Lecture 5 Flashcards
(35 cards)
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
