Lecture 4 Flashcards
Where are most volcanoes found?
- located near plate boundaries
- 2/3 of all volcanoes are found on the “ring of fire” surrounding the Pacific Ocean
- subduction zones and mid-ocean ridges allow molten rock to reach the surface
What is magma?
-it is found deep within the crust and upper mantle
What are cinder cone volcanoes?
- these are relatively small volcanoes composed of small pieces of tephra
- they are around to oval-shaped and typically contain a crater at the top
- these volcanoes are found in Mexico
What is lava?
- it is found flowing from an erupting volcano
- lava is magma on the Earth’s surface
What are the components of magma?
- the most abundant elements in magma are silicon and oxygen; when combined they are referred to as silica
- volcanic rocks are named based on the amount of silica present
- types of volcanic rocks: basalt, andesite, dacite, rhyolite (low silica–> high silica)
What is magma viscosity determined by?
- contains small amounts of gases (water vapour and CO2)
- volcanoes have different shapes based on the chemistry and viscosity of their magma
- magma viscosity is determined by silica content and temperature
What are characteristics of magma with high silica content?
- cooler
- more viscous
- more gases
- produce the most explosive eruptions
- as magma approaches the surface the pressure lowers allowing gases to bubble up and escape
What are characteristics of magma with low silica content?
- hotter
- less viscous
- fewer gases
What kind of eruptions do rhyolitic and dacitic magma produce?
-explosive eruptions
What kind of eruptions do basaltic and andesitic magmas produce?
-eruptions that tend to flow rather than explode
What is a shield volcano?
- largest volcanoes on Earth and are shaped as broad arcs built from lava
- they are associated with basaltic magma
- eruptions are non-explosive and consist of gentle flows
- some eruptions contain tephra
- accumulations of tephra are referred to as pyroclastic deposits; if compacted together these deposits are called pyroclastic rock
- common in Hawaii, Iceland, and around Indian Ocean
What are composite volcanoes?
- cone-shaped and are built from a combination of lava flows and pyroclastic deposits
- also called stratovolcanoes; comes from layers of lava and deposits
- eruptions are more dangerous and explosive but less frequent than shield volcanoes
- common along west coast from Alaska to Oregon
- Mt.Rainier and Mt. St. Helens are the most well known composite volcanoes in NA
What are volcanic domes?
- contain highly viscous rhyolite magma
- steep-sided mounts that form around vents
What are maars?
- 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
- Maar derives from Latin “mare” meaning sea and resembles a large lake
What are ice-contact volcanoes?
- 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
- ice contact volcanoes are found in Iceland and BC
- evidence of the Mt. Garibaldi eruption 12000 years ago in BC is preserved in currently exposed rock
What is a crater?
-a depression formed by the explosion of a volcano top; they can be up to 2km in diameter
What is a 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
What is a caldera?
- a circular to oval depression formed during the collapse of a volcano
- they can be up to 25km in diameter
- eruptions that form caldera are the largest and most deadly eruptions on Earth
How do calderas form?
-collapse of a magma chamber below a composite volcano during an explosive eruption
What is a hot spring? What is a geyser?
hot spring: heated groundwater can discharge at the surface as a hot spring
geyser: groundwater that boils in an underground chamber to periodically produce a release of stream or water
- approx 1000 geysers on Earth and nearly half are located in Yellowstone National Park
What is Old Faithful?
- most famous geyser in the world
- erupts to a height up to 50m with eruptions lasting for 2-3 minutes
- the average interval between eruptions is 70 minutes
What are super eruptions?
- products of supervolcanoes and are extremely rare events
- supervolcano: a volcano capable of producing an eruption with ejected material covering thousands of square km with a lot of ash, can wipe out cities, crop land
- occur when a large volume of magma rises to shallow depths in the continental crust over a hot spot
- the magma is unable to break through the crust; pressure builds until the crust can no longer contain it
Describe the Yellowstone Supervolcano
- Yellowstone National Park sits on a massive caldera created from the last eruption of the Yellowstone Supervolcano
- the area is located over a continental hot spot
- super eruptions occurred 2.2 million years ago, 1.3 million years ago, and 640 000 years ago
- super eruption could last for weeks and spread ash over half of the US- would be over 1000 times the ash fall released by Mt St Helens
- millions would die from ash suffocation and US agriculture would be destroyed
What is the dominant volcanic rock and where does it originate?
