A: The Earth Flashcards
(79 cards)
1
Q
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How old is Earth
A
4.567bn years
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2
Q
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Older volcanoes erupted with a greater % of what
A
Olivine
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3
Q
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What % of Earth volume does mantle comprise
A
84%
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4
Q
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Where are most volcanoes found
A
On oceanic ridges formed as tectonic plates separate from each other
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5
Q
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Why is there less explosive volcanism in oceans
A
Water exerts greater pressure as 2.5km of water = 250x air pressure
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6
Q
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Magma
A
Molten rock below the surface
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7
Q
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Where does almost all volcanism begin
A
Mantle
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8
Q
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What is the mantle mainly composed of (a type of rock)
A
Peridotite
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9
Q
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What is Peridotite comprised of
A
Crystalline minerals
- Olivine
- Pyrixen
- Garet
- Feldspar
- Metal oxides
- Oxygen, silicon, magnesium, iron, aluminium, coopper - compose 99% of mass
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10
Q
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What is the state of matter of the mantle
A
Solid
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11
Q
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What causes the mantle to flow through creep
A
Heat and gravity
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12
Q
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What is primordial heat
A
Heat originating from when Earth was first formed
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13
Q
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What continues to release heat into Earth’s interior
A
Radioactive decay of isotopes of uranium, potassium and thorium
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14
Q
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How is heat transferred out of Earth to surface
A
Convection
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15
Q
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Why do volcanoes form? (basic)
A
Because the mantle melts
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16
Q
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2 proceses for mantle melting
A
- Oceanic ridges and hotspots
- Subduction zones
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17
Q
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Decomprssion melting (1)
A
- Occurs at oceanic ridges and hotspots
- within 300km of surface
- minerals melt at different temp (e.g., magnesium rich vs iron rich olivines)
- Melting points are sensitive
- less pressure = lower melting point as less energy needed to break intermolecular forces
- as rises to surface, less pressure = mantle melts
- Partial melting as only minerals with lowest melting points melt -> 1-20% of peridotite melts
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18
Q
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Partial melting
A
Where 1-20% of minerals in Peridotite melt via decompression cooling
- Occurs at subduction zones
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19
Q
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What % silica does Basaltic lava have
A
45%
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20
Q
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Silica chemical symbol
A
SiO2
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21
Q
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What type of melting forms oceanic ridges
A
Adiabatic deocompression melting
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22
Q
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What forms hotspots
A
Mantle plumes formed by adiabatic decompression melting
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23
Q
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What % of heat and magma can be attributed to mantle plumes
A
5-10%
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24
Q
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What are great outpourings of lava scientifically referred to as
A
Large Igneous Provinces (LIPs)
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25
# Oppenheimer (2011)
Average rate of spreading at oceanic ridges per year
5cm
## Footnote
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# Oppenheimer (2011)
What forms is magma found in
Solid, liquid and gas
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# Oppenheimer (2011)
What is solid magma composed of?
Minerals - olivine, feldspar, pyroxene and quartz
Silicate melt - loose arrangements of silicon and oxygen atoms mainly
Volatile components - water, CO2, sulphur and halogens
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# Oppenheimer (2011)
What strong bonds form between Oxygen and Silicon
Covalent
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# Oppenheimer (2011)
What is a major factor affecting how magma moves, volcanic hazards and eruptions
The polymerisation of silica tetrahedra
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# Oppenheimer (2011)
Primary source of greenhouse gases into atmospehre on geological timescales
Volcanic outgassing
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# Oppenheimer (2011)
How can magma accumulation lead to eruptions
- Magma finds level of neutral buoyancy where gravity can no longer propel it upwards
- Magma ammasses in Magma chambers
- Often causes fissure eruptions
## Footnote
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# Oppenheimer (2011)
Fractional distillation
- As magma cools, crystals grow are composed of minerals with the highest melting/freezing point
- Can increase eruptions as magma enriched by volatiles such as water and CO2
- Cooling of volatiles creates bubbles
- Bubble expansion increases pressure of magma chamber
## Footnote
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33
# Oppenheimer (2011)
How many main types of magma are there
3
- Basaltic (45-52% SiO2 by mass(
- Intermediate (52-63% SiO2 by mass)
- Silicic (>63% SiO2 by mass)
## Footnote
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34
# Oppenheimer (2011)
What substance largely determined the effusivity of eruptions
Water
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# Oppenheimer (2011)
What plate boundaries have the highest concentrtaion of dissolved volatiles such as water
Subduction zones
- derive lots of water from water, sulphur and chlorine from subducted oceanic rocks
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# Oppenheimer (2011)
Impact of bubble formation
- Lowers density and increases volume
- Reduces viscocity
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# Oppenheimer (2011)
Impact of water on viscosity
- increases it
- water inhibits covalent bonding between silca tetrahedra which causes it to string together in chains
- Increases viscosity
- contributes to more explosive eruptions
## Footnote
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# Oppenheimer (2011)
Anaolgy for eruptions
Uncorking of champagne
- dissolved carbon dioxide is released as chamber walls fail
- reduction of pressure insude bottle reduces the solubility of CO2, which releases CO2 gas
- CO2 gas expands which releases foam out of the bottle
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# Oppenheimer (2011)
What are hydrovolcanic eruptions
Where magma meets water causing steam eruptions
- commonly associated with the reawakening of domant volcanoes
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What do explosive eruptions produce
Tephra/pyroclasts - fragmented rocks including pumice, ash and bombs
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# Oppenheimer (2011)
What do effusive eruptions produce
lavas
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Why are eruptions called Plinian
Like the 79CE eruption of Vesuvius recorded by Pliny the Younger
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# Oppenheimer (2011)
What are two fundemantal eruption parameters
- Magnitude
- Intensity
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# Oppenheimer (2011)
What is Earth's interior doing
Cooling down
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45
# Oppenheimer (2011)
What % of Earth's volume does the mantle comprise
84%
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46
# Oppenheimer (2011)
Where do most volcanoes lie
Oceanic ridges at the boundaries of tectonic plates
## Footnote
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47
# Oppenheimer (2011)
Why is volcanism less violent 2.5km at the depths of the ocean
2.5km water column exerts a pressure 250x sea level pressure
This inhibits explosive volcanism
## Footnote
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# Oppenheimer (2011)
Magma
Moltenr rock below Earth's surface
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# Oppenheimer (2011)
Lava
What comes out of a volcano
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# Oppenheimer (2011)
What is Earth's core rich in
Iron
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# Oppenheimer (2011)
What is the mantle comprised largely of
Rock called peridotite
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# Oppenheimer (2011)
How does the mantle 'flow'?
