Lecture 5 Flashcards
What shape is the earth
Obligate spheroid flattened at the poles
Flattening quantified
(Re - Rp)/ Re =1/298
Where R is the radius
E is the equatorial one and the P is the polar radius
Gravity of the Earth
At equator = 978 gal
Poles = 983 gal
gal is 10-2 ms-2
Ideal gravity field
Assume the earth is fluid and then use the plausible density-depth variation and angular velocity to give a reference spheroid shape—>provides reference gravity values
What is the geoid
The reference surface for gravity observations using the sea surface
What is the geoid anomaly
This is the difference in height between the geoid and the theoretical reference surface
There is no correlation with oceasns and continents trying to explain the geoid anomaly so what does that mean
x 3
- on a planetary scale the Earth closely resembles a perfect fluid
- Geoid anomalies must reflect deep structure
- Continents and oceans must be hydrostatically balanced otherwise that would show up as anomalies
What is isostasy?
The effect where the ‘excess mass’ of mountains must be compensated by the mass deficiency beneath them
What are the two compensation mechanisms of isostacy
- Airy’s Hypothesis : Mountains have roots like icebergs
2. Pratt’s Hypothesis : Mountains have lower density than surrounding rocks
What is flexure and why is it not an example of isostacy
- big gravity anomaly
- not isostatic equilibrium
- topography supported by elastic bending of the plate
- not lateral density change
Where does Airy’s Hypothesis generally work
Continents as continental mountains resemble icebergs ‘floating’ on denser mantle as Moho is much further from crust the thicker that crust is
where does Pratt’s compensation generally apply
Oceanic crust where there is a relatively constant thickness but water-depth varies so oceanic mountains must be compensated by density variations in the mantle and that the high oceanic mountains are underlain by less dense mantle that the lower areas
With oceanic islands, they can be supported by either flexure or isostacy. Where does isostacy work?
Where volcano added to hot, weak lithosphere and there is no gravity anomaly which means Pratt and Airy mechanisms working together
The relationship between average height and crustal thickness is preserved in mountain belt erosion
So at all times in approximate isostatic balance. The end results of this erosion is a continent at sea level so the continental root is replaced through time by mantle material.
The mantle flow required to do this occurs by creep
Present-day magnetic field
Approximates to a dipole and is presently at 11.5 degrees spin axis.
Removing the best fitting dipole from the observed field to get non-dipole field
What is secular variation
non-dipole components of the field migrate on a time-scale of tens of years which is very rapid
What is paleomagnetism
Using rocks to study the history of the earth’s magnetic field
What are the three ways in which natural Remnant Magnetism (RM) is preserved in rocks
- Thermo-remnant (TRM)
- Chemo-remnant (CRM)
- Depositional or detrital remnant (DRM)
What is TRM
Thermo-remnant is the cooling from high temperatures through the Curie point and when below this temperature there is a ‘frozen’ magnetism in the rock
What is CRM
Chemo-remnant
- Magnetisation acquired by chemical action of phase changing during formation of iron oxides at low temperatures (below Curie point)
What is DRM
Depositional or detrital remnant
- alignment of magnetised particles in a sediment, falling through water or rotating in water-filled interstitial holes in wet sediment
What is the model for the interior magnetism mechanism
A solid core surrounded by tubular convective cells which are partially controlled by thermal convection
Inclination angle
the angle between the horizontal and the Earth’s magnetic field lines
Declination
Bearing so the angle between the geographical north and geomagnetic north
Geocentric axial dipole
Assume that secular variation averages to zero
dipole axis = rotation axis
roughly true over very long geological time
Magnetic reversal
Where the dipole compenent completely reverses and the geographical south pole becomes the magnetic north pole and vice versa
Average time between pole reversals
1 million years but they are random and not regular
- this is evidence for the fact the earth is not a perminent ‘bar’ magnet
Origin of the agnetic field
- likely from a part of the earth that is fluid (to account for rapidity of secular variation and reversals)
- Magnetism cannot be ferromagnetism as temperatures too high
- Arises from self-exciting dynamo- so core has to be a good electrical conductor
Long term average behaviour of magnetic field
the axial dipole field where
tan(i) = 2tan(λ)
I is inclination and λ is paleoaltitude
How to find paleolatitude?
The inclination of the fields in the sample
- the declination gives the direction of the paleo-north
What is the assumption when using paleomagnetism to determine distance and direction to paleo-north pole
That the field averages to an axial geocentric dipole, aligned to the Earth’s rotation axis
What is an apparent polar wander path?
APWP is the sequence of paleomagnetic poles from rocks of known ages that give the latitude and orientation in time for a continent
Ways to interpret APWP’s
x 4
- similar APWPs then continents moving as part of same mass
- IF differ significantly then continents were in relative motion
- if two continents were fixed relative to each other during some past time interval and then have moved relative to each other then can be repositioned by superimposing the APWPs for the interval during which they were not in relative motion
- APWPs dont give longitudes but can work out relative longitudes as long as the two continents were in relative motion
Example
- Phanerozoic APWPs for North America and Europe
Obervation-
differ but show no obvious patten when plotted to present-day coordinates
— so NA and WE have been in relative motion at some time in past 570 myr
Observation-
When plotted on best-fit reconstruction of continental edges then the paths converge in silurian and diverge in Jurassic
— consistent with techtonic model in NA and western E collided around 400ma forming one continent until 200ma
Pangea
The supercontinent where all the land fits together into one land mass
