LEC.171 The Earth's Interior Flashcards
What are the 4 eons from earliest to latest and which are part of the Precambrian period?
Hadean –> Archean –> Proterozoic –> Phanerozoic, Precambrian = first 3
What is the Earth’s core composed of?
Fe and Ni
Which planet collided into Earth to create the Moon 4.51Ga ago?
Theia
What was the Earth’s early atmosphere composed of?
O3, N2, and O2
How many Ga ago was the universe formed?
13-15 Ga
What can S waves not travel through and why not?
Liquids (the outer core) and gases, shear modulus is zero
What is seismic discontinuity?
A boundary between 2 layers with different seismic velocities
What 3 things can happen to S and P waves at seismic discontinuities?
- Reflection
- Refraction into a different layer (diving waves)
- Transformation
What is reflection/refraction dependant on?
The angle of incidence between incoming seismic wave and boundary, and the density/velocity contrast between 2 layers
Why does seismic velocity change between layers?
Differing rock density/elasticity
What triggered the dramatic increase in atmospheric O2 levels 580 Ma ago?
Continental collisions that increased rates of carbon burial by seafloor sediment
What is the Moho the boundary between?
Crust and upper mantle
What is S-wave velocity inversely proportional to the square root of?
Density
What is bulk modulus and shear modulus?
Bulk: Pressure change needed to give volume change
Shear: Pressure change needed to give SHAPE change
What 2 things does the water on Earth mainly originate from?
- Volcanic outgassing
- Comets that bombarded early Earth
What is the area on the Earth’s surface 105-142° away from an earthquake where no DIRECT P-waves are recorded called?
The shadow zone
What do shadow zones show the presence of at 2900 km?
Core-Mantle Boundary
What is the mantle and asthenosphere made of?
Mantle: Peridotite
Asthenosphere: Peridotite and melt (lower elasticity so lower seismic velocity)
What are the meanings of P, S, K, I, J, c, and i in terms of P and S-wave movement e.g. PKIKP?
P: P-wave in mantle
S: S-wave in mantle
K: P-wave in outer core
I: P-wave in inner core
J: S-wave in inner core (solid)
c: Reflection off outer core
i: Reflection off inner core
What was discovered using identification of the PKIKP phase and why?
Inner core, S-waves reappear so inner core must be SOLID
What type of movement do P and S-waves have and which wave type is faster?
P: Push-pull
S: Side-to-side
P-waves are faster
What type of rock do seismic waves travel faster through?
Igneous
What type of boundary is the Lithosphere-Asthenosphere Boundary?
Thermal boundary layer (mantle becomes partly molten)
What do mantle transition zones reflect with examples?
Changing mineral structure as pressure increases
Olivine –> spinel –> perovskite (increase in density/elasticity/seismic velocity)
What are 3 ways we know the mantle’s composition?
- Ophiolite suites
- Xenoliths
- Meteorites (similiar appearance)
How do ophiolite suites allow us to know the mantle’s composition?
Mantle peridotite brought to the surface by tectonics = peridotite mantle
How do xenoliths allow us to know the mantle’s composition?
Mantle peridotite fragments carried to the surface in magma
What does the Earth’s magnetic field require the convection of and how is this affected?
Electrically conducting liquid in the outer core (self-exciting dynamo), fluid flow affected by solid inner core and rotation
What are magnetic field reversals caused by and when do they happen?
Caused by change in core convection pattern, happen at random time periods
Where is the Earth’s magnetic field currently weakening?
Bolivia
What are 5 oceanographic research methods?
- Echo sounding
- Seismic reflection profiling
- Dredging
- Drilling
- Submersibles
What features of ocean basins have been discovered using echo sounding and seismics?
Contain abyssal plains, trenches with max depth of 11km and networks of mid-ocean ridges (MORs)
What features of ocean basins have been discovered using dredging, drilling, and submersibles?
Seafloor is mostly sediment-covered with volcanic rocks beneath and recent volcanic activity at MORs
What do active and passive continental margins have?
Active: Steep continental shelf + ocean trenches (more EQs/tsunamis)
Passive: Sloped continental shelf
What are MORs offset by and what does this form?
Fracture zones, perpendicular steep scarps
What are transform faults?
