directly beneath the lithosphere
What factors affect the formation of
magma?
1. Temperature -- temps need to be
between 800˚C and 1200˚C
2. Pressure – as pressure decreases, the
temperature at which a substance melts
also decreases
3. Water content of the rocks – rocks melt at
lower temperature in the presence of
water
4. Composition of the rocks – different types
of rock melt at different temperatures
What is Viscosity?
• A substance’s resistance to flow is called its
viscosity.
• Ex: cold syrup is more viscous than hot
syrup (cold syrup does not flow easily)
• Ex: basaltic magma has lower silica
therefore has lower viscosity, so it flows
more quickly than Rhyolitic magma which
has higher viscosity
Gases in Magma
• Most important: Water vapor, carbon dioxide, and sulfur
• Also has: hydrogen (which can combine with oxygen to
form steam)
• Amount of gas determines type of eruption
– Large amt of gases = more explosive eruptions
• VISCOSITY – (Very Important Term!!!)
– Definition: Resistance to Flow
• Viscous Magma – like pudding – doesn’t flow well
– Determined by High Silica Content - FELSIC
• Non-Viscous Magma – like maple syrup – flows well
– Determined by Low Silica Content – MAFIC
Three types of magma
Composition
Source
Material
Viscosity
Gas
Content
Silica
Content
Location of
Magma
Least
Both
oceanic and
continental
crust
Basaltic
Upper
mantle
Andesitic
Oceanic
crust and
oceanic
sediments
Intermediate
3–4%
About
60%
Intermediate
Continental
margins –
subduction
zones
Rhyolitic
Continenta
l crust
High
4–6%
About
70%
greatest
Continental
crust
Low
1–2%
About
50%
Explosive
Lava
• What is it?...MOLTEN ROCK ABOVE
GROUND
• A little different than magma b/c
some gas escapes, some new
materials added
• Also felsic and mafic
– Mafic: gases escape quickly &
lava comes out smoother
• Pahoehoe – fluid, “ropy”
lava
– Felsic: gases can’t move easily,
explosive eruption
• Aa – blocky, chunky, lava
that topples over itself
This lava flow is smooth, fluid, and “runny” – What type is it…MAFIC or FELSIC?
Lava Fragments
• Tephra: solid fragments of lava
produced by explosive eruptions
–Smallest pieces: ash
–Larger pieces: lapilli
–Largest pieces: blocks (solid), bombs
(liquid then harden)
Tephra
Vent
Chamber
Plume
Volcanic Hazards
• Pyroclastic Flows – clouds of ash, dust, and gases (AKA – Nuee
Ardente – “Glowing Cloud”)
– Move at speeds of up to 200 km/h
• Denser than surrounding air – so it stays close to ground
• Poisonous
• Superheated (5X the temperature of boiling water-~7000C)
• Lahar – heated snow covered mountaintop – snow melts  super
fast mudslide – more dangerous than nuee ardente (more
common)
• Lava Flows – obviously dangerous due to heat – but can be
diverted or chilled Ash – threat of suffocation
• Poisonous gases – kill due to toxicity – but the most common and
dangerous is CO2 (drives away oxygen)
http://dsc.discovery.com/video-topics/other/other-topics-volcanovideos.htm
Types of Volcanoes
• Type of Volcano depends on:
– Type of material that forms it and type of eruptions that occur
• We will focus on (3) most common
– Shield Volcano (Mauna Loa)
• Mountain with broad, gently sloping sides
• Generally non-explosive
– Cinder-Cone Volcano (Izalco – El Salvador)
• Material that is ejected high into the air – piles back up around the
vent
• Steep sides, but generally small in size
• Water and silica laden magmas, and full of gas – somewhat explosive
– Composite Volcano (Mount St. Helens)
• Large amounts of water, silica, and gases
• VERY LARGE – Due to their large size – pose the greatest threat
• These can blow their tops completely off forming a CALDERA –
Crater Lake Oregon
Just to put the size of the Mount Saint Helens eruption in perspective,
this figure compares the size of that eruption (as measured by the
volume of ejected material) to some bigger ones in the past. Notice how
small Mt. St. Helens was compared to the others!!
http://dsc.disco
very.com/tvshows/discover
ypresents/videos
/understanding
-volcanoeslava-flow.htm
Living vs. Dead Volcanoes
• Active Volcano: A volcano that is erupting. Also, a
volcano that is not presently erupting, but that has
erupted within historical time and is considered likely
to do so in the future.
• Dormant Volcano: Literally, "sleeping." The term is
used to describe a volcano which is presently inactive
but which may erupt again. Most of the major
Cascade volcanoes are believed to be dormant rather
than extinct.
• Extinct Volcano: A volcano that is not presently
erupting and is not likely to do so for a very long time
in the future
Volcano Prediction
• Tectonic Activity
– The volcanic mountain will “rumble” prior to an eruption
• Magma is working its way up the vent or volcanic pipe, and pushes
stuff around during its ascent
• Topographic “Bulges”
– The magma can push up/out on the land as it rises
• Gas emissions – volcanic “burps” of sulfur, water vapor,
and carbon dioxide
• Temperature Spike – the temperature around and inside
the volcano will rise
• ~Questionable~: Animals will bug out of the area –