Lecture 8: Volcanoes
• Most volcanoes are located near plate boundaries.
• 2/3 of all volcanoes are found on the ‘Ring of Fire’ surrounding the Pacific Ocean.
• Subduction zones and midocean ridges allow molten rock to reach the surface
Types of Molten Rock
• Magma: it is found deep within the crust and upper mantle
• Lava: it is flowing from an erupting volcano (lava is magma that has reached the surface of the volcano)
o Essentially, lava is magma on the Earth’s surface.
• The most abundant elements in magma are silicon and oxygen; when combined they are referred to as silica.
• Volcanic rocks are named based on amount of silica present
Types of volcanic rocks:
• Basalt, Andesite, Dacite, Rhyolite
• low silica high silica
• Magma also contains small amounts of gases (water vapour and carbon dioxide).
• Volcanoes have different shapes based on the chemistry and viscosity of their magma.
• Magma viscosity is determined by silica content and temperature.
• Magma with high silica content:
o Cooler, more viscous, more gases
• Magma with low silica content:
o Hotter, less viscous, fewer gases
• Volcanoes with high silica magma produce the most explosive eruptions.
• As magma approaches the surface, the pressure lowers allowing gases to bubble up and escape
• Therefore, rhyolitic and dacitic magmas produce explosive eruptions.
• Basaltic and andesitic magmas produce eruptions that tend to flow rather than explode
Types of Volcanoes
• Volcanoes are classified into 4 different types.
• The classification is based on their shape, appearance, and style of eruptions.
• Types of volcanoes:
o volcanic dome
o cinder cone
• These are the largest volcanoes on Earth and are shaped as broad arcs (like warrior shields) built from
• Associated with basaltic magma • Eruptions are nonexplosive and consist of gentle flows
• Some eruptions can contain tephra.
• Accumulations of tephra are referred to as pyroclastic deposits.
• If compacted together, these deposits are called pyroclastic rock.
• These volcanoes are common in hawaii, Iceland, and around the Indian Ocean
• These volcanoes are coneshaped and are built from a combination of lava flows and pyroclastic
• They are also called stratovolcanoes; this term comes from the layers of lava and deposits.
• The magma is andesitic and dacitic
• Eruptions are more dangerous and explosive but less frequent than shield volcanoes
• These volcanoes are common along the west coast from Alaska to Oregon.
• Mt. St. Helens is the most famous composite volcano in North America
• These volcanoes contain highly viscous rhyolite magma.
• They are steepsided mounts that form around vents
Cinder Cone Volcanoes
• These are relatively small volcanoes composed of small pieces of tephra.
• They are round to oval shaped and typically contain a crater at the top
• They are found in Mexico
• Definition: A circular volcanic crater produced by an explosive eruption and filled with water
• They are caused by groundwater coming in contact with magma creating an explosion.
• Some volcanoes erupt beneath or against glaciers.
• These eruptions melt huge quantities of ice producing floods known as jokulhlaups
• When lava contacts glaciers, it quickly cools to form pyroclastic rock.
• Icecontact volcanoes are found in Iceland and British Columbia
• Evidence of the Mt. Garibaldi eruption 12,000 years ago in British Columbia is preserved in currently
• Crater: A depression formed by the explosion or collapse of a volcano top. They can be up to 2 km in
• Volcanic Vent: An opening on the surface through which lava and pyroclastic debris erupt
• Most vents are circular but some are elongated cracks called fissures.
Caldera: A circular to oval depression formed during the collapse of a volcano
• the entire volcano collapsed up to 25 km in diameter
Eruptions that form caldera are the largest and most deadly eruptions on Earth.
• we need to pay attention in this area
Formation of Calderas • Calderas form by the collapse of a magma chamber below a composite volcano during an explosive
Hot Springs and Geysers
• Heated groundwater can discharge at the surface as a hot spring
• Groundwater that boils in an underground chamber to periodically produce
a release of steam or water is called a geyser.
o a lot of pressure build up
o very rare
• There are approximately 1000 geysers on Earth and nearly half are located
in Yellowstone National Park (Located in a very small area)
• This is the most famous geyser in the world.
• it erupts to a height up to 50 m with eruptions lasting for 23 minutes
• The average interval between eruptions is 70 minutes.
• These are the products of supervolcanoes and are extremely rare events.
• They occur when large volume of magma rises to shallow depths in the
crust over a hot spot (rare)
• Yellowstone is a super volcano
• Yellowstone is inland from the coast
• The magma is unable to break through the crust; pressure builds until the
crust can no longer contain it. Only a matter of time before the magma
breaks through the tough threshold
• Super eruptions occurred about 600,000 years ago in Yellowstone National
Park and about 700,000 years ago in California.
• A massive amount of land is covered by ash fall from super eruptions.
• Yellowstone National Park sits on a massive caldera created from the last eruption of the Yellowstone
• The area is located over a continental hot spot
• Super eruptions occurred 2.2 Mya, 1.3 Mya, and 640,000 years ago
• A super eruption could last for weeks and spread ash over half of the United States. Ash fall would be
over 1000 times that released by Mt. St. Helens.
• Millions of people would die from ash suffocation and the US agriculture economy would be
• The park is continuously monitored for geologic activity.
Volcanoes and Plate Tectonics
• Volcanism is directly related to plate tectonics.
Volcanoes and Plate Tectonics
• The dominant volcanic rock is andesite that o