Transcript 19.01.2009 Plate tectonic intro (updated 28.01.2009)

Introduction to Earth System
Solid Earth part
Rocco Malservisi
[email protected]
Phone: 2180 4201
Magnetic Field is a vector
It has an intensity (can be measured looking
At the oscillation of a compass)
And a direction
The direction change with the position
Magnetic Pole:
The place where the compass is pointing
Down
Magnetic Equator:
The place where the compass is horizontal
The Magnetic Poles are close to the geographical
Poles but do not coincide (~11 off)
The Earth’s Magnetic Field
B = (X, Y, Z)
Or
B = (F, I, D )
Or
B = (D, H, Z)
F: intensity
I: inclination
D: declination
H: Horizontal component
The seven elements of the (local) magnetic field
in the geographic coordinate system
I. Geomagnetic field – Local Geomagnetic Field Vector
The place where the axis of the dipole intersect the surface
Of the earth are called geomagnetic poles
other secular variation: reversal
From Butler,
Palaeomagnetism, 1992.
Slide
I. Geomagnetic field – Temporal Variations
PaleoMagnetic Field:
Magnetization of Rocks
DRM
Detrital
Remanent
Magnetization
TRM
Thermal
Remanent
Magnetization
Gesteinsmagnetisierung:
Curie Temperatur: etwa 580 Grad C für Magnetit
680 Grad C für Hämatit
Blocking Temperatur:
Typische Schmelztemperaturen liegen allerdings bei
1100 – 800 Grad C, also wesentlich höher.
Das heißt, Gesteine können eine Magnetisierung im
Umfeld annehmen, und diese bei Abkühlung unter die
Blocking-Temperatur auf geologische Zeiträume hinweg
behalten.
Wir unterscheiden:
Thermoremanente Magnetisierung: TRM
Depositionale Magnetisierung (in Sedimenten): DRM
Chemoremanente Magnetisierung: CRM
DRM entsteht durch die geordnete Ablagerung magnetischer
Minerale in Sedimentgesteinen zur Zeit der Deposition.
CRM entsteht durch das langsame Mineralwachstum nach der
Ablagerung oder Erstarrung.
A tape recorder
“An essay of GeoPoetry”
Submarine Lava flow at ridge
From
www.ridge2000.org/science/tcs/epr06activity.php
Dating the Magnetic Reversal
PaleoPole position
If we measure I we can compute
The original Latitude, from this
If we know the position of the
Sample we compute the
Position of the paleopole.
Note if the plate move we have an
Apparent Polar Wander
Geomagnetic inclination (IGRF)
tan I = 2 tan 
I. Geomagnetic field – Worldwide Variation of I
Hierbei ist θ die sogenannte magnetische Co-Latitude.
Beispiel-Rechnung:
Basalt-Probe aus einem gegenwärtigen Ort an 47 S, 20 E.
Die remanente Magnetisierung ergibt eine Paleo-Inklination
von 30 Grad. Paleo-Deklination N30E.
Bestimmen wir die Paläolatitude
Position: 47S 20E
StereoNet
Position: 47S 20E
Declination:
N30E
Position: 47S 20E
Declination:
N30E
Inclination: 30
grad
Paleolat:16 grad
Distance Pole:74
grad
Position: 47S 20E
Declination:
N30E
Inclination: 30
grad
Paleolat:16 grad
Distance Pole:74
grad
APW
Apparent Polar Wander
Since the
mechanism of
generation of the
magnetic field is
influenced by the
rotation the dipole is
mainly oriented
along the rotation
axis and people use
the magnetic pole as
past proxy for the
rotation axis
APW
Apparent Polar Wander
Ocean Spreading
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.
A tape recorder
“An essay of GeoPoetry”
A tape recorder
“An essay of GeoPoetry”
Dating the Magnetic Reversal
Chron
A tape recorder
“An essay of GeoPoetry”
Isochron (or chron)
Continental Drift
In 1912 Wegener suggested that the current position of the continents has
evolved and that the continents were once fit together in a supercontinent
called PANGAEA.
Evidences for Continental Drift
• The shape of the continent
seems to fit together.
Evidences for Continental Drift
• The shape of the continent
seems to fit together.
• Matching Geological Units
Evidences for Continental Drift
• The shape of the continent
seems to fit together.
• Matching Geological Units
• Distribution of Fossils
Evidences for Continental Drift
• The shape of the continent
seems to fit together.
• Matching Geological Units
• Distribution of Fossils
• Distribution of Climatic Belts
Evidences for Continental Drift
• The shape of the continent
seems to fit together.
• Matching Geological Units
• Distribution of Fossils
• Distribution of Climatic Belts
• Location of past glacial
regions.
Evidences for Continental Drift
• The shape of the continent
seems to fit together.
• Matching Geological Units
• Distribution of Fossils
• Distribution of Climatic Belts
• Location of past glacial
regions.
BIG PROBLEM!!!!
HOW CAN THE WEAK CONTINENTS MOVE TROUGH THE
STRONG OCEANS?? WHERE THE FORCES TO MOVE
THE CONTINENTS COME FROM???
APW
Apparent Polar Wander
Since the
mechanism of
generation of the
magnetic field is
influenced by the
rotation the dipole is
mainly oriented
along the rotation
axis and people use
the magnetic pole as
past proxy for the
rotation axis
A tape recorder
“An essay of GeoPoetry”
Magnetic Reversal on Sea Floor
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.
Brittle Ductile behavior
brittle
ductile
The behavior is dependent on the temperature, the material
But also by the RATE!!!!