Plate boundaries/fracture zones between offset ridges
What are the 3 main sources of deep sea sediment?
- Terrigenous sediment - transported from land to ocean by rivers
- Pelagic clays - fine particles that settle in deep, calm water
- Pelagic oozes - remains of microscopic organisms (calcareous/radiolarian)
What do calcareous and radiolarian mean?
Calcareous: CaCO3
Radiolarian: SiO2
What are basaltic pillow lavas?
Blobs of lava that rapidly cool on contact with seawater
What are sheeted dikes?
Vertical blades of cooled magma
What is gabbro?
Cooled, coarse-grained igneous rock with the same chemical composition as basalt (layered at bottom due to crystal settling)
Where do 95% of earthquakes happen and what types of faulting are present?
Ocean trenches, extensional (normal) and compressional (thrust) faulting
What do Wadati-Benioff zones show?
Lithosphere remains solid as subducted until below 700km where it is heated, softened, and flows instead of fracturing
Why is it difficult to date seafloor basalt samples using radioactive isotopes and what technique is used instead?
Radioactive isotopes altered by seawater, drilling to retrieve sediment cores (sediment age/thickness increases away from MORs)
Which magnetic mineral is present in basalts and gabbros?
Magnetite (Fe3O4)
How is the magnetism of oceanic crust determined?
Lavas on land dated to establish global magnetic stratigraphy and applied to oceans
What are chrons and subchrons?
Chrons: Long periods of mostly normal/reversed magnetism
Subchrons: Shorter reversals within chrons
How can any ocean floor be dateable?
Counting stripes from ocean magnetic anomalies parallel to MORs
What are 4 pieces of evidence of continental drift?
- Jigsaw fit of continental coastlines
- Similar geological histories
- Plant/animal fossil distribution
- Polar wander (looking at ancient lavas, magnetic poles appear to have wandered over time)
What is an example of an oceanic divergent plate boundary?
Mid-Atlantic Ridge
What is an example of a continental divergent plate boundary (thinning lithosphere, chains of volcanoes)?
East African Rift Zone
What is an example of an oceanic convergent plate boundary (island arcs)?
Lesser Antilles
What is an island arc?
Chain of volcanic islands parallel to an oceanic trench at an oceanic convergent plate boundary
What feature do continental convergent plate boundaries have?
Continental collision zones - continental lithosphere too thick/buoyant to subduct)
What feature do oceanic conservative plate boundaries have?
Transform faults (can offset ridges, trenches, or both)
What feature do continental conservative plate boundaries have with an example?
Transform fault systems (complex band of almost parallel faults), San Andreas Fault
What is a triple junction?
Point on Earth where 3 plates meet
What are displacements on a sphere surface equal to?
Rotations about the Euler pole/pole of rotation (each plate has a different Euler pole
What indicates relative plate motion?
Orientation of transform faults between plates
What is the relative speed of plates to a neighbouring plate or MOR revealed by?
Magnetic anomalies/stripes on seafloor (but MORs/other plate boundaries aren’t stationary)
Why does relative plate speed vary along the length of the plate boundary?
Greater distance from Euler pole
What type of quantity is plate motion?
Vector
How are absolute plate velocities measured?
Using radiometric dating of lavas from different Hawaiian islands plotted against distance (plate motions carry extinct volcanoes away from stationary hotspots)
What are present-day plate motions measured with?
GPS - satellites emit radio signal with precise atomic clock and precise position independent of plates, repeated to detect movement over the years
What caused the Early Earth lithosphere to form?
Cooling of the magma ocean
What are 2 sources of heat in the Earth?
- Kinetic energy in Earth’s accretion
- Radioactive decay
How is the Earth’s interior slowly cooling?
Via heat flow from interior to surface (varies across Earth’s surface)
Where is there the most heat flow?
At O-O divergent plate boundaries
Which heat flow mechanism is the main mechanism on Earth?
Convection
Why is there less heat flow at the continental crust and what fraction of continental heat flow is produced in situ?
Thicker, more complex structure/composition, much older, 2/3
Where is there more continental heat flow?
Where there has been recent rifting/volcanism
What % of the planet’s total surface heat flow is hydrothermal circulation responsible for?
