Volcanoes Flashcards
Magma
Magma is molten or partially molten rock beneath the Earth’s surface
We refer to magma as lava (from the Italian, lavare, meaning “stream”) when it erupts onto the Earth’s surface
Extrusive igneous rocks
produced from the cooling and solidification of magma or lava at the Earth’s surface (e.g. basalt)
Intrusive igneous rocks
produced by from the cooling and solidification of magma within the Earth (e.g. granite)
Decompression Melting
Occurs at divergent plate boundaries, continental rifts, and hot spots
Great pressures are created at depth due to the weight of overlying rock
Thinning and stretching of the crust at rifts and divergent boundaries causes the mantle to well up towards the surface where pressures are lower
Plumes of hot rock well up to shallow depths at hotspots
Mid-Oceanic Ridges
At spreading ridges, mafic magma derived from the asthenosphere rises to the ocean floor to create new crust
When magma erupts underwater, it forms pillow lava, formed by repeated oozing and quenching of mafic magma
A flexible glass crust forms around the newly extruded lava, forming an expanded pillow
Pressure builds until the crust breaks and new magma extrudes like toothpaste, forming another pillow
In Iceland, a spreading ridge occurs on land; active volcanism associated with this diverging plate boundary
Continental Rifts Valleys
Tectonic forces stretch the crust causing Earth’s surface to fracture into normal faults
As the crust thins, the hot mantle can rise closer to the surface, producing magma through decompression melting
The magma travels through fractures in the crust, often along normal faults
Felsic
Felsic magmas are produced by heating and hydration of the continental crust
Horst and Graben Topography
This results in blocks that tilt and alternatively drop and rise (horst and graben topography)
Hotspots
As rising plumes of hot mantle migrate upwards they begin to melt under low pressure (decompression melting) to form magma; the magma rises to the surface and forms a volcano
Hotspots are fixed positions, as the plate carrying the volcano moves away from the hot spot volcanism ceases and a new shield volcano forms in the position over the hot spot producing island arcs (oceanic) or volcanic arcs (continental)
Volcanism associated with hot spots occurs in both the Atlantic and Pacific Oceans but is more common in the Atlantic because it moves at a higher velocity
Oceanic Hotspots
Oceanic hotspots produce mafic magmas (e.g. Hawaii) whereas continental hotspots produce a mixture of mafic and felsic magmas (e.g. Yellowstone)
Stratovolcanoes
Occur at subduction zones around Pacific Rim
Oceanic Island Arcs
Form at oceanic-oceanic convergent plate boundaries
Continental island arcs
Form at oceanic-continental convergent plate boundaries
Assimilation
Occurs when the temperature of rocks exceeds the melting temperature of silicate rocks at that depth
This heat melts adjacent rocks which then melt and change the composition of the melting magma, a process called assimilation
Composition of Magma
Magmas are composed of melted silicate minerals (SiO2; referred to as silica) and dissolved gases
Differentiated on the basis of how much silica the magma contains (see next slide)
Viscosity of Magma (Resistance to flow)
Stickiness factor: low viscosity = high fluidity
Mafic magmas have lower viscosity (higher fluidity) due to their chemical composition (less silica) than felsic magmas (in which silica tends to form strongly bonded chains)
Differences in viscosity influence the mobility of the magma when it is erupted onto the surface as well as the style of the eruption (effusive vs explosive)
Basaltic magma
Basaltic magma: 1000 to 1200° Celsius
Basaltic lava has a low viscosity when it first is erupted onto the Earth’s surface; as the lava cools away from the vent, its viscosity increases
Andesitic Magma
Andesitic magma: 800 to 1000° Celsius
Rhyolitic Magma
Rhyolitic magma: 650 to 800° Celsius
Gases of Magma
The percentage and type of volatiles within a magma influence its buoyancy and explosivity
The main volcanic gases are H2O (water vapor) and CO2 (carbon dioxide)
The volatile content of magma increases with corresponding increases in silica content
andesitic and rhyolitic magmas are more prone to explosive eruptions because they contain more dissolved gas (2-5 wt%) than basaltic magmas (<1 wt%)
Volatiles
Dissolved gases
Magma chamber or reservoir
large underground pool of magma
Country rock
surrounding rock that may be heated and mix with the magma
Dike/Sill
conduits along which magma reaches surface
Flank
often refers to the sides of the volcano
Fissure
a narrow opening or crack along which magma erupts often on a volcano’s flanks
Fumarole
opening through which volcanic gases emerge
Effusive Eruptions
are characterized by the outpouring of basaltic lava onto the surface (e.g. Kilaeua, Hawaii). These eruptions tend to be non-explosive because steam bubbles in the rising mafic magma are able to expand and burst
Explosive Eruptions
are characterized by the violent fragmentation of magma (e.g. Mount St. Helens, WA, 1985). These eruptions are explosive because the high viscosity of intermediate or felsic magmas do not allow trapped steam bubbles to escape leading to an increase in pressure. The main products of explosive eruptions are referred to as tephra (unconsolidated) or pyroclastic deposits (consolidated).
