Planetary morphologic processes - Introduction to Planetary Geomorphology - page 35

Figure 3.1. Road-cut near Death Valley, California, showing rocks that
have been sheared by a reverse fault as a result of compression; the
fault plane extends toward the upper right from the gure standing by
the road; note that the light gray rocks (A) have been shoved upward
some 5m vertically (arrows indicate direction of relative motion).

or the crust, consists of chemically differentiated materials
termed sima (referring to the elements Si, silicon, and
Mg, magnesium), which floors the ocean basins, and a
lower-density sial (Si and Al, aluminum) that forms the
continental crust. Analyses of the physical properties of
the Earth ' s interior zones from seismic data led to the
recognition of the lithosphere and asthenosphere.
The lithosphere includes the crust and the upper part of
the mantle, which together behave as a relatively rigid,
solid shell. The lithosphere rests on the asthenosphere,
which is the mechanically plastic (or “ slippery ” ) layer of
the mantle. The mantle is volumetrically the largest of
Earth ' is interior zones, beneath which is the core, consist-
ing of an outer liquid zone and an inner solid zone.
Heat sources within Earth ' s interior generate convec-
tion cells within the mantle and plastic ow that can
upwarp zones in the lithosphere and concentrate heat.
These zones can fracture, pull apart, and become sites of
volcanism, which introduces new rock to the surface.
Lateral flow of the “ slippery ” asthenosphere can, in turn,
drag segments of the lithosphere outward from the frac-
tures, forming rift zones. However, heat sources are not
evenly distributed within the mantle; nor are they of equal
magnitude, resulting in convection cells of different sizes
and geometries. On a global scale, this leads to individual
segments of the lithosphere, or plates ( Fig. 3.3 ), that are
of different sizes, moving at different rates.
Zones of upward-converging convection tend to be
sites of ma c (magnesium- and iron-rich) volcanism,
which re ect the sources of magma derived from the
upper mantle. Such volcanism generates new sima-style

Figure 3.2. These rocks in southern Israel were originally deposited
in flat-lying, horizontal beds, but subsequently have been deformed
into folds by compression.

crust and commonly occurs in oceanic settings. Up-
arching of the crust and accumulation of lava form sym-
metric mid-ocean ridges and fracture systems, such as the
mid-Atlantic rift. Lateral flow away from central rifts,
termed sea- oor spreading, has been measured to be as
rapid as 16 cm/yr along the East Paci c Rise, equivalent to
a pencil-length each year. In one million years (a geologic
“ blink of the eye ” ), its separation is some 160 km.
Downward-converging convection cells drag litho-
spheric plates toward one another into collision. Any
one of several styles of plate collision can occur, depend-
ing on the composition of the crustal segments that are
involved and the angle of the collision. Downward drag-
ging of slabs of crust, or subduction, generates earth-
quakes and possible remelting of the crust, leading to
volcanism. Because this melt, or magma, is formed at
least partly from continental crustal materials or from
oceanic sediments, the volcanism in subduction zones