Tectonics Flashcards
Which planets have a mantle (i.e. not a solid core) in the solar system?
The Earth, Venus (but no plate tectonics, so just hotspot volcanism), Mars (one plate formed from several)
What, a priori, is evidence of plate tectonics?
Parts of the earth’s surface are contracting whilst other bits are extending
What is Global Hypsometry used for?
To show the distribution of the world’s topography by surface area
What is significant about global hypsometry?
A bimodal distribution due to continental and oceanic crust differences;
continental crust is thicker and less dense than oceanic crust, which is older
How does paleomagnetism work?
As the seafloor spreads and cools to <100 degrees, the magnetic minerals in the magma align themselves to the magnetic pole
Why is continental rifting significant?
Forms new oceanic crust over time
Why is seafloor spreading significant?
Effusive volcanism releases CO2, a major flux in the geological carbon cycle
Why is plate subduction important?
- Ensures that the world does not expand
- Sight of earthquakes and volcanism
What are the 2 main sources of internal heat?
1) Primordial (latent) heat from planetary accretion
2) Heat in core due to long- and short-lived radioactivity (major), (Francis, P. and Oppenheimer, C., Volcanoes, 2nd ed., 2004, Oxford UP)
Why is studying hotspots important?
- Allows plate velocity to be determined
- Allows plate direction to be determined
What methods are there of studying the interior of the Earth?
- Chemically with rocks
- Seismic shadows
What is the lithosphere?
All of the crust and upper part of mantle (asthenosphere being plastic part of mantle)
Which plate type (continental vs oceanic) is more rigid? Why is this significant?
Oceanic more rigid, so seismicity is less diffused at margins involving oceanic plates
Is the mantle liquid?
No, it is solid. It can still flow though, just as glaciers flow. Pitch used as a metaphor in Holmes’ Principles of physical geology 4th ed
How do we know the mantle flows?
Isostatic rebound in the Holocene
How do we know convection is driving mantle flow?
Rayleigh number strongly favours mantle convection (Ra is > 10^6) (Kearey, et al., 2009)
What are the two categories of forces acting on plates?
1) Edge force mechanisms: Ridge push and slab pull
2) Mantle drag mechanisms (driven by convection)
(Kearey, et al., 2009)
Name 3 key types of faults
1) Normal faults (extension forces)
2) Reverse faults (compression forces)
3) strike-slip faults (conservative boundaries)
By what process does the mantle flow?
Creep formation as crystal structures deform (Oppenheimer and Francis, 2004)
What is believed to cause excessive effusive eruptions and continental flood basalts (CFBs)?
Plumes rising from the core under thermal instability (lower density etc) can adhere to one another, propagating the rise of magma
What are trench forces?
- Edge forces at subduction cells driven by localised convection cells (from heated material, esp. water, above subducting plate) in the mantle wedge (also Conrad and Lithgow-Bertelloni, 2002)
- “Sucks” in the subducting plate
Open University, 2019
What is the main role of plate tectonics?
To transfer heat away from the core (Kump et al., 2014)
What is the main role of mantle convection?
To transfer heat from the core to the lithosphere (Kump et al., 2014)
How is heat transferred from the core?
- Convection occurs as the solid mantle heats, expands and rises due to positive buoyancy induced by a lower density, before being replaced by cooler material
- Warmer mantle material reaches the moho discontinuity at the base of the lithosphere
- Heat is transferred by conduction through the lithosphere to the surface
- Mantle material cools and descends back towards the core
(Kump, et al., 2014)
Is there just one layer in the mantle?
Possibly, though there could be two (upper/lower) if the chemical composition of differs (Kump, et al., 2014)
Where do upwellings in convection currents start?
At the D” layer (D-double-prime layer) a discontinuity where the mantle meets the core (Kearey, et al., 2009)
Are there any contemporary critiques of what is known about mantle convection?
Yes!
