Chapter 4 – Volcanoes
Mount Unzen, 1991
• One of the most destructive of Japan’s 19 volcanoes, more than 200 years ago
killed an estimated 15,000 people.
• Lay dormant until June 3, 1991 when another eruption forced the evacuation of
o Produced 0.2 km of lava and more than 10,000 superheated flows of hot
gas, ash and large rock fragments.
o More flows than any other volcano in recent time.
4.1 Introduction to Volcanoes
• Volcanic activity, or volcanism, is directly related to plate tectonics.
o Most active volcanoes located near plate boundaries
• Mid-ocean ridges and subduction zones are sites where molten rock, or magma,
reaches the surface and erupts as lava.
o Magma – The molten rock formed deep within earth’s crust or in the upper
o Lava – The molten rock (magma) that flows from the volcano
• Two-thirds of all active volcanoes on land are located along the Pacific Ring of
• Composed of melted silicate minerals and dissolved gases.
o Main elements include: Oxygen, silicon, aluminum, iron, magnesium,
calcium, sodium and potassium.
Oxygen and silicon most abundant and combine to make silica
• Names geologists apply to volcanic rocks depend on the amount of silica
o Basalt – most common volcanic rock. (45-52% SiO₂).
o Andesite – not as common. (52-63% SiO₂).
o Dacite (63-68%) very uncommon
o Rhyolite (>68%) very uncommon
• Magma also contains small but significant amounts of dissolved gases, mostly
water vapour and carbon dioxide.
• Volcanoes have different shapes and eruptive styles that are related to the
chemistry and viscosity, or fluidity of their magmas.
o Magma viscosity determined by silica content and temperature.
• Shield Volcanoes – A gently sloping, broad, convex volcano consisting of basalt
lava flows; the largest type of volcano 2
o Largest volcanoes on the Earth. Gentle sloped sides and broad summits
o Among the tallest mountains on Earth when measured from their bases.
o Common on the Hawaiian Islands, Iceland and some islands in the Indian
o Non-explosive eruptions of very hot, low-viscosity, basaltic magmas.
o Magma of basaltic composition erupts as lava through openings, or vents,
in the volcano and flows down its sides.
Lava can also flow many kilometres away from the vent through
underground lava tubes.
o Shield volcanoes consist almost entirely of lava flows but fragment debris
called tephra can also be erupted.
Tephra – A general term for fragmented volcanic material blown out
of a volcano; includes ash, lapilli, blocks and bombs.
• Composite Volcanoes – The steep-sided volcanic cone produced by alternation
layers of pyroclastic debris and lava flows. Also know as a stratovolcanoe.
o Cone shaped with steep sides.
o Erupt less frequently than active shield volcanoes but eruptions involve
andestic or dacitic magma and are explosive.
o Lavas are thick and viscous and therefore rarely flow more than a few kms
away from the vents.
o Eruptions are deadly, they are responsible for most of the death and
destructions caused by volcanoes throughout history
Destroyed the roman towns of Pompeii and Herculaneum in 79
A.D. killing 10,000-24,000 people.
• Volcanic Domes – A volcano formed from viscous magma with a high silica
content; eruptive activity is generally explosive.
o Steep-sided. Not very common. Highly explosive.
o Most recent serious of eruptions was from Lassen peak between 1914
• Cinder Cones – A conical volcano consisting of pyroclastic deposits. Also called
a soria cone.
o Relatively small volcanoes made of nut to fist sized pieces of red or black
o Round top, commonly have a crater at their top.
o Usually found on the flanks of larger volcanoes, along some normal faults,
and along cracks or fissures.
o Can be very fast growing
• Maars – A flat bottomed, roughly circular volcanic crater produced by a single
explosive eruption and commonly filled with water.
• Ice-Contact Volcanoes
o Many volcanoes and lava flows erupt beneath or against glaciers.
o When they erupt large quantities of ice are melting creating huge outburst
o Many are found in British Columbia.
• Volcanoes are very complex. Commonly include craters, calderas, volcanic
vents, geysers, and hot springs.
• Craters – A bowl-shaped depression at the top of a volcano that has formed by
an explosion or collapse of the summit area.
o Usually hundreds of metres to a couple of kilometers in diameter.
• Calderas – Large crater produced by a violent volcanic eruption or the collapse
of the summit area of a volcano after an eruption
o Few dozen kilometers in diameter
o Produced by the largest and most deadly type of volcanic eruption called a
Rare, only occur once every 200 to 1000 years
• Volcanic Vent – A circular or elongate opening in the ground through which lava
and pyroclastic debris are erupted.
o Elongated cracks are called fissures.
o Fissures produce lava flows called flood basalts
• Geyser – A spring that ejects hot water and steam at regular or irregular
intervals. The most famous geyser is Old Faithful in Yellowstone National Park.
• Hot Spring – A natural discharge of groundwater at a temperature higher than
that of the human body.
• Supervolcano – A volcano that produces extremely large, but rare explosive
eruptions with hemispheric or global impacts.
o Produce more than 1000km^3 of ash and fragmented rock, enough to
cover PEI to a depth of 175m. 4
Volcanoes and Plate Tectonics
• Volcanism is directly related to plate tectonics. The tectonic setting determines
the type of volcano that will be present.
• 1.) Subduction Zones.
o Oceanic crust, which is denser than continental crust is carried into Earth’s
o Rising heat and pressure dry out the subducting crust, called dehydration.
o At depths of several dozen km’s, the expelled water from the descending
crust rises and changes the chemical composition of the overlying mantle.
o Presence of water lowers the melting temperature of the overlying mantle
rock, causing them to melt and form magma that rises through the Earth’s
crust and erupts on the surface.
o Stratovolcanoes occur at subduction zones. Most common on Pacific Rim.
o More than 80% of terrestrial volcanic eruptions have come from volcanoes
above subduction zones.
