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Lecture 1 volcanoes Flashcards

1
Q

Volcano definition

A

*a vent in the Earth’s crust through which lava, rock fragments, vapour and gases erupt
*a mountain formed by volcanic material

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2
Q

Pyroclasts Definition

A

all solid fragments ejected from volcanoes

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3
Q

Lava

A

magma that erupts on Earth’s surface

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4
Q

Tephra

A

all pyroclasts that fall to the ground from eruption columns

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5
Q

Magma

A

molten rock inside the earth

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6
Q

What are monogenetic volcanoes?

A

Volcanoes that erupt only once

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7
Q

What are polygenetic volcanoes?

A

Volcanoes that erupt many times

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8
Q

How do volcanoes erupt?

A

*Magma is buoyant (has a lower density than
surrounding rocks) and will rise up through the crust to erupt on the surface.

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9
Q

What type of gas and magma will form from effusive eruptions?

A
  • Small amount of entrapped gases and low viscosity magma will form effusive eruptions.
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10
Q

What type of gas and magma will form from explosive eruptions?

A
  • Large amount of entrapped gas and high viscosity magma will form explosive eruptions.
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11
Q

How does a volcano erupt? (compared to other volcanoes - what changes how it erupts?)

A

*When magma reaches the surface it depends on how easily it flows (i.e. viscosity) and the amount of gases (e.g. CO2, S, H2O) it contains as to how it erupts.

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12
Q

What controls viscosity?

A

Silica content, gas content and magma temperature.

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13
Q

how does viscosity effect explosiveness?

A

more viscous = more explosive

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14
Q

Why do volcanoes exist?

A

Primordial heat (all of the heat left over from when the Earth formed)
- radioactive decay from
40K, 235U, 238U, and 232Th

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15
Q

What is the mantle?

A

~1800 miles (2,900 km) thick
(80% of Earth’s volume)

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16
Q

What is the upper mantle?

A

Upper Mantle
-(Fe, Mg silicates)
- Two sub-layers
- Bottom layer, 1400 -
3000°C, deforms plastically
- Upper layer = same rock
but stiffer because of lower T

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17
Q

What is the lower mantle?

A
  • Solid rock at 3000°C (high
    pressure)
  • Silicon, magnesium oxides
    and sulphides
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18
Q

Why does melting occur? (3)

A

Due to heating, decompression and hydration

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19
Q

What are the three types of plate boundaries? for volcanoes

A

spreading ridge volcanism: ~75%
subduction zone volcanism: ~15%
intraplate volcanism: ~10%

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20
Q

How would you describe volcanism at divergent plate boundaries?

A

plat eboundaries move away from eachother, boyaunt magma moves upwards.
Along mid-ocean ridges
* ~75% of annual output of magma on
Earth
* ~4,000 volcanoes per million square
km on the floor of Pacific Ocean 
probably over a million submarine
volcanoes on the ocean bottoms

21
Q

How would you describe volcanism at hot spots?

A
  • Characteristics: shallow slopes, smooth broad shield volcanoes
  • Usually non-explosive, basaltic lava flows if related to oceanic crust
  • Mostly basaltic lava flows
  • Where hotspots occur below continental crust, continental crust melts to form rhyolitic lavas, which can cause explosive eruptions (e.g. yellow stone Caldera)
22
Q

What are ocean islands often linked to?

A

Volcanism at hotspots
Hot spots long lived & static
* Age progression in the volcanism
* Hawaii: presently active volcanoes at
SE end of chain
* Best explained by a plate moving over a stationary
hotspot or plume for the last 70 Myr

23
Q

LIPS

A

Large igneous provinces/ continental flood basalts
Volcanism at hot spots: LIPS
Large igneous provinces / continental flood basalts Fissure volcanism on land, mostly associated with the past
 continental flood basalts
e.g. Columbia River Basalt, USA (0.25x106 km3); Deccan Traps, India (>2.0x106 km3)

24
Q

Why is Iceland interesting?