Convection currents could play a role in supercontinent formation. Plates move to cooler regions of the upper mantle due to currents. This could create warmer currents in the new location, thus a positive feedback loop (Kearey, et al., 2009)
Why is magma at ridges and hotspots liquid?
The pressure is lower
What are the names of eruptions taking place on and below the surface?
Eruptive = On surface
Intrusive = Below the surface (eg. magma chamber formation)
What are the two main types of eruptions?
Explosive and effusive
What is the difference between magnitude and intensity of eruptions?
Magnitude = Total mass/volume of deposits
Intensity = Rate of mass/volume ejected over time
How do eruption size and rate vary?
Eruptions of various sizes occur at a constant rate. Larger eruptions less frequent
How is eruption magnitude measured?
Volcanic Eruption Index (VEI) - a log scale
What is a fountain collapse?
Then an ash cloud collapses due to lost energy, creating pyroclastic density currents
Why are Plinian eruptions said to be “self-propagating”?
High heat and energy draws in air (and thus more chemical energy)
Who proposed the Cenozoic cooling hypothesis?
Raymo, et al., 1988; also Raymo and Ruddiman, 1992
What is an example of a proxy used to determine past climates and atmospheres?
- Forams (Foraminifera)
- Single-celled organisms with calcium carbonate shells (more common than others)
- Oxygen isotopes taken up in shell
- Oxygen isotope concentration in atmosphere depends on temperature
(see e.g. Pearson, P.N., 2017)
What are the two oxygen isotopes taken up in Foram shells? Which is most common in cooler climates?
- 18O and 16O
- As climate cools, 16O more common in ocean, so thus taken up by foram shells
- 16O evaporates more rapidly under warmer temperatures
(see e.g. Pearson, P.N., 2017; Raymo and Ruddiman, 1992)
GOOD LINKS TO QUARTERNARY
Outline average temperatures during the Cenozoic
- Last 66 Mya
- 50 mya temp peaked
- Cooled since, esp. last 6 mya
- Formed ice sheets at poles
What rocks are a carbon sink when weathered by carbonic acid?
Silicate rocks (granites)
Why has the Indian plate subducted beneath the Eurasian plate?
Slab pull of Indian plate in Indonesia has driven the Indian plate into, and under, with subduction, the Eurasian plate (against its will)
How was rock pulverised in the Himalayas, thus exposing it to chemical weathering?
Mechanical erosion by glaciers
What process enhanced chemical weathering of the Himalayas?
Mountain uplift (possibly caused by isostaic rebound, not necessarily tectonics) may have increased monsoon rainfall and thus carbonic acid dissolved in rainwater increased
What is the main counter-argument to the Cenozoic uplift/cooling hypothesis (Raymo, 1998)?
- Chicken or the egg (Molnar & England, 1990)
- Cooler climate (due to Milankovitch cycles or BLAG hypothesis) in the Cenozoic drove (non-tectonic) uplift through glacial and fluvial erosion increasing isostatic rebound
- Isostatic rebound continued after erosion
- Left a distinctly pulverised “new” looking surface
What was the main mechanism of the Cenozoic cooling?
Chemical weathering of silica-rich rocks (granites)
Give a criticism of the Cenozoic cooling hypothesis?
- Growth of Tibet plateau was not absolutely due to collision of Indian plate and Eurasian plate
- Instead due to collision within the Eurasian plate and creating relief (Himalayas existed before Cenozoic) (Spicer, et al., 2020)
What is uplift?
- Movement in the direction opposite the vector of gravity
- Relative to a reference point (Geoid)
(Molnar and England, 1990)
How do we know silicate weathering took place in the Cenozoic?
- Strontium isotopes (Raymo and Ruddiman, 1992)
- Although not clear how much comes from silicate weathering versus carbonate weathering (Garzione, 2008)
Why is studying cenozic cooling important?
- Studies previous climates
- Proves to climate change deniers that the climate has always changed, but that the most recent warming is anomalous
What is the BLAG hypothesis?
