200 Encyclopedia of the Solar System
FIGURE 10 (a) Sand dunes in Namibia. Namib-Naukluft National Park is an ecological preserve in Namibia’s vast Namib Desert,
and is the largest game park in Africa. Coastal winds create the tallest sand dunes in the world here, with some dunes reaching
300 meters in height. This ASTER perspective view was created by draping an ASTER color image over an ASTER-derived digital
elevation model. The image was acquired October 14, 2002. In the great deserts of the world, sand sheets are the dominant
morphology and are wind driven. In open desert areas (e.g., Sahara or Arabian Peninsula), dune trains may stretch for tens or
hundreds of miles.(Courtesy NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team). (b) Deadly landslide in
La Conchita, California. Large 1995 landslide and more recent 2005 debris flow that initiated from the slide above the town of La
Conchita, California. It destroyed or seriously damaged 36 houses and killed 10 people. Loss of coherence in water-saturated ancient
marine sediments was triggered by heavy rain. Landslides observed on Mars are typically 1 to as many as 3 orders of magnitude larger
and may indicate the past presence of water. Alternatively substantial atmospheric lubrication is possible as is thought to have
occurred during the ancient gigantic Blackhawk Slide on the slopes of the San Bernardino Mountains in California. (Courtesy of the
U.S. Geological Survey).slides and slumps, volcano-tectonic sector collapses, and
scour related to the action of glaciers. Mass-wasting pro-
cesses tend to affect a relatively minor proportion of the
Earth’s surface at any given time, however, such as volcanic
eruptions (with which they are often associated), when they
occur near population areas, their effects can be devastating
(Fig. 10b). On Mars, massive landslides, similar in morphol-
ogy and scale to the largest terrestrial submarine landslides,
are commonly seen within Vallis Marineris and its tributary
canyons.
4. Tools for Studying Earth’s Deep Interior
In comparison with other planets, the interior of the Earth
can be studied in unprecedented detail. This is because
of the existence of sources of energy, such as earthquakes
or magnetic and electric disturbances. Seismic waves, for
example, can penetrate deep inside the Earth, and the
time they travel between the source (earthquake or an ex-
plosion) and the receiver (seismographic station) depends
on the physical properties of the Earth. The same is true
with respect to electromagnetic induction, although obser-
vations are different in this case.
Observation and interpretation of seismic waves provide
the principal source of information on the structure of the
deep interior of the Earth. Both compressional (P waves)
and shear (S waves) can propagate in a solid, only P waves in
a liquid. Compressional waves propagate faster than shear
waves by, roughly, a ratio of√- Velocities, generally, in-
crease with depth because of the increasing pressure; hence
the curved ray paths (Fig. 11).
At the discontinuities (which include the Earth’s sur-
face), waves may be converted from one type to another.
Figure 11a shows P waves emanating from the source
(“Focus”). The P waves can propagate downward (right part
of the figure) and are observed as PP, PS, PPP, PPS, for ex-
ample. They can also propagate upward, be reflected from
the surface, and then observed as so-called “depth phases”:
PP, PPS. Depth phases are very helpful for a precise deter-
mination of the depth of focus.
Figure 11a shows rays in the mantle; there are also the
outer core and inner core. The outer core is liquid and has
distinctly different composition; the P-wave speed is some