- andesite that originates from a melting tectonic plate
- composite volcanoes are found at these locations
- over 80% of volcanic eruptions are above subduction zones (ex: cascade mountains)
Where are shield volcanoes found (in terms of plate tectonics)?
- mid ocean ridges
- these are composed of basaltic magma that originates in the athenosphere
- shield volcanoes are found at these locations
ex: Iceland
What results from hot spots beneath oceans?
- chains of shield volcanoes containing basaltic magma form as a plate moves over a hot spot
- ex: Hawaiian Islands
- island of Hawaii is still near the hot spot and contains active volcanoes
- volcanoes on Oahu and Maui are no longer active
What do hot spots beneath continents result in?
- produce explosive eruptions composed of rhyolitic and dacitic magma
- eruptions often result in the formation of calderas
- ex: Yellowstone National park
Where are volcanoes in Canada?
- generally restricted to BC and southern Yukon
- Mt Baker in Washington State provides the greatest actual risk to Canada
What are primary effects of volcanoes?
-lava flows, lateral blasts, pyroclastic flows, ash fall, poisonous gases, sector collapse
What are secondary effects of volcanoes?
-landslides, floods, fires, tsunamis, and lahars
How are volcanic eruptions measured?
- Volcanic Explosivity Index (VEI)
- scale ranges from 0-8 and is based on tephra ejected
- logarithmic scale
- no VEI 8 eruption has likely occurred in the last 10 000 years
- Yellowstone was VEI 8 and Mt St Helens was VEI 5
How can volcanoes affect climate?
- powerful eruptions can impact global climate
- ash and gases reflect solar radiation causing a cooling
- notable eruptions impacting climate:
Mt Tambora (1815) Indonesia- 1816 is known worldwide as the year without summer
Mt Pinatubo (1991) Philippines- 1992 is one of the coolest years worldwide in the 20th century
What are lava flows? What are the types of lava?
- these occur when magma flows out of a central crater or a fissure on the side of a volcano
- pahoehoe basaltic lava: low viscosity (a few km/hr), high temperature, when hardened it has a smooth texture
- aa basaltic lava: high viscosity (few metres per day), low temperature, when hardened it has a blocky texture
Found in Hawaii
What are lateral blasts?
- an eruption directed away from a volcano where materials are blown parallel to the surface
- ex: a lateral blast from Mt St Helens flattened forests for over 20km
What are pyroclastic flows?
- pyroclastic flows are avalanches of ash, gas, and rock fragments that travel down the slopes of a volcano during an explosive eruption
- speeds can reach 150km/h and the flow can travel up to 30km from the source
- most people have been killed by pyroclastic flows than any other volcanic phenomenon
What is ash fall? What are hazards of ash fall?
- particles of ash can be carried downwind hundreds of kms from an eruption site
- destroys vegetation, contaminates surface water, health hazards to people and animals, causes aircraft engine failure
What is sector collapse?
- the flank of a volcano can collapse at any time
- as magma travels up the channel, the volcano can inflate and its slopes can become steepened
What are lahars?
- large amounts of material that become saturated with water and move downslope
- also referred to as mudflows
- explosive eruptions can destroy nearby vegetation therefore enhancing the risk from lahars
What occurred at the beginning of Mt St Helens eruption?
- before its eruption, the volcano had been dormant for 120 years
- in march 1980 small explosions were evident due to groundwater contacting magma
- a bulge began growing on the flank of the mountain
What triggered Mt St Helens eruption?
- M 5.1 earthquake on May 18 1980 caused the area of the bulge to break off and fall downslope
- a lateral blast occurred from the area of the formed bulge and the entire north slope was destroyed
- ash was ejected from the central crater, reached heights of 19km, and travelled around the world
- did not affect climate
- killed 57 people mainly from pyroclastic flows
- left behind barren landscape that is slowly reforesting naturally
What are natural service functions of volcanoes?
- ancient volcanoes provided the gases that now form the atmosphere and sustain life on Earth
- internal heat from volcanoes can produce renewable geothermal energy
- volcanic soils can be good for agriculture since they are porous, well-drained, and contain nutrients
- volcanic landscapes attract tourism and recreation
- eruptions have created new land (Hawaii, Iceland)
What is usually the first sign of an impending volcanic eruption?
-small earthquakes