- AKA spreading rate hypothesis
- The association between seafloor spreading rates and rates of weathering
- Example: More seafloor spreading = more atmospheric CO2 = warmer = more weathering = sequestration of CO2
- Could explain the cooling before uplift hypothesis (Molnar and England, 1990)
What is an isostatic anomaly?
AKA residual rebound (Benn and Evans, 2010)
Isostatic adjustment taking place after weight on the crust is removed. Hydrostatic balance (equilibrium) is not yet reached and the crust is not at the level of compensation
(Holmes’ Principles of physical geology 4th ed)
What is isostatic compensation?
AKA isostatic adjustment: the crust rising up after weight is removed from the surface (often ice, but can be sediment, or both)
What is a forebulge?
- A part of the crust that is forced upwards by mantle material displaced by the weight of material added to the surface (ice sheet/ sediment)
- When the material is removed, the forebulge moves inwards (towards the centre) and downwards (into mantle), assuming its original position
- Forebulge currently moving down in SE UK, resulting in an enhanced localised sea level rise
(Benn and Evans, 2010)
Why are landforms (raised beaches and caves) not suitable evidence of isostatic adjustment?
- Can be created by other drivers
- Can be eroded
- Sediments and fossils dated, a more accurate and reliable method
- Recent rebound measured with GPS
(ALL Benn and Evans, 2010)
Could isostatic rebound (and melting ice sheets in general) cause more hazards?
Possibly
- More faulting (Benn and Evans)
- Vent and conduit plugging (ice) could be removed from volcanoes (Bill McGuire
When did Pinatubo erupt?
- Activity started in April 1991
- Major Plinian eruption 15th June 1991
- Largest eruption in 100 years (Oppenheimer, 2011)
Why is the Mt Pinatubo event significant?
- Large injection of SO2 into stratosphere
- Advent of satellite remote sensing techniques, allowing empirical evidence to be collected, contributing to a previously theoretical field
(Oppenheimer 2011)
Why is it believed that the Tambora 1815 eruption had a greater impact on society than Pinatubo, despite sharing similar characteristics?
Agricultural productivity was depended upon more within localised communities. Today worldwide food network
What distinguishes radiative forcing from radiative balance?
- A change in the radiation budget away from equilibrium
+ve forcing = warmer, energy retained
-ve forcing = cooler, energy lost from the system
(Grainger & Highwood 2003)
What is the chemical equation for the formation of sulphate aerosol (“sulphates”)?
OH (hydroxide radical) + SO2 -> H2SO4 (g) -> H2SO4 - H2O
What is sedimentation (of aerosol particles)?
When aerosol particles are too large (heavy) to remain aloft in the stratosphere, so fall out into troposphere
How long do SO2 and sulphates remain in the stratosphere for?
- SO2 a few weeks (days/weeks in troposphere)
- Aerosol a few years, though depends on location of injection (months in troposphere)
By what mechanism is sulphate aerosol removed from the troposphere?
Precipitation removes the sulphates in deposition
Why do sulphates in the stratosphere at higher latitudes sediment more rapidly than those at lower latitudes?
Stratospheric circulation moves air masses polewards, so if SO2 is injected closer to the pole (and turns into sulphur aerosol), then it will sediment faster
What is an obvious anomaly of the supposed correlation between VEI and cooling?
- Mt St Helens
- Large eruption, but minimal injection into stratosphere
- Eruption was not vertical, exhibiting no self-propagating Plinian column
How long did sulphate aerosol from the Pinatubo eruption stay in the stratosphere for?
How do we know?
- 5 years
- LIDAR observations from NZ + Japan
(Oppenheimer, 2011)
How large do aerosol particles need to be to cause negative radiative forcing and thus net cooling?
- 0.5um (micrometres), causing net cooling
- Same size as visible light wavelength, hence scattering takes place
(Oppenheimer, 2011)
How by how much did the Earth’s albedo increase following Pinatubo?
How did this affect the radiation budget?
- 5% increase in albedo
- 20-30% less radiation into the system
- -0.7W/m^2 net cooling
(Oppenheimer, 2011)
How much did temperatures decrease following Pinatubo?
