Page 35 - Global Tectonics

22 CHAPTER 2

center. The differentiation prevents any estimate being bulk of the remainder comprising calcium, aluminum,
made of the overall composition of the Earth by direct nickel, sodium, and possibly sulfur.
sampling. However, it is believed that meteorites are
representatives of material within the solar nebula and
that estimates of the Earth’s composition can be made
from them. The presence of metallic and silicate phases 2.4 THE CRUST
in meteorites is taken to indicate that the Earth consists
of an iron/nickel core surrounded by a lower density
silicate mantle and crust. 2.4.1 The continental crust
Seismic data, combined with knowledge of the mass
and moment of inertia of the Earth, have revealed that Only the uppermost part of the crust is available for
the mean atomic weight of the Earth is about 27, with direct sampling at the surface or from boreholes. At
a contribution of 22.4 from the mantle and crust and greater depths within the crust, virtually all information
47.0 from the core. No single type of meteorite pos- about its composition and structure is indirect. Geologic
sesses an atomic weight of 27, the various types of studies of high grade metamorphic rocks that once
chondrite being somewhat lower and iron meteorites resided at depths of 20–50 km and have been brought to
considerably higher. However, it is possible to mix the the surface by subsequent tectonic activity provide some
proportions of different meteorite compositions in such useful information (Miller & Paterson, 2001a; Clarke et
a way as to give both the correct atomic weight and al., 2005). Foreign rock fragments, or xenoliths, that are
core/mantle ratio. Three such models are given in carried from great depths to the Earth’s surface by fast-
Table 2.1. rising magmas (Rudnick, 1992) also provide samples of
It is apparent that at least 90% of the Earth is made deep crustal material. In addition, much information
up of iron, silicon, magnesium, and oxygen, with the about the crust has been derived from knowledge of the
variation of seismic velocities with depth and how these
correspond to experimental determinations of veloci-
Table 2.1 Estimates of the bulk composition of the ties measured over ranges of temperature and pressure
Earth and Moon (in weight percent) (from Condie, consistent with crustal conditions. Pressure increases
1982a). −1
with depth at a rate of about 30 MPa km , mainly due

to the lithostatic confining pressure of the overlying
Earth Moon rocks, but also, in some regions, with a contribution
from tectonic forces. Temperature increases at an
1 2 3 4
−1
average rate of about 25°C km , but decreases to about
Fe 34.6 29.3 29.9 9.3 half this value at the Moho because of the presence of
O 29.5 30.7 30.9 42.0 radioactive heat sources within the crust (Section 2.13).
Si 15.2 14.7 17.4 19.6 Collectively, the observations from both geologic and
Mg 12.7 15.8 15.9 18.7 geophysical studies show that the continental crust is

Ca 1.1 1.5 1.9 4.3 vertically stratified in terms of its chemical composition
Al 1.1 1.3 1.4 4.2 (Rudnick & Gao, 2003).
Ni 2.4 1.7 1.7 0.6 The variation of seismic velocities with depth
Na 0.6 0.3 0.9 0.07 (Section 2.2) results from a number of factors. The
S 1.9 4.7 – 0.3 increase of pressure with depth causes a rapid increase
in incompressibility, rigidity, and density over the
1: 32.4% iron meteorite (with 5.3% FeS) and 67.6% oxide portion topmost 5 km as pores and fractures are closed. There-
of bronzite chondrites. after the increase of these parameters with pressure is
2: 40% type I carbonaceous chondrite, 50% ordinary chondrite,
and 10% iron meteorite (containing 15% sulfur). balanced by the decrease resulting from thermal
3: Nonvolatile portion of type I carbonaceous chondrites with expansion with increasing temperature so that there is
FeO/FeO + MgO of 0.12 and suffi cient SiO reduced to Si to yield little further change in velocity with depth. Velocities
2
a metal/silicate ratio of 32/68. change with chemical composition, and also with
4: Based on Ca, Al, Ti = 5 × type I carbonaceous chondrites, FeO
= 12% to accommodate lunar density, and Si/Mg = chondritic changes in mineralogy resulting from phase changes.
ratio. Abrupt velocity discontinuities are usually caused by