Global Geophysics

1
Global Geophysics

• Lecture part
• Rocco Malservisi
• roccom_at_lmu.de
• Phone [phone]

2
NASA photo from Apollo 17
3

• Is it really a sphere?
• Is it an ellipsoid?
• Is it flat?
• The shape of our system depend on the problem we
  want to study and the required precision!!

4

• How big is the planet?
• By definition 40000 km!!
• 1m1/4e6 length of a meridian.
• Or 60x360 nautical miles (21600 nm).

5
(No Transcript)
6

• How big is the planet?
• By definition 40000 km!!
• The first one to measure it correctly
  Erstosthenes measuring the distance from
  Alexandria and Syene (5000 stadia) and the angles
  in figure, he computed a circumference of 250000
  stadia 39300km

From Marshak, 2005
7

• How big is the planet?
• By definition 40000 km!!
• Today we say that the radius of the Spherical
  Earth equivalent to the volume of the planet is
• 6371 km
• We also know that an ellipsoid is a better
  approximation
• Eq radius 6378 km
• Pol Radius 6356 km
• Flattening 1/298

From Marshak, 2005
8

• We are attracted to it and there is gravity

http//www.jadetower.org/muses/wlsimages/gravity-n
ewton.jpg
9

• We are attracted to it and there is gravity
• And if we look at the satellite it is falling in
  to the Earth attracted by a force that is
  proportional to the mass of the planet.
• From the orbit parameter we can compute the mass

http//www.jadetower.org/muses/wlsimages/gravity-n
ewton.jpg
10

• Well even from the ground if we know the constant
  G and our distance from the center of the Earth
  we can measure the gravity acceleration thus the
  mass of the Earth (ex a pendulum)
• MgR2/G
• g9.8ms-2
• G6.67e-11 m3kg-1s-2
• R6371km
• M6e24 kg

http//www.jadetower.org/muses/wlsimages/gravity-n
ewton.jpg
11

• So can we say something about the interior of the
  planet?
• what is the average density?
• Mass 6e24 kg
• Volume ?

12

• So what is the average density?
• Mass 6e24 kg
• Volume4/3pR3
• 1.08e21 m3
• So its density is
• 5500 kg m-3
• Density of surface rocks?

13

• So what is the average density?
• Mass 6e24 kg
• Volume4/3pR3
• 1.08e21 m3
• So its density is
• 5500 kg m-3
• Density of surface rocks?
• 1.5-3.5 g/cc
• Density Iron 7g/cc

14
http//earthguide.ucsd.edu/mar/dec5/earth.html
15

• Which shape has the planet?
• What can we see (colors)?
• What can we measure?

16

• We have a magnetic field that it is very similar
  to the one of a dipole.
• Well in reality this is true close to the surface
  if we go far away enough it looks more complex

Magnetopause 10Re Moon 60Re
17
Variation of Magnetic field on oceans
Looking for subs the British and Americans
developed a map of Magnetic anomalies of the sea
floor, in 1961 Harry Hess explained It using
seafloor spreading theory.
18
Müller et al. 2006
Magnetic and gravity anomalies and
topo/bathymetry
GGM01S, GRACE mission www.csr.utexas.edu/grace/gra
vity/
19
Lets try to look more in details the brownish
regions
Some area are flat and some are rough
Some areas look like if someone enjoyed to fold it
Some area are more brownish then other
20
Lets try to look more in details the gravity
What does influence this variation of gravity
field?
21

• So as first approximation the Earth is a planet
  that looks like a sphere with a density higher
  than we would expect looking only at the surface,
  with regions of different colors at the surface
  and able to generate a dipolar magnetic field.
  That does not appear to be constant.

22
So it does not look any longer as an homogeneous
sphere!! On second approximation it is an
ellipsoid 6357 km (polar) 6378 km
(equatorial) And it looks like if some processes
are shaping the brownish regions. We will spend
the rest of the semester to look at these
processes and to figure out how we can observe
them
23
PLATE TECTONICS
Plate Tectonic is a theory that unify different
previous geological theories (CONTINENTAL DRIFT
and OCEAN SPREADING) and that can explain the
majority of the solid earth system
observations. Basic concept The outermost
layer (LITHOSPHERE) is divided in a small number
of rigid plates in relative motion one respect
to the other and that are moving on a weak
ASTHENOSPHERE
24
PLATE TECTONICS

• Basic concept
• The outermost layer (LITHOSPHERE) is divided in a
  small number of rigid plates in relative motion
  one respect to the other and that are moving on a
  weak ASTHENOSPHERE
• Basic Assumptions
• The astenosphere viscosity is low enough to allow
  on long time scale for viscous flow
• The generation of new plate material occurs by
  sea floor spreading
• The new oceanic lithosphere form part of a rigid
  plate that may or may not include continental
  material
• Earths surface area remains constant this means
  that seafloor spreading must be balanced by
  consumption of plate elsewhere
• Lithospheric plate are capable of transmitting
  stress over great horizontal scale. In other
  words plates are rigid and the deformation is
  concentrated along the boundaries.