Geoscience Reference
In-Depth Information
Another system of classification, which dates back to the 1930s but has intro-
duced names that are deeply embedded in the literature, is to study a suite of
rocks for their CaO and (Na
2
O
K
2
O) contents. Rock suites that are rich in the
alkalis are called alkaline; rock suites that are rich in CaO are calcic. Andesite
volcanoes typically have calc-alkaline trends.
For geophysicists, the difference between basalt and andesite is especially
important. Basalts must have less than 52% SiO
2
,whereas andesites have more.
Basalts have over 40% by weight of dark minerals, typically pyroxene, whereas
andesites have less than 40% of dark minerals and often (though not always)
contain hornblende. The plagioclase in basalt is more calcium-rich than that in
andesite. In thin section, basalt and andesite can be seen to have different micro-
scopic textures. The rock types between basalt and andesite are called andesitic
basalts and basaltic andesite.
Basalts are, in fact, a family of lavas, divided according to their content of
normative minerals into tholeiites (which have slightly more silica relative to
some other components) and
alkali-olivine basalts
(which are relatively more
alkaline), with a transitional group known as
olivine tholeiites
. Most ocean-floor
basalts and most but not all basalts in Hawaii are olivine tholeiites. Alkali basalts
occur in zones of continental rifting. For more detail, the reader should consult
any textbook on igneous petrology.
+
9.2 The oceanic lithosphere
9.2.1 Oceanic crust
Worldwide gravity surveys indicate that the oceanic regions are in approximate
isostatic equilibrium with the continents; that is, the pressure at an arbitrary
depth in the mantle is the same beneath continents and oceans. This means that, at
arbitrary depth in the mantle, a column of continental crust and underlying mantle
and a column of oceanic crust and its underlying mantle have the same mass. This
fact enables us to make a simple estimate of the thickness of the oceanic crust
(see Section 5.5 for the method). If we assume Airy-type compensation, densities
of sea water, crust and mantle of 1.03
10
3
kg m
−
3
,
respectively, and an average ocean-basin depth of 5 km, then a typical 35-km-
thick continental crust would be in isostatic equilibrium with an oceanic crust
6.6 km in thickness. This rough calculation tells us the important fact that the
oceanic crust is approximately one-fifth the thickness of the continental crust.
The details of the seismic structure of the oceanic crust have been determined
by using seismic-refraction and -reflection profiling and wide-angle-reflection
techniques. In the absence of direct sampling of the crust, its composition must be
estimated from measurements of its physical properties (e.g., seismic velocity and
density), which vary with lithology. These estimates are frequently ambiguous.
The most direct way to determine the composition of the oceanic crust is to
collect rock samples from each of the oceanic plates. Dredging samples from the
seabed is not particularly difficult or expensive, but it is often frustrating: imagine
10
3
, 2.9
10
3
×
×
and 3.3
×