Shield Volanoes
Named for the resemblance to a warrior’s shield
Largest volcanoes on Earth with gently sloping sides (<5-10°) and broad summits
The shape is created from fluid basaltic lava that pours out in all directions from the central summit
Formed mostly from effusive eruptions (contains very little tephra)
Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the volcano (e.g. Pu’u ‘O’o’ cinder cone on Kilauea, Hawaii)
Composite Volcanoes
Also referred to as stratovolcanoes
Characterized by a steep, conical shape (6-10° on flanks to 30° near top)
Built mostly from tephra (up to 50%) but characterized by both effusive and explosive eruptions leading to the interlayering of lava flows and tephra
Owing to the higher viscosity of felsic magmas, they are usually more explosive (and dangerous) and erupt less often than shield volcanoes
Characterized by long periods of repose or inactivity lasting for hundreds or thousands of years
Cinder or Scoria Cones
These are steep-sided small volume cones (<300 m) with a round to oval surface and a crater on the top that often occur in association with shield volcanoes
They form from the accumulation of pyroclastic debris, usually cinders or scoria
Cinders or scoria form when basaltic or andesitic magma containing abundant volatiles is thrown out explosively from the volcanic vent
Cinder Cones
Cinder cones typically are monogenetic (erupting only once) and therefore have short life spans (decades)
Scoria Cones
Scoria is recognizable from its texture of abundant cavities produced from expanding steam bubbles called vesicles
Volcanic Domes
Volcanic domes are steep-sided mounds of lava that form around vents from the eruption of highly viscous high volatile felsic magmas (rich in silica) such as rhyolite and dacite
Lava piles up near vent, forming a very rough, spiny dome over the vent that is pushed up from magma below
Very dangerous as domes may produce lateral blasts (e.g. Mount St Helens, 1980) or pyroclastic flows
Craters
Less than 1 km in diameter
Circular to oval depressions at the top of volcanoes that form by the explosive ejecta of magma or from collapse when magma is withdrawn from a shallow magma chamber
Calderas
Larger than >1km usually several km in size
Circular to oval depressions at the top of volcanoes that form by the explosive ejecta of magma or from collapse when magma is withdrawn from a shallow magma chamber
Volcanic Vents
openings at the summit or flanks of the volcano where lava erupts or tephra or pyroclastic debris is ejected
Fissures or rift zones
Some vents are circular and others form along linear cracks known as fissures or rift zones
Resurgent Calderas
are >24 km in diameter that produce more than 1000 km3 of ash and fragmented rock
Produce super eruptions that are larger than any eruptions known in historic times
Supervolcanoes
originate when large volumes of magma rise to shallow depths in the crust above a mantle hot spot; pressure then builds until the eruption occurs
Supervolcanoes are associated with the eruption of large volumes of felsic magma at continental hotspots
Maars
are broad low relief craters that have formed by the violent interaction of magma and groundwater that form a crater by violent explosion (phreatic eruptions
Often fill with water along with craters and calderas to form crater lakes
Crater Lakes
Crater lakes may be deadly if a breach in the crater wall produces a lahar (Mount Ruapehu, New Zealand) or when they produce limnic eruptions in which dissolved CO2 suddenly erupts from deep later water
