Global Geophysics
Lecture part
Rocco Malservisi
roccom@lmu.de
Phone: 2180 4201
NASA photo from Apollo 17

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!!
How big is the planet?
By definition 40000 km!!
1m=1/4e6 length of a
meridian.
Or 60x360 nautical miles
(21600 nm).
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
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
We are attracted to it and
there is gravity
http://www.jadetower.org/muses/wlsimages/gravity-newton.jpg
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-newton.jpg
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)
M=gR2/G
g=9.8ms-2
G=6.67e-11 m3kg-1s-2
R=6371km
M=6e24 kg
http://www.jadetower.org/muses/wlsimages/gravity-newton.jpg
So can we say
something about the
interior of the planet?
what is the average
density?
Mass = 6e24 kg
Volume = ?
So what is the average
density?
Mass 6e24 kg
Volume=4/3pR3=
1.08e21 m3
So its density is
~5500 kg m-3
Density of surface rocks?
So what is the average
density?
Mass 6e24 kg
Volume=4/3pR3=
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
http://earthguide.ucsd.edu/mar/dec5/earth.html
Which shape has the
planet?
What can we see
(colors)?
What can we measure?
We have a magnetic field
that it is very similar to
the one of a dipole.
Magnetopause 10Re Moon 60Re
Well in reality this is true
close to the surface if
we go far away
enough it looks more
complex
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.
Müller et al. 2006
Magnetic and gravity anomalies
and topo/bathymetry
GGM01S, GRACE mission
www.csr.utexas.edu/grace/gravity/
Let’s try to look more in details the brownish regions
Some
Somearea
areasare
look
flat
like
and
ifsome
someone
are
rough
enjoyed
Some
area
are
more
brownish
then
other to fold it
Let’s try to look more in details the gravity
What does influence this variation of gravity field?
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.
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
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
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;
Earth’s 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.