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Exam 2 > Lecture 1 volcanoes > Flashcards

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
Q

describe Basalt/ basaltic lava

A
  • 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
Q

Name two types of basaltic lava?

A

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
Q

Describe a shield volcano

A

(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
Q

Convergent plate boundaries - what are the three types?

A
  • Subduction of oceanic crust below
    continental crust
  • Subduction of oceanic plate below
    another oceanic plate
  • Collision of two continents
29
Q
  • Subduction of oceanic crust below
    continental crust
A
  • Formation of deep trench and
    volcanic arc
  • Earthquakes
  • e.g. California, Japan
30
Q
  • Collision of two continents
A
  • Formation of a mountain range
  • e.g. Alps, Himalayas
31
Q
  • Subduction of oceanic plate below
    another oceanic plate
A
  • Formation of deep trench and island arc
  • Earthquakes
  • e.g. Philippines, Mariana Trench
32
Q

describe the process of the volcanoes at the convergent plate boundary

A

as the plate subducts the water and minerals are squeezed and water is removed, this hydrates = melting.

33
Q

Describe typical stratovolcano - volcanic arc

A

Typical stratovolcanoes
(e.g. Mt. Fuji, Arenal, Cotopaxi, Mt. St. Helens)
* Tend to be more andesitic to rhyolitic in composition.

34
Q

Typical stratovolcano - island arc

A

Typical stratovolcanoes (e.g. Pinatubo)
* Incipient island arcs tend to be more basaltic in composition

35
Q

Explain volcanism at convergent plate boundaries

A
  • 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
Q

trend with viscosity and bubbles

A

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
Q

Explain the VEI

A

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
Q

Strombolian eruption

A
  • 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
Q

Vulcanian eruption

A

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
Q

Plinian (or vesuvian)
eruption

A
  • 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
Q

What are the typical rock types from subduction zone related volcanism

A

Andesite/Dacite/Rhyolite

42
Q

Andesite

A
  • 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
Q

Dacite

A

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
Q

Rhyolite

A

Light-colored rock with >68 weight % SiO2
* Quartz, feldspar and biotite in glassy matrix
* Erupt at temperatures of 700 to 850°C

45
Q

volcanism at divergent plate boundaries summery

A

Volcanism at divergent plate boundaries is
generally non-explosive and of low viscosity
basaltic lava.

46
Q

What are the Typical volcanoes at divergent plate boundaries and hot spots

A

Typical volcanoes at divergent plate boundaries
and hot spots are fissure, and shield volcanoes

47
Q

Subduction related volcanism is generally …

A

Subduction related volcanism is generally explosive with typical stratovolcanoes and andesitic to rhyolitic lava composition

48
Q

What is Explosiveness of volcanoes a function of / caused by

A

Explosiveness of volcanoes is a function of gas
content and viscosity of magma.

Exam 2 (21 decks)

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