formed by filling of water in maars, craters, and calderas
Food Basalts
produced by extensive fissure eruptions over millions of years that result in thick, nearly horizontal lava flows covering thousands of km2 (e.g. North Mountain Basalt, Nova Scotia)
Ice-Contact Volcanoes
Many volcanoes erupt subglacially
Water leaks from the lake into the volcano below, destabilizing the magma and resulting in pyroclastic debris explosively ejected by hydrovolcanic eruptions
Eventually the volcano breaks through the glacier surface and basaltic lava erupts to form the flat-topped tuya
Tuyas
Tuyas form when a basaltic eruption melts part of the overlying ice and produces pillow basalt in the subglacial lake
In British Columbia, there is evidence of tuyas that were developed between 10,000-740,000 years ago
Shield Volcanoes
Shield volcanoes are typically found along divergent plate boundaries where crust is pulling apart (Iceland, East African Rift, Basin and Range) or oceanic hotspots
Stratovolcanoes
Stratovolcanoes occur above subduction zones; lava dome collapse often occurs in final stages of activity
Cinder cones
Cinder cones may occur in a variety of settings- commonly in association with or on the flanks of larger volcanoes
Effects of Volcanoes
Most active volcanoes are located near plate boundaries
2/3 of all active volcanoes on land are located along the Ring of Fire surrounding the Pacific Ocean
50 to 60 volcanoes erupt each year
Most eruptions are in sparsely populated areas
Nearly 100,000 people have been killed by eruptions in the past 100 years
500 million people live in the vicinity of volcanoes
Active volcanic regions around the world include Japan, Mexico, Philippines, and Indonesia
Western North America cities with populations of more than 100,000 include Vancouver (Mt Baker), Seattle and Tacoma (Mt Rainier), and Portland (Mt Hood)
Vesuvius, Italy, A.D. 79:
First extensively documented volcanic eruption by Pliny the Younger (16,000 killed)
Krakatau, Indonesia, 1883
Devastating link between tsunami and volcanic eruptions (36,000 killed)
Nevado del Ruiz, Columbia, 1985:
Devastating link between lahars and volcanic eruptions (23,000 killed)
Mt Pinatubo, Phillippines, 1991:
Lessons learned regarding volcano forecasting (60,000 evacuated; 300 killed)
Eruptive Styles
The Volcanic Explosivity Index (VEI) outlines several types of magmatic volcanic eruptions and quantifies their eruption size, volume, and strength
Styles of eruptions can range from frequent and mild (predominantly effusive) to infrequent and violent (predominantly explosive)
Styles are often named after famous volcanoes that exhibit a principal type of behaviour
Some volcanoes change style over the duration of one eruptive event yet others will exhibit the same style over several eruptive events
Magmatic Eruptions
defined by the VEI based on their eruptive mechanism and strength
Phreatic Eruptions
non-magmatic eruptions, driven by the superheating of steam in contact with magma (sources of water may include non-volcanic lakes, crater lakes, or shallow seas)
produces maars
Phreatomagmatic Eruptions
steam-driven explosions arising from the interaction of magma with ground water
hazards include ash falls and base surges
Hawaiian Eruptions (VEI 0)
very mild effusive eruptions characterized by lava flows (low silica, volatiles, viscosity)
Produces shield volcanoes