Net global decrease of 0.5K (Oppenheimer, 2011)
Why was El Nino significant for Pinatubo?
El Nino year, so the cooling effect of Pinatubo was less pronounced (Oppenheimer, 2011)
Why did temperatures increase in continental Europe and Asia after Pinatubo?
- A smaller pressure difference
- Due to temperature
- Between Icelandic low and Azores high
- Enhanced polar vortex
- Negative North Atlantic Oscillation of the jet stream (NAO)
(Oppenheimer, 2011; Timmreck, 2012)
How did Pinatubo affect the oceans?
- Temp decrease of 0.4K (Oppenheimer, 2011)
- Thermal contraction caused sea level to fall 0.5cm (Oppenheimer, 2011)
- Thermal inertia of oceans prolonged the cooling effect (Oppenheimer, 2011)
How did Pinatubo affect the biosphere?
- Photosynthesis increased 1991-93 (Oppenheimer, 2011)
- Plants did not need to close leaves etc
- Cooler ocean temperatures increased deep sea mixing in the Red Sea, creating algal blooms (but created hydrogen sulphide, killing coral) (Oppenheimer, 2011)
How good was our knowledge of the effects of explosive volcanic eruptions prior to Pinatubo?
- Very (surprisingly so) accurate
- Alan Robock (Rutgers Uni) had already predicted the effects of a large, explosive eruptions on climate (Oppenheimer, 2011)
Why are sulphur species important regarding the (cooling) climatic effects of volcanic eruptions?
- Create sulphate aerosol
- Ash and other materials/species sediment v Quickly, contrary to Davidson, et al., 2002
(Oppenheimer, 2011) - Ash small effect globally, but strong -ve radiative forcing locally (Grainger & Highwood 2003)
What types of magma release the most SO2?
- Highly oxidising + reducing magmas (eg basalt)
Oppenheimer, 2011
How big do eruptions need to be to cause net cooling?
- Big, Plinian eruptions to reach tropopause
- Once in stratosphere, less weather influence, so less deposition by rainfall
(Oppenheimer, 2011)
How does latitude influence climate-change (cooling) inducing eruptions?
- At high latitudes, tropopause is lower (10Km vs 17km on average in tropics) (Oppenheimer, 2011)
- BUT less sunlight per unit area and more rapid sedimentation at high latitudes
- At tropics the oscillation of ITCZ allows material to be injected into both hemispheres
(Oppenheimer, 2011)
How do effusive eruptions affect climate in the long term and in the early evolution of Earth’s atmosphere?
- CO2 vented as part of geological carbon cycle
- In the early atmosphere H2O, CO2, N2 etc vented, creating ideal conditions for life (O2 from photosynthesis later)
(Davidson et al., 2002)
What is a stratospherically significant eruption?
An eruption column that extends beyond the tropopause and into the stratosphere
(Grainger & Highwood 2003)
What is the exception to effusive eruptions causing global cooling?
- An injection of material into stratosphere from large igneous provinces (rare)
- Lots of material and species vented into troposphere, enough to cool without rapid deposition. Example: laki, Iceland, 1783 (Grainger & Highwood 2003)
Why is magma composition an important factor in determining the extent of negative radiative forcing from an explosive volcanic eruption?
- Controls viscosity
- Controls gas content and thus the amount of degassing taking place
- Less viscous liquids release more gas when exposed to ambient atmospheric pressures
(Grainger & Highwood 2003)
Was the cooling effect of the Pinatubo eruption uniform?
No, the impacts were heterogenous
Grainger & Highwood 2003
Did stratospheric temperatures decrease following 1991 Pinatubo eruption?
No, the temperature of the stratosphere increased by 1K (absorption of incoming solar radiation)
(Grainger & Highwood 2003)
What VEI is believed to be sufficient to inject SO2 into the stratosphere?
