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GEOLOGY 12
CHAPTER 4 NOTES
VOLCANOES (WEB VERSION)
A.
Name __________________
TYPES OF VOLCANOES
Shield Volcanoes
• basaltic lavas – low in silica, high in iron & magnesium
• fluidic, flow freely, flow long distances
• build up a flat volcano
• lava flows individually thin sheets
• often produced by hot spots
• Hawaiian Island – Mauna Loa – largest peak, still active – 10 km high, 100 km in
diameter (measured from sea floor)
Composite Volcanoes or Stratovolcanoes
• usually more dangerous
• erupt different materials at different times and so are built up by layers of
different material
• usually over subduction zones
• fairly stiff magma (more silica), containing much gas
• may erupt explosively due to trapped gases or water vapor
• steep sided, combination of pyroclastic and viscous lavas.
Cinder Cones
• volcano built of pyroclastic materials
• magmas may contain dissolved water and gases trapped under great pressure
in lithosphere.
• as it rises, pressure is released and gases escape explosively throwing out bits
of magma and rock
• Size ranges of solid materials (pyroclastics):
- dust and flow-like ash
- coarse gritty volcanic ash
- cinder – up to golf ball size
- blocks – up to house size (rocks)
- bombs – blobs of magma which are streamlined in flight
- tuff - rock made up primarily of ash sized pyroclastic fragments
- volcanic breccia - coarser rock containing large angular blocks (i.e./dallasite)
Volcanic Domes
• more silica lavas (andesitic and rhyolitic)
• more viscous, thicker, oozing, potentially explosive
• makes compact steep-sided volcanoes – but over time can become quite tall
• example - Mt. St. Helens
• covers small in area compared to shield
Lava Plateaus
• a.k.a. fissure eruptions
•
•
very basaltic, fluid magma coming from a fissure rather than a single pipe
examples - Columbia Plateau – Washington, Oregon, Idaho (50000km2) – layer
after layer, up to 1.5 km thick.
Hot Spots
• stationary magma plums from mantle
• Hawaiian Islands
• Yellow Stone National Park and north western states
Columnar Joints
• lava & volcanic rock contracts as it cools and may fracture into a mass of
polygonal (hexagonal) columns.
• Ireland’s Devil’s Causeway
B.
TYPES OF MAGMA
Magma is melted underground rock. It forms wherever the temperatures are high
enough to melt the rock. This happens in the asthenoshpere and at plate
boundaries. When it melts, it becomes larger, is lighter, and rises to the surface as
volcanoes.
How fast it moves depends on its silica content. High silica content magmas (felsic
magmas) are thick, light colored and move slowly. Mafic magmas have lower silica
content, are thinner, and flow more easily. Magma contains water vapor, carbon
dioxide, and sulfur gases as well as hydrogen (to make steam), carbon monoxide
(to make carbon dioxide), chlorine, and fluorine. The sulfur gas becomes hydrogen
sulfide and sulfur dioxide. The gases are dissolved in the magma and bubble off
when it reaches the surface. If the magma is thick, this bubbling off when it reaches
the surface. If the magma is thick, this bubbling may be explosive. The more gas,
the more explosive the eruption. When magma reaches the surface, it becomes
lava.
Types of Lava
1. Temperatures generally range 5000C – 14000C
2. Mafic Lavas (basaltic composition)
a) Pahoehoe – smooth, billowy or ropy texture (Hawaii)
b) Aa – blocky and rough
c) Pillow Lava – glassy rind, coarse grained interior, formed in water
3. Felsic – thicker and more explosive eruptions, granitic or andesitic composition
Types of Pyroclastics
Explosive eruptions produce solid fragments called tephra.
• ash – tephra less than 2mm in diameter.
• lapelli – tephra 2 – 64 mm in diameter
• blocks – tephra greater than 64 mm – erupted as solids
• bombs – tephra greater than 64 mm – erupted as liquid and solidifies in the
air
Types of Hazards
1. Nueé Ardente – "glowing cloud"
Tephra may combine with gases to form a dense superheated cloud that flows
down the flanks of the volcano at speeds of more than 100 km per hour.
Eg. Mount Pelée (1902) destroyed St. Pierre in the Caribbean and killed 30,000
people within minutes.
C.
2.