A

It is where hotspot and MOR meet
* Large volumes
* Low viscosity
* Hot (~1200°C) basaltic lava
* Fissure volcano type (fissure is the type of vent)
* Usually non-explosive, effusive activity
* Fissure volcanism on land mostly associated with the past continental flood basalts

25
describe Basalt/ basaltic lava
* Most common rock type in Earth’s crust * Covers 2/3 of the Earth’s surface * e.g. Large Igneous Provinces (continental flood basalt, ocean basin flood basalt, submarine ridges, ocean islands - in red.
26
Name two types of basaltic lava?
A a lava Eruption temperature 1000°C to 1100°C. pahoehoe lava Eruption temperature 1100°C to 1200°C low viscosity low dissolved gas content
27
Describe a shield volcano
(e.g. Hawaii, Galapagos) * Low viscosity lava flows * Low silica magma – basalt (pahoehoe & a’a) * Gently sloping flanks – between 2 and 10 degrees * Tend to be very large * Ratio of height and half-width of the base <0.18
28
Convergent plate boundaries - what are the three types?
* Subduction of oceanic crust below continental crust * Subduction of oceanic plate below another oceanic plate * Collision of two continents
29
* Subduction of oceanic crust below continental crust
* Formation of deep trench and volcanic arc * Earthquakes * e.g. California, Japan
30
* Collision of two continents
* Formation of a mountain range * e.g. Alps, Himalayas
31
* Subduction of oceanic plate below another oceanic plate
* Formation of deep trench and island arc * Earthquakes * e.g. Philippines, Mariana Trench
32
describe the process of the volcanoes at the convergent plate boundary
as the plate subducts the water and minerals are squeezed and water is removed, this hydrates = melting.
33
Describe typical stratovolcano - volcanic arc
Typical stratovolcanoes (e.g. Mt. Fuji, Arenal, Cotopaxi, Mt. St. Helens) * Tend to be more andesitic to rhyolitic in composition.
34
Typical stratovolcano - island arc
Typical stratovolcanoes (e.g. Pinatubo) * Incipient island arcs tend to be more basaltic in composition
35
Explain volcanism at convergent plate boundaries
* Central composite (strato-) volcanoes. * Usually constructed through a mixture of lava and ash deposits * Steep slopes * Ratio of height to half-width of base >0.18
36
trend with viscosity and bubbles
Volcanic activity in subduction zones is explosive and devastating Magmas = more viscous  Main feeders to volcano often plugged and high viscosity inhibits escape of expanding bubbles  Release of volatiles can result in violent volcanism as plug is breached  Gives rise to explosive pyroclastic deposits
37
Explain the VEI
Volcanic explosivity index * Describes intensity of volcanic eruption * Increase in number represents an increase around a factor of 10 * Includes: volume of erupted pyroclastic material, height of eruption column, duration in hours and qualitative descriptive terms, such as (name for eruption, number of those eruptions in human history)
38
Strombolian eruption
* basaltic eruptions with low viscosity lava * episodic explosive eruptions, accompanied with loud blast (burst of gas bubbles) * ejection of volcanic bombs and lapilli fragments * building of cinder cone. e.g. Mt. Etna, Stromboli
39
Vulcanian eruption
highly viscous magma * high gas pressure -> popping of the cap that holds magma down * eruption columns between 5 to max. 20 km high * deposit of andesitic to dacitic material
40
Plinian (or vesuvian) eruption
* Si-rich magma with very high dissolved gas content * Gases in magma chamber vesiculate, accumulated and agglutinate during rise in magma conduit eruption columns >20 km * Deposition of dacitic to rhyolitic material * Typical heavy pumice airfall * Associated with large pyroclastic flows * e.g. Mt. St. Helen, Pinatubo
41
What are the typical rock types from subduction zone related volcanism
Andesite/Dacite/Rhyolite
42
Andesite
* Black to gray rock with 53 - 63 weight % SiO2 * Most common rock type on stratovolcanoes * Plagioclase feldspar, pyroxene, iron oxide * Erupt at temperatures of 900 to 1100°C
43
Dacite
Light gray rock with 63 – 68 weight % SiO2 * Most common rock type associated with plinian eruptions * Plagioclase feldspar, pyroxene, amphibole * Erupt at temperatures of 800 to 1000°C
44
Rhyolite
Light-colored rock with >68 weight % SiO2 * Quartz, feldspar and biotite in glassy matrix * Erupt at temperatures of 700 to 850°C
45
volcanism at divergent plate boundaries summery
Volcanism at divergent plate boundaries is generally non-explosive and of low viscosity basaltic lava.
46
What are the Typical volcanoes at divergent plate boundaries and hot spots
Typical volcanoes at divergent plate boundaries and hot spots are fissure, and shield volcanoes
47
Subduction related volcanism is generally ...
Subduction related volcanism is generally explosive with typical stratovolcanoes and andesitic to rhyolitic lava composition
48
What is Explosiveness of volcanoes a function of / caused by
Explosiveness of volcanoes is a function of gas content and viscosity of magma.

Exam 2 (21 decks)

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