Occur at cinder cones
Strombolian Eruptions (VEI 1-2)
mildly explosive eruptions characterized by the production of cinder or scoria
Produced from the interaction of fluid magma (intermediate viscosity) with ground or sea water
Occur at cinder cones such as Paricutin, Mexico and typically at stratovolcanoes (e.g. Mt. Etna and Stromboli, in Italy)
Vulcanian Eruptions ( VEI 2-3)
more explosive than Strombolian eruptions due to andesitic to dacitic type magmas (high silica, volatiles, viscosity)
Produces small/several lava domes and collapse
Occur at stratovolcanoes such as Sakurajima, Japan; famous example is Nevado del Ruiz, Columbia (1985)
Pelean Eruptions (VEI 4)
characterized by devastating pyroclastic flows and a larger eruption column than seen in Vulcanian events
Named for the 1902 eruption of Mount Pelée in Martinique
Produces a much larger lava dome and collapse
Occur typically at stratovolcanoes such as Mayon Volcano, Philippines; Iceland (2010)
Plinian Eruptions (VEI 5-8)
highly explosive eruptions produced from dacitic to rhyolitic magmas (high silica, volatiles, viscosity) that generate massive ash fallout in addition to pyroclastic flows
Prevailing winds drive the eruptive column of gases, ash, and fragments away from the volcano (ashfall) covering regions from 0.5-50 km3 in size
Pyroclastic flows are generated at speeds up to 700 km/hr and extending hundreds of kilometers from the vent
Hot materials ejected from the summit may mix with snowfall, ice, and other volcanic deposits to form lahars
Named for Pliny the Younger who documented the A.D. 79 eruption of Mount Vesuvius, Italy
Occur typically at stratovolcanoes; famous examples include Mt St. Helens, WA (1980), Hekla, Iceland (1947-1948), Mt. Pinatubo, Phillippines (1991)
Eruptive Column
An eruptive column is produced in the magma chamber due to the build-up of pressure propelling magma up and out of the volcanic vent into the stratosphere
Ultra-Plinian Eruptions (VEI 8)
Same as Plinian characteristics but produce a much greater volume of eruptive materials (e.g.) Toba, Indonesia, 740,000 years ago
Perception of Volcanic Risk
People live near volcanoes for a variety of reasons
Place of birth
On some islands, all land is volcanic
Fertile land for farming
Believe an eruption is unlikely
Unaware of risk
Economic limitations
Perception of risk in Canada
Relatively low levels of volcanic activity and the remoteness of Canadian volcanoes has resulted in a general false perception that Canada’s volcanoes are extinct
Historic records in Canada (since 1867) are short in comparison to other regions of the world, however Indigenous oral traditions extend much farther back than the written record
Volcanoes have much longer lifespans than humans; they are generally active over thousands of years
Active
currently erupting, showing signs of unrest, or has erupted in historic time
Dormant
not currently active but could become restless or erupt again
Extinct
unlikely to erupt again, however keep in mind this is difficult to determine
Volcanoes in Canada
200 potentially active volcanoes in Canada
49 have erupted in the past 10, 000 years
Primary Effects
lava flows, ashfalls, volcanic bombs, pyroclastic flows and surges, lateral blasts, and poisonous gases
Secondary Effects
lahars, debris avalanches, landslides, groundwater and surface contamination, floods, fires, and tsunamis
Tertiary Effects
global cooling, famine, disease
Basaltic flows are the most common but least..