- VEI 4 or above to inject material into stratosphere
(Grainger & Highwood 2003) - < VEI4 = effusive eruptions (Schmidt & Robock, 2015)
Why do modern-day eruptions, despite mainly releasing CO2 and H2O, have a minimal effect on atmospheric concentrations of these gasses?
CO2 and H2O have very high atmospheric concentrations, so therefore the amount added is negligible (Timmreck, 2012)
What is the process called whereby sulphate aerosol particles group together?
Coagulation
Why do very large (and stratospherically) explosive eruptions not contribute to -ve radiative forcing much?
- More coagulation with more aerosol particles present (= more sedimentation)
- OH in stratosphere limits the chemical reaction (delays formation)
(Timmreck, 2012; Schmidt & Robock, 2015)
Is the height of the tropopause temporally static?
No, it changes seasonally. In summer hemisphere it is higher
Why did the temperature decrease induced by Pinatubo last so long, despite surface cooling ending within a few years?
- Thermal inertia of oceans (cooler temps perpetuated in deepest layers) (Timmreck, 2012)
- Biological response (see Oppenheimer, 2011)
- SAM altered, increasing sea ice extents and thus albedo (Timmreck, 2012)
All +ve feedback mechanisms causing knock-on indirect impacts
What are 2 good texts on volcanism and climate?
- Oppenheimer, 2001
- Timmreck, 2012
What are the 3 most common gasses emitted by volcanic eruptions?
- H2O
- CO2
- SO2
(Grainger & Highwood 2003)
What is the main reason why effusive eruptions do not cause cooling?
Smaller, so do not penetrate the tropopause (Robock, 2000)
Is basaltic lava less explosive?
Yes, compared to silicic lavas (Self, et al., 2015)
What proportion of the global carbon flux comes from volcanoes?
Only 4% of Anthropogenic flux from volcanism (for comparison)
(Self, et al., 2015)
Why is ash important in eruption columns?
- High SHC (? CHECK)
- Propagates eruption column by increasing rate of rise and thus creating low pressure
(Self, et al., 2015)
Which magma (basalt/andesitic) releases more SO2?
Basalt (just not injected into stratosphere)
Self, et al., 2015
Under what circumstances can effusive eruptions inject material into the stratosphere?
Continental flood basalts can inject material into stratosphere intermittently over several years
(Self, et al., 2015)
What are large igneous provinces (LIPs)?
- Long-lasting, large-scale eruptions with the potential to alter the carbon concentration of the atmosphere over geological timescales
- Important in geological carbon cycle
(Tasmin, et al., 2015)
What recent evidence suggests that, contrary to previous beliefs, extratropical eruptions can cause more -ve radiative forcing?
- Extratropical eruptions = 80% more -ve radiative forcing
- More material in one hemisphere
- Strengthens hemispheric warming in stratosphere
(Toohey, et al., 2019)
How do we know that mantle convection does not drive all plate movements?
Mantle convection cells do not match up with plate movements
(Open University, 2019)
What forces, and where, resist tectonic plate movements?
- Friction at subduction zones (viscous drag)
- Transform faults (v. small - Chapple and Tullis, 1977)
- Mantle convection producing friction (if mantle flow direction opposes the direction of plate movement)
(Richardson, et al., 1979)
Does the mantle drive plate movement?
Not clear…
- Can’t just be the mantle because plate movements do not correspond to mantle flow (Open Uni, 2019)
- Hotspots only drive plate movement at constructive boundaries because the mantle material radiates outwards in all directions (Chapple and Tullis, 1977)
What force (besides friction at the subduction zone) could resist slab pull?
The more viscous lower mantle “buttressing” the subducting plate, particularly detached slab fragments
(Conrad and Lithgow-Bertelloni, 2002)
Does subduction only cause the subducting plate to converge?
No
Also overriding plate converges, at a slower rate due to slab suction (Conrad and Lithgow-Bertelloni, 2002)
Give an example of a paper where observed and theoretical do not match up, regarding plate movements
Conrad and Lithgow-Bertelloni, 2002