• 2.) Mid-ocean ridges and continental drift Zones.
o Occur in areas where plates are spreading away from one another.
o Usually occurs in the ocean, but can occur on land.
o Magma rises to the ocean floor to create new crust.
• 3.) Hot spots beneath the oceans.
o Hot mantle material wells up beneath a plate at a stationary point rather
than a boundary.
o The upwelling mantle material, focused on a single spot, creates a
o As the plate slowly moves a series of volcanoes form in a row, termed hot-
I.e. Plate shifts, hot spot doesn’t, with time get a bunch of
o Most famous example is the Hawaiian Islands. Source of the magma that
created the islands is a hot spot that has been stationary for millions of
• 4.) Hot spots beneath continents. 5
o Caldera-forming eruptions. Extremely explosive and violent
4.2 Geographic Regions with Active Regions
• Most volcanoes on Earth’s land surface occur along the Ring of Fire, which
borders the Pacific Ocean.
o Do occur in other settings: hot spots (Hawaii, Long Valley, Yellowstone),
mid-ocean ridges (Iceland), and continental rift zones (East Africa)
• Highest risk of volcanic activity in Canada and US is Pacific Northwest, although
more than 90% of North America is free of risk from volcanic activity.
• Young volcanoes in Canada are restricted to BC.
o Only Mount Meager has had an eruption in the past 10,000 years.
• The volcano that poses the greatest risk to Canada is in the US (assholes).
o Mount Baker in northern Washington, last erupted in 1843, would spread
ash over Vancouver and other parts of BC if it erupted. Landslides and
flooding could occur.
4.3 Effects of Volcanoes
• 50 to 60 volcanoes erupt each year, including 2 or 3 in the US, mainly in Alaska.
• Many eruptions are in sparsely populated areas causing little loss of life or
• Approximately 500 million people live close to volcanoes.
• In the past 100 years 100,000 people have been killed by volcanic eruptions
o 28,500 in the 1980s alone.
• Densely populated countries, with active volcanoes such as Japan, Mexico, the
Philippines and Indonesia are the most vulnerable.
4.4 Hazards Associated with Volcanoes
• Volcanic hazards can be subdivided into primary (direct) or secondary (indirect)
effects of eruptions.
• Primary: Lava flows, ash fall, volcanic bombs, pyroclastic flows, pyroclastic
surges, lateral blasts and poisonous gases
• Secondary: lahars, debris avalanches, other landslides, groundwater and
surface water contamination, floods, fires and tsunamis.
o Large eruptions can also cool Earth’s atmosphere for a year or so 6
• The size of a volcanic eruption can be measured using a scale called the
Volcanic Explosivity index (VEI).
o Takes into account the volume of material erupted, the height of the
eruption cloud, the duration of the main eruptive phase and other factors.
o Assign’s a number on a linear scale from 0 to 9. (MLS notes say there is
no upper limit)
0 represents non explosive. 9 means we’re all fucked
Mount St. Helens which was a very large event registered a 5
The largest in recorded history was Tambora, Indonesia in 1815; it
was recorded as a 7.
• Lava Flows – The molten rock (magma) that flows downslope from a volcanic
vent, cools, and solidifies
• Lava Flows are one of the most common products of volcanic activity.
o Occur when magma reaches the surface and flows out of a central crater
or erupts from a fissure or vent.
• Can be fluid and move rapidly, or can be viscous and slow moving.
• Three major types of lava flows: pillow, pahoehoe and aa
• Pillow (not in textbook): elongated, interconnected flow lobes that are elliptical or
circular in cross-section
o volumetrically the most abundant type; they are erupted at mid-ocean
ridges and because they make up the submarine portion of seamounts
and large intraplate volcanoes
• Pahoehoe: characterized by a smooth, billowy, or ropy surface
o ropy surface develops when a thin skin of cooler lava at the surface of the
flow is pushed into folds by the faster moving, fluid lava just below the
o flows tend to be relatively thin, from a few inches to a few feet thick
• Aa: characterized by a rough, jagged surface
o flows advance much like the tread of a bulldozer.
• Most of the time the travel slowly enough that people and animals can move out
of the way 7
o However property and environmental damage can be extensive
• Lateral Blasts – a volcanic eruption characterized by an explosion directed away
from the volcano more or less parallel to the ground surface; may occur when the
side of a volcano collapses.
o Eject debris at tremendous spends and can be very destructive
o A lateral blast from the eruption of Mount St. Helens (1980) flattened forest
up to 25km away from the vent.
Pyroclastic Flows and Surges
• Pyroclastic Flow – A rapid flow of incandescent ash, blocks, and gas that have
been explosively erupted from a volcano. Pyroclastic flows result from the
collapse of an eruption column or lava dome. Also called ash flow or nuee
o Most lethal eruptive phenomena of a volcano
• Some Pyroclastic flows form when the towering column of ash rising above the
volcano collapses. Others form when a less energetic eruption boils over the
edge of a crater or caldera, or when a lava flow or dome on a steep slope
• Can move at speeds up to 150km/h, and run dozens of km’s away from their
• Destroy buildings and other objects in their paths by impact, burial, or
o People have no chance of survival if caught in a pyroclastic flow.
o Most deadly event occurred when Mount Pelee erupted in 1902. Killed
30,000 people on Caribbean island of Martinique. The only 2 survivors
were severely burned (no shit)
• Pyroclastic surge – A hot, gaseous, turbulent flow of pyroclastic material that
travels at high velocities down the flank of a volcano. Pyroclastic surges are less
dense than pyroclastic flows.
o more violent and travel much faster than pyroclastic flows (360km/h)