Download Notes-Volcanoes

Volcanoes
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-Volcanoes represent venting of the Earth’s interior
-Molten magma rises within the Earth and is erupted either quietly (lavas) or
violently (pyroclastics, solid blocks of material)
Terminology
Magma – molten rock sometimes containing suspended minerals and
dissolved gases. Magma forms when temperatures rise sufficiently high for
melting to occur in the Earth’s crust or mantle.
Volcano – a vent at the surface in which magma, solid rock, and gases
erupt.
Lava – magma that reaches the surface and pours out over the landscape.
Characteristics of Magma
Composition –
Controlled by the abundant elements in Earth (Si, Al, Fe, Ca, Mg, Na, K, H,
and O).
Most common types of magma are:
basaltic (~50% SiO2), andesitic (60% SiO2) and rhyolitic (70% SiO2)
Magma that solidify on the surface are called extrusive rocks and rocks that
solidify below the surface are called intrusive.
~70 - 75% of all magma erupted by volcanoes is basaltic the rest is split
between andesitic and rhyolitic.
- SiO2 = Silica
Dissolved Gases
Comprise a small percentage of the magma (0.2 to 3 wt.%). Although not
present in abundance these gases strongly influence the eruption style and
explosiveness of the magma.
Dominantly H2O and CO2 with small amounts of nitrogen, chlorine, sulfur
and argon.
Temperature
Ranges from ~800 C to ~1200 C
Viscosity
a substances resistance to flow
**dependent on temperature and composition
Important Rock Components
Bassalt, Andesite, Dacite, Rhyolite (Lava: Extrusive Surface Flows)
Diorite, Granite (Plutonic Rocks: Subsurface Intrusive Product)
Explosive vs. Effusive Eruptions
Three factors effect the explosivity of a volcano:
1) Temperature of magma
High-temperature, less explosive
2) Composition of magma
Less silica, less explosive
3) Gas content of magma
Less gas, less explosive
- Temperature + Composition collectively contribute to the fluidity of the
magma
Rhyolitic Volcanism
- Nobody has ever witnessed a rhyolitic volcano erupt
- SiO2 contents are even greater than is the case in andesitic magmas, (70%
vs. 60% respectively) therefore viscosities are even greater.
- So, Incredible resistance to flow!
- In addition, rhyolitic magmas tend to be richer in H2O, because they form
by partial melting of the crust, and melting is only possibly there if H2O is
present.
- In other words, rhyolitic magmas exsolve more H2O (more bubbles form as
the magmas rise), yet the bubbles cannot expand owing to the high viscosity
of the magma (they must expand as pressure decreases, i.e., as the
magmas rise through the crust)
- Recipe for a major disaster!!!
SUMMARY
Calderas are primarily rhyolitic
–largest explosive eruptions are caldera-related
–lava is cool and viscous, rises slowly, allowing pressure to build up
–gas percolates slowly through the viscous magma, does not have an easy
way to vent
Stratovolcanoes are dacitic-andesitic
–have eruptions of intermediate explosivity
–can undergo lava flows if lava is mafic enough and hot enough
–lava domes if lava is more felsic and cooler
–gas does vent, but slowly, through fissures
Shield volcanoes are usually basaltic
–lava is very hot and fluid
–gases easily pass through magma to be released into the atmosphere
–experience gentle, effusive activity
•fountaining if pressure builds, usually at the start of an eruption
Types
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5 Main Types of Volcanoes:
1) Caldera – A large depression created by the collapse of a volcano
2) Cinder Cone – A simple volcano built form blobs of lava ejected from a
single vent
3) Shield Volcano – Built from countless outpourings of fluid lava flows
4) Stratovolcano – Built of layers of lava, ash and volcanic debris
5) Lava Dome – Mound formed when viscous lava piles up around a vent
Cinder Cones
- Cinder Cones are volcanoes, which erupt but only during one episode (they
erupt once and never erupt again)
- They are small in size but incredibly explosive
- The cone is a pile of pyroclastic debris, which piles up at the angle of
repose
- Generally of basaltic composition (50% silica)
- Eruptive activity typically lasts a few months or years
Shield Volcanoes
-
Broad, gently sloping volcanoes
Composed mainly of basaltic lava flows
Also present on other planets (Mars)
Shield volcanoes and mars are MUCH larger than those on earth
Stratovolcanoes (composite volcanoes)
- Consist of alternating layers of lava and pyroclastics (for this reason
sometimes called composite volcanoes)
- Dominantly Andesitic in composition (60% SiO2)
- Typical of subduction zones
- Explosive eruptions
- Viscous lava
- Built of interlayered lava and pyroclastic material
Calderas
- Large depressions generally caused by the removal of large quantities of
magma from beneath a volcano causing the ground to collapse into an
empty space
- A Caldera is not a crater, a crater is a vent or opening atop a volcano
which is a common characteristic of all volcanoes however not every volcano
necessarily has a caldera but all have a crater
- Wizard island = “volcano within a volcano”, small cinder cone rising nearly
2,400 feet above the lake bottom
Formation
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How and where do magmas and volcanoes form ?
What tectonic environment do these volcanoes occur in and why?