hazardous processes associated with volcanoes
travel at a few km/hr
devastating in populated regions
lava flows may have durations of many years
Pahoehoe flows
Smooth, hummocky, or ropy surface that forms as fluid lava drags cooled skin on the surface into small wrinkles and folds
Higher in temperature and volatiles but less viscous than aa flows
May develop lava tubes
AA flows
Rubbly surface composed of broken partially crystallized blocks called clinkers
Lower in temperature and volatiles but higher in viscosity; thus are much thicker and slower than pahoehoe flows
Lava Tubes
Lava tubes: develop when the surface of a pahoehoe flow solidifies but molten lava continues to flow in a channel beneath
Often consist of a main tube with several small tubes that supply lava to the front of one or more separate lava flows
Lava tubes may transport lava up to several km from the vent
Ways to Detect Lava Flows
1) Bombing
2) Hydraulic Chilling
3) Wall Construction
Siliceous Lava Flows
Thick stubby flows of andesite, dacite, or rhyolite that do not travel far from volcanic vent because of high viscosity
Hazardous because they induce pyroclastic flows due to lava dome collapse (Vulcanian, Peléan , and Plinian style events)
Tephra
general term for any fragments of rock and ash blasted into the air
Eruption cloud
tephra and gases downwind of the volcano
Eruption column:
tephra and gases directly above volcanic vent
Ash
<2 mm fragments
Lapilli
2-64 mm fragments
Blocks and bombs
> 64 mm fragments
Tephra falls
Secondary Effects
Reduction in visibility/causes darkness and atmospheric haze
Buildings may be damaged as tephra piles up on roofs Health hazards (respiratory illnesses)
Mechanical and electrical equipment can be damaged disrupting electrical power
Aircraft engines can experience failure (e.g. 1989, Mt Redoubt, Alaska; 2010 Eyjafjallajökull, Iceland)
Tephra Falls
Tertiary Effects
Atmospheric haze (causing brilliant sunsets)
Surface water may be contaminated
Damage to prime agricultural lands (famine)
Contributes to temporary global cooling (e.g. Laki 1783; Tambora, 1815; Krakatau, 1883; Pinatubo, 1991)
Tephra Fall in Canada
White River Ash
Produced the 2 largest ashfall deposits in the past 2,000 years
East lobe was produced 1149 years BP and north lobe was produced 1887 yrs BP from Mt Bona-Churchill, in Alaska
Ash from the east lobe eruption was dispersed from 25 km west of the Alaska-Yukon border to Great Slave Lake, NWT
Pyroclastic Flows
avalanches of hot rock, ash, volcanic rock fragments
Can move at speeds up to 150 km/h
Pyroclastic Surges
dense clouds of hot gas and rock debris produced by explosive interaction of water and magma
Can move at speeds of more than 360 km/h
Due to their speed surges are better able to overtop topographic barriers and cross bodies of water than pyroclastic flows
Lateral Blasts
Primary Effect
Rock fragments, gas, and ash that are blown horizontally from side of volcano, mixing with avalanche deposits that sweeps down the side of the volcano and reaches up to 25 km away
Poisonous Gases
Edifice or Sector Collapse
Debris Flows and Other Mass Movements
- Lahars (volcanic ash + pyroclastic material become saturated with water and rapidly move downslope.)
Tsunami
Interactions with Snow and Ice (CREATES LAHARS)
Poisonous Gases
Magma chambers may leak gas into overlying crater lakes or limnic eruptions may occur
Volcanoes release sulphur dioxide which mixes with water to form acid rain and “vog” (a type of smog)
Volcanoes also release fluorine which may contaminate pastures affecting livestock
Lahar
Lahar is an Indonesian term for a mudflow or debris flow that originates on the slope of a volcano
Secondary effect when lahars occur days, weeks or years after an eruption
Jökulhaups
Volcanic eruptions that occur subglacially or near glaciers produce jökulhaups
Jökulhaup is an Icelandic term for any sudden burst of water released from subglacial lakes or reservoirs
Jökulhaups may be triggered by melting of snow and ice from a volcanic eruption or geothermal heating
Iceland is at the greatest risk from these types of eruption-triggered floods
Minimizing Volcanic Hazards
- Monitoring Seismic Activity
Shallow earthquakes can precede eruptions
May not provide enough time for evacuation
Minimizing Volcanic Hazards
- Thermal, magnetic, and hydrologic monitoring
Accumulation of hot magma changes temperatures, magnetic properties, and chemical properties of rocks and groundwater
Minimizing Volcanic Hazards
- Land Surface Monitoring Gas Emissions
Monitoring the growth of bulges or domes
Minimizing Volcanic Hazards
Monitoring Volcanic Gas Emissions
Changes in carbon dioxide and sulphur dioxide emission rates may indicate movement of magma toward the surface
Minimizing Volcanic Hazards
Geologic History
Mapping of lava flows and pyroclastic deposits can be helpful in predicting future eruptive behaviour