25%
What is mantle convection driven by?
Density contrast (hotter material = less dense/more buoyant)
Where is seismic velocity faster at (seismic tomography)?
Cool, thick continents
What are 2 features of fast-moving plates?
Little continental area, long subduction zones at margin
From slab pull and continental drag, which is the main driving force and which is the main retarding force?
Driving force: slab pull
Retarding force: continental drag
What do subducting plates provide?
Main driving force for plate tectonics
What is isostasy?
Lithosphere sticks down into asthenosphere much more than it sticks up above sea level
What is a Wilson cycle?
Continents grouping into supercontinents, then rifting apart via the opening and closing of oceans
What is the evidence for Wilson cycles?
Present day North Atlantic margin lies in close proximity to much older faunal divide
What are the 8 steps to a Wilson cycle?
- Rift basin
- Young passive margins
- Mature passive margins
- Oceanic subduction
- Mature subduction/early collision
- Continental orogen
- Stable continent
- Cratonic sag basins
How did continental crust form?
Early Earth magma pooled in mafic crust which increased silica content, making crust less dense –> crustal emersion
Which 2 of the past supercontinents can be reconstructed and what was there an increase of for each supercontinent?
Pangea and Rodinia, zircon crystals
How could supercontinents be considered self-destructive?
Continental collisions = unusually thick crust –> insulates mantle so heat builds up –> drives expansion –> uplift/rifting –> MOR
What minerals is peridotite in the mantle made up of?
Olivine, pyroxenes, and plagioclase/spinel/garnet
Which mineral has the higher melting point out of olivine and pyroxene, what is the gap between their melting points called, and what is melting temp. dependant on?
Olivine, melting range, pressure, composition and temp.
What do solidus and liquidus mean and where does melting first take place?
Solidus: Entire mantle is solid
Liquidus: Entire mantle is liquid
Mineral boundary
What does the partial melting of peridotite produce?
Basaltic magma
What type of melting takes place at MORs and continental rift zones and how does this happen?
Decompression melting, plates diverge so mantle rises to fill gap so pressure decreases and melting point reached
What type of melting takes place at hotspots and how does this happen?
Decompression, hot mantle plume rises so pressure decreases and melting point reached
What type of melting takes place at subduction zones and how does this happen?
Fluid-induced, hydrothermal circulation at MORs chemically alters crust so rich in hydrated minerals –> H2O released, rises and percolates through lithosphere –> peridotite in NON-SUBDUCTING lithosphere melts
Explain fractional crystallisation
Basaltic magma cools so dense mafic minerals (olivine/pyroxene) crystallise + settle so remaining melt more silica-rich and less dense
What is the order of types of rock from least to most silica content?
Basalt –> andesite –> dacite –> rhyolite
Are silicic or basaltic LIPs (Large Igneous Provinces) more common?
Basaltic
Where are most flood basalts found?
Distant/extinct end of hotspot chain (aseismic ridges)
What 2 features do the origins of magma plumes have?
- Very hot
- Much slower S-waves
What is the plume model?
Buoyant rise of anonymously hot material through mantle –> head approaches surface + decompression melting produces high volumes of magma –> flood basalt eruptions
What carries the LIP away from the tail of the magma plume?
Plate movements
What is an example of a volcanic aerosol and does it cool the Earth?
H2SO4, scatter incoming solar radiation
Which gases released from volcanic eruptions react with H2O to form acid rain?
CO2, HCl, SO2
Which layer of the atmosphere has far higher wind velocities?
Stratosphere
What do the atmospheric impacts of a volcanic eruption depend on?
- Explosivity (injection height)
- Magma SO2/CO2 content
- Eruption duration/latitude (equatorial = global impact)
How do Wilson cycles affect climate?
Moist air rises over mountains formed from continental collisions so high ppt. –> enhanced chemical weathering removes atmospheric CO2 –> global cooling
34 Ma ago, what did the abrupt climatic cooling coincide with?
India-Eurasia collision
What % of marine species disappeared during the Permian-Triassic extinction, what was it caused by, and what could have been a trigger?
90%, acid rain/global warming/ocean anoxia, Siberian Traps LIP erupted around this time (increased CO2)