1-Magma generation at hot spots (Shield Volcanoes)
- Basaltic magmas at hot spots are derived from deep within the mantle
- The magmas are fed by deep mantle plumes which are stationary relative
to the drifting tectonic plates
Intraplate Volcanism
- Volcanoes can also form where there is interplate volcanism (stretching &
thinning of the earth’s crust)
- Best example of intraplate volcanism is Hawaii
– More lava is extruded here constantly than anywhere else on Earth!
Hot Spots
- Some mantle anomaly allows the oceanic or continental lithosphere to melt
where it would not normally melt
- The anomaly (usually) stays stationary
- The plate(s) moves over it
- Islands and seamounts get older the further away you move from the hot
spot
2-Magma generation at mid-ocean ridges (Shield Volcanoes)
- In these zones, the mantle rises and melts, producing magma of silicate
composition
- The magma continues to rise, and erupts mainly as basaltic lava flows
-
This rifting process is dramatic on Iceland
Iceland is literally being torn apart by rifting of the two plates…
Yet its center is continually renewed by new magma from the mantle…
The same thing is going on under the ocean
Tensional forces opening the plates
3-Magma generation at subduction zones (Stratovolcanoes)
- Subduction is process that takes place at convergent bounrdries by which
one tectonic plate moves under another tectonic plate, sinking into the
earth’s mantle as the plates converge. A subduction zone is an area on Earth
where two tectonic plates move towards one another and subduction occurs
- During subduction, the subducted oceanic plate is heated as it plunges into
the mantle
- At a depth of 80-120 km, melting begins, and volcanoes are produced
which parallel the subduction zone
Volcanic Activity
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Lava Flows
-This is a basalt lava flow in a channel
-Due to its low silica content and high temperature, it is quite
fluid (but stickier than maple syrup)
-Yet lava usually flows fairly slowly
Pahoehoe lava
- This is a Hawaiian term for smooth, ropy lava
- It generally exhibits fluid-like textures
Aa lava
- This type of lava is quite blocky on the surface, and comparatively cool
- Yet below the surface, the lava is fairly massive and much hotter
Fire fountaining
- Sometimes, basaltic lava can contain lots of gas
- Then, small explosive eruptions form fire fountains
- As partially liquid drops fall back to the ground, they may coalesce to form
a lava flow
Flood basalts
- The previous examples represent small-scale activity
- But basaltic eruptions can be huge, forming lava plateaus
- These huge outpourings may occur quickly (1-3 Ma) and may contribute to
mass extinctions
Lava domes
- Form when the lava is silica-rich and thus highly viscous (sticky) and
cannot easily flow
- Thus it tends to form steep-sided domal structures
Pyroclastic falls
- During explosive volcanic eruptions, ash falls downwind of the volcano
- In the case of very large eruptions, the ash may be deposited over a vast
area
-Pyroclastic flows are suspensions of hot pyroclastic material, air, and gas
which descend under the influence of gravity
-Their velocity is generally very high (50-500 km/hr)
Pyroclastic Fall Hazards
Death
Structural damage
Contaminated drinking water
Climate change
Lahars
- Lahar is an Indonesian word for volcanic debris flow
- Lahars are flows of water and loose volcanic debris
- They are especially prevalent at snow-clad and ice-clad volcanoes
-Lahars are volcanic mudflows
- They are triggered in one of three ways:
1) Volcanic activity melting snow and ice on the volcano
2) Or a summit crater lake rupturing and draining due to volcanic activity
3) Torrential rainfall providing the water source
- Do not require an active volcano
- Rainwater mixes with existing ash to create the lahar
–Rocks and ash from a landslide entering an existing drainage route
- Can destroy dams, unleashing more water
-As they progress, lahars can undergo a process called “bulking” in which
they:
–Incorporate material from the area they flow over
- Erode sides of existing drainage channel
As they progress, they are also diluted by existing river water
–A dilute lahar is called a hyperconcentrated flow
- Following a river valley, these fast-moving flows can transport ash and
sediments over 250 km away from the source
Impact of Lahars
- During bulking, large objects like rocks, cars, houses can be picked up
–These often end up destroying bridges as the lahar progresses
- People living in river valley drainage systems of volcanoes are at risk
- Sediment load from the volcano can affect the way a river flows, fish
populations, etc.
Debris Avalanches
-
Sometimes a volcanic edifice is weakened
Wholesale collapse of part of the volcano may ensue
During collapse a debris avalanche occurs and a scalloped scar remains
Part of the volcano, flank, is weakened and collapses
Gases
- Volcanic gases are typically highly acidic
- Major constituents include H2O, CO2, HCL, SO2 and HF
- Powerful explosive eruptions can inject large amounts of gas into the
stratosphere causing climate change
Sizes of Volcanic Eruptions
- The volcano explosivity index (VEI) is similar to the richter scale for quakes
- It’s logarithmic and emphasizes the degree of the explosivity of eruptions
Eruptive styles
–Hawaiian eruptions can produce lava, but not much ash –Strombolian eruptions produce bombs and lapilli and more
–Vulcanian eruptions have an ash plume and only minor
–Plinian eruptions have lots of ash, minimal lapilli and blocks
ash
blocks
Volcanic Hazards of Canada
- Canada has “active” (bi-dormant, some are extinct some are dormant)
volcanoes which pose a potential threat in B.C
- Another major hazard is ashfall from explosive eruptions of Cascade
volcanoes in Washington state