Introduction to the Earth (Lectures 1-4) Flashcards
What are the differences between P-waves and S-waves released in an earthquake?
P-waves are faster
P-waves longitudinal, S-waves transverse
What are the P and S wave velocity equations?
Vₚ = sqrt((κ + 4μ/3)/ρ) Vₛ = sqrt(μ/ρ) κ is compressibility μ is shear modulus ρ is density
Why do S-waves not travel through liquids?
μ = 0 in a liquid
What are excited by big earthquakes?
What are they useful for?
Normal modes of the Earth
Interior density and velocity structure of the Earth
What are the shadow zones for P and S waves?
Direct P and S shadow zone from 103° to 142°
Direct S shadow zone 142° to 180°
What do the shadow zones for P and S waves reveal?
The outer core is liquid
The inner core is solid
Which law do P and S waves obey?
Snell’s Law
How was the Moho found?
Beyond 250 km the first P wave arrival was earlier than if it travelled purely in the crust
Must be a jump from 6 km/s to 8 km/s at crust/mantle boundary
What did the jump in velocity at the Moho indicate?
A change in composition or mineral structure
What is the Low Velocity Zone (LVZ)?
A drop in velocity for S and P waves at 100-150 km depth from partial melting of mostly solid mantle
Why are carbonaceous chondrites significant when discussing the Earth?
Close in composition to the original nebula the solar system was formed from
Richer in iron than rocks in the mantle and crust
What evidence is there that the chondritic model of the Earth is applicable?
Comparing abundances of elements in carbonaceous chondrites and the Sun matches up with most elements bar volatile ones
What does the chondritic model imply?
How is this consistent with seismological data?
The mantle is mostly silicates
The core is >90% iron and some nickel
The core-mantle boundary at 2800km gives correct bulk composition and average density for Earth
Where does the Earth’s magnetic field originate from?
Interior is too hot (4000°C at the core-mantle boundary) for a permanent magnet
Sustained by a self-exciting dynamo: core conducts electricity (made of iron), motions maintained by a heat source (crystallisation of the inner core) driving convection currents (outer core as viscous as water)
Outline the Geological Cycle
Sedimentation -> burial deep in crust -> folding, faulting, mountain-building and igneous intrusion -> erosion and flattening of mountains ->
What is an effect that arises from the Geological cycle?
An unconformity
Bottom layers are vertical, top layers are horizontal
What are facies?
An environment of deposition
What are ooids?
Sand size, white ellipsoids of concentric layers of the mineral calcite deposited on a nucleus
How do ooids form?
Shallow turbulent water (waves) by precipitation
What are glacial sediments comprised of?
Boulder clay and gravels
What are the two climate states the Earth has gone between over the last 2 Ma?
Interglacials: a little ice, like today
Glacials: lots of ice, lower sea levels
What are foraminifera? What are they used for?
Small marine micro-organisms, typically <1 mm
Paleoenvironment indicators: in glacial periods sea water is very enriched in Oxygen-18 and ice enriched in Oxygen-16
What orbital effects are there for global temperature?
Eccentricity
Tilt
Precession
What features are there that suggest an ice age?
Glacial striations Varves Drop-stones Boulder clays Tills
Rocks behaving elastically means what?
The deformation is recovered if the forces are removed
What do brittle substances do?
Fracture without appreciable permanent deformation away from the cracked surface
Like slip on faults
What do ductile substances do?
Deform appreciably by flow
What is cataclastic flow?
Repeated fractures which progressively reduce the grain size and allow the fragments to slide over each other
Where is cataclastic flow common?
Fault zones
Crystalline solids flow by what?
Creep
What is creep?
Very slow flow under a constant load
What are the three ways creep can happen at the crystal level?
Movement of dislocations, which are imperfections in the atomic arrangement within crystals
Sliding of crystals on grain boundaries
Recrystallisation
What are creep mechanisms sensitive to and how?
Temperature
Rocks creep more easily when hotter
What is homologous temperature?
Ratio of actual temperature to melting temperature
T/Tm
When does creep appear?
Homologous temperature above 0.6
What is power law creep?
When does it occur?
Motion on polygonal cellular patterns of dislocations within grains, with grain boundary sliding and some recrystallisation
Happens above homologous temperature of 0.55
What is diffusion creep?
When does it occur?
Migration of atoms within crystals or along grain boundaries
Homologous temperature above 0.85
When is power-law creep dominant?
Relatively low homologous temperature and relatively high stress
When is diffusion creep dominant?
Relatively high homologous temperature and relatively low stress
What is the asthenosphere?
Homologous temperature is about 0.85 at these depths
Most creep by diffusion
What is the lithosphere? (5 points)
Top 100-125 km of the Earth Relative strong and cold Rigid spherical caps of plates Top part is brittle Lower part is warm and has dislocation creep
What is the mantle made of?
Peridotite
Why are crust and mantle not the same as lithosphere and asthenosphere?
Crust and mantle refer to compositional structure
Sharp boundary
Lithosphere and asthenosphere refer to mechanical layering
Gradational boundary
What is viscosity?
The ratio of shear stress to strain rate
How well something flows (low = runny)