Creep
- Crystals slop past each other and atoms and ions diffuse from one place to another
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# Oppenheimer (2011)
Examples of primordial heat
- Kinetic energy of meterorite hails
- Chemical reactions
- Decay of radioactive elements (uranium, potassium, thorium)
## Footnote
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54
# Oppenheimer (2011)
What is the background temperature in space
-270.43 degrees c
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55
# Oppenheimer (2011)
What is essential to volcanoes
- Convection of magma in the mantle. The hotter the mantle the lower its density, causing rising; colder regions sink.
- It is a circulation of solid mantle
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56
# Oppenheimer (2011)
When was the last glacial maximum
20,000 years ago
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57
# Oppenheimer (2011)
What causes hotspot volcanoes
Mantle plumes
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# Oppenheimer (2011)
What % magma coming from mantle comes from mantle plumes
5-10%
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# Oppenheimer (2011)
Total length of ridges globally
50,000km
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# Oppenheimer (2011)
What is the average spreading rate of divergent plates
5cm per year
--> 2.5sqkm more ocean crust created each year
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# Oppenheimer (2011)
Hydration
- Seawater reacts with hot volcanic rocks extracting sulphur
- Seawater hydrates minerals such as olivine
- Basalt transformed into slippery green rock called serpeninite
- Water perolates into overlying mantle
- Water in mantle reduces its melting point, causing partial melting
## Footnote
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# Oppenheimer (2011)
Why do bubbles in magma form
Less pressure towards surface - process called exsolution
- Produce bubbles of CO2 (causing long term atmospheric changes), waer, sulphur dioxide and hydrogen flouride
## Footnote
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# Oppenheimer (2011)
Neutral bouyancy
Where the density of magma = density of Earth's crust
- Tend sto be 3-30km below the surface
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# Oppenheimer (2011)
How does magma cooling in the magma chamber contribute to eruptions
- Fractional crystaliation
- Crystals with highest meltin points freeze
- Leaves behind more silica rich minerals
- More volatiles left behind
- Cooling and fractional crystalation lead to bubble formation
- Bubbles expand magma chamber due to increased pressure
- Increased pressure fractures the rock wall and initiates and eruption
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# Oppenheimer (2011)
What % of volatiles can magmas at high pressures in Earth's crust contain
10%
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66
# Oppenheimer (2011)
What plate boundaries have the most dissolved volatiles
Subduction zones
- Due to subducted oceanic rock
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# Oppenheimer (2011)
How can magma encountering water cause eruptions
Production of steam
- Hydrovolcanic
- Example: Iceland 2010
- Commonly associated with the re-awakening of dormant volcanoes
## Footnote
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# Oppenheimer (2011)
What are less explosive eruprions called
Effusive
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# Oppenheimer (2011)
Two broad categories for products of explosive eruptions
Tephra (fragmented rock: pumice, ash or bombs)
Pyroclasts
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# Oppenheimer (2011)
What does the intensity of an eruptions refer to
The rate at which magma is erupted
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# Oppenheimer (2011)
How high do most intense ash columns reach
20-30km
Intro stratosphere
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# Oppenheimer (2011)
Volcanic hazards
- Ash
- Gas clouds
- Lava flows
- Pyroclastic currents
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# Oppenheimer (2011)
Why does intensity and magnitude of eruptions only loosely correlate to human impacts
Expsosure and vulnerability of societies varies
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# Oppenheimer (2011)
Impact of volcanic eruptions in C20th on people
- 500 events had a human impact
- 6 million homeless or evauated
- Fatalities in 250 eruptions
- Death toll of 100,000
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# Oppenheimer (2011)
How could volcanoes present a greater threat in the future
Due to greater urbanisation
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76
# Oppenheimer (2011)
How many people were injured in volcanic eruptions in 20th century
- 12,000
- Low survival rates
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77
# Oppenheimer (2011)
What is a classic manifestation of a volcanic eruption
Cloud of ash, rock and gases
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78
# Oppenheimer (2011)
What is Strombolian activity
In volcanology, a Strombolian eruption is a type of volcanic eruption with relatively mild blasts, typically having a Volcanic Explosivity Index of 1 or 2.
## Footnote
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