Toxic Gases
ƒ may be CO (carbon monoxide), sulfur gases, HCl & HF acids
ƒ 1986 Lake Nyos, Cameroon - 1700 killed by CO2 gasses released from lake
3.
Steam Explosions
ƒ water (especially seawater) seaps into rock, becomes superheated to steam
and blows up volcano as PHREATIC ERUPTION
ƒ Krakatoa 1883 – 100 Mt. of TNT, heard 3000 km away. Dust shot 80 km up
into atmosphere.
ƒ shockwave made tsunami 40 m high which killed 3600 people on other
islands
4.
Lahars
ƒ ash, ice and snow melts (causes mudslides)
ƒ 20,000 died 1985 Nevada del Ruiz (Columbia)
5.
Climate Changes
ƒ dust may cause cooling
ƒ gas may make acid rain
VOLCANOES & TECTONICS
Divergent Boundaries
• spreading seafloor ridges, basaltic, mafic lava
• spreading continental ridges – mostly basaltic but some silicic
Convergent Boundaries
• usually undergoing slab is mafic (from seafloor) and some sediments (more
silicic) which contain water
• process of "dewatering" involves:
- releasing water from sediments which run into upper mantle between plates
and lowers melting temp. of material and induces magma formation
- andesitic to basaltic lava
- making island arcs and volcanoes on continental edges
- volcanoes above the subduction zones of continental edges are andesitic
(and explosive)
Intraplate Volcanism
• believed to originate from mantle plumes or Hot Spots
• examples: Hawaii, Yellowstone
• magma depends on original source rocks
Structure of a Volcano
Dike
Volcanic Hazards
CANADIAN VOLCANOES
The volcanoes in southwestern British Columbia were formed by the subduction of the
Juan de Fuca and the Explorer Plate which are remnants of the much larger Farallon
Plate, under the North American Plate and has produced the Cascadia subduction zone. It
is a 680-mile long fault, running 50 miles off the west-coast of the Pacific Northwest. It
starts from Northern California and stretches to northern Vancouver Island, British
Columbia. The subduction zone has created large earthquakes, including the Cascadia
earthquake, which took place at the evening of January 26, 1700 by a magnitude 8.7 - 9.2
megathrust earthquake. Unlike in most subduction zones, there is no trench present along
the continental margin. Instead, terranes and the accretionary wedge have been uplifted to
form a series of coast ranges and exotic mountains. The subduction zone moves at rate of
over 10 mm per year in a north-easterly direction and the outer margin is slowly being
compressed, similar to a giant spring.
Garibaldi Volcanic Belt
The Garibaldi Volcanic Belt was formed by the subduction zone and contains the most
explosive young volcanoes in Canada. It is a north-south range of volcanoes and is the
northern extension of the Cascade Volcanoes in the United States. The eruption styles
within the belt range from effusive to explosive, with compositions from basalt to rhyolite. A
major catastrophic eruption occurred in the Garibaldi Volcanic Belt about 2,350 years ago
at a complex volcano, known as Mount Meager. The eruption produced a ash column at
least 20 km high into the stratosphere.
MT ST. HELENS
Mount St. Helens is an active stratovolcano in Washington, in the Pacific Northwest region
of the United States. The mountain is located in the Cascade Range and is part the
Cascade Volcanic Arc, a segment of the Pacific Ring of Fire that includes over 160 active
volcanoes. This volcano is well known for its ash explosions and pyroclastic flows.
Mount St. Helens is most famous for its catastrophic eruption on May 18, 1980, which was
the deadliest and most economically destructive volcanic event in the history of the United
States. Fifty-seven people were killed; and 250 homes, 47 bridges, 24 km of railways and
300 km of highway were destroyed. The eruption caused a massive debris avalanche,
reducing the elevation of the mountain's summit from 2,950 m to 2,550 m, and replacing it
with a 1.5 km-wide horseshoe-shaped crater. The debris avalanche was the largest in
recorded history. However, the scale of the blast is considered minor when compared with
past debris avalanches elsewhere on Earth.
As with most other volcanoes in the Cascade Range, Mount St. Helens is a large eruptive
cone consisting of lava rock interlayered with ash, pumice, and other deposits. The
mountain includes layers of basalt and andesite through which several domes of dacite
lava have erupted. The largest of the dacite domes formed the previous summit; and off of
its northern flank sat the smaller Goat Rocks dome. Both were destroyed in the 1980
eruption.