Download Earth`s Structure

Introduction to Geophysics and
Planetary Physics
(5) Structure of the Earth
Geophysics
104
Geophysics
105
Refraction of Seismic Waves
P&S, 1994
P&S, 1994
At interfaces between different materials seismic waves will be reflected an refracted (just like light beams).
When the material properties change continuously (like in the upper mantle) the waves will also be refracted
continuously. Here the speed of the seismic waves increases with depth, resulting in continuous bending towards
the surface (governed locally by Snell’s law of refraction, and globally by Fermat’s principle of least time).
At the core-mantle boundary vp decreases abruptly, to increase then again with increasing depth (Source, also for
the next slides: Press & Siever, „Allg. Geologie“).
PP waves experience one internal reflection at the Earth’s surface (left). PcP waves are reflected at the core.
PKP waves travel also through the molten outer core, PKIKP even through the solid inner core (right).
Geophysics
106
Shadow Zones for P- and S-Waves
P&S, 1994
P&S, 1994
S-Waves (left) cannot travel through the liquid outer core, therefore they will never emerge beyond 105° from
the focus. P-Waves (right) can penetrate the outer core, but due to the strong bending at the core-mantle
boundary they also have a shadow zone between 105° und 142°. For a PKIKP wave it takes ~ 20 minutes to
traverse the whole Earth.
Geophysics
107
Velocity Profiles
P&S, 1994
The speed change with
depth of P- and S-Waves
reflects different layers of
the Earth (left).
Note that there are Swaves in the (solid) inner
core, which cannot penetrate the outer core and do
not originate in the inner
core – but wave type
conversion can happen at
boundaries.
Right: The – partially –
molten asthenosphere is
characterized by low SWave velocity (“low
velocity zone”. Phase
transitions (Olivine–
Spinel–Perovskite) cause
stepwise increases of SWave velocity – and deep
earthquakes.
P&S, 1994
Geophysics
108
Earths’ Most Common Mineral
The high-pressure modification
of Olivine – a Silicate-Perovscite
(which is known from high
pressure experiments) has just
(2014) been discovered in nature –
not on Earth (where it is hidden in
the lower mantle), but in the
Tenham Meteorite.
The official name is now
Bridgmanite (“Bridgmanit” in
German). Credit: Chi Ma, Caltech.
Geophysics
109
Igneous Rocks
Basalt
Rhyolite
Igneous rocks are classified according to their
silica content and texture.
The most important igneous rocks (top right).
Continental crust is considerably thicker than
oceanic crust (right). Source: P & S, 1994.
Gabbro
Granite
Geophysics
110
Earth Crust
Global crust-thickness (satellite GOCE, ESA).
Geophysics
111
Eruption Types
With increasing viscosity of the magma volcanoes become more and more explosive …
Geophysics
112
Eruption Types
Source: USGS
Geophysics
113
Mt. Peleé, 1902
Until the morning of May 8, 1902 St. Pierre with ~ 30000 inhabitants was the largest
city on the island of Martinique. St. Pierre was completely destroyed by a pyroklastic
cloud (left and right, pictures: Heilprin). There were only two survivors, Louis-Auguste
Cyparis survived behind the tick walls – later on he became part of Barnum and Bailey’s
“Greatest Show on Earth” (top left: Circus World Museum).