Page 43 - Global Tectonics

30 CHAPTER 2

Conrad discontinuity, but this is not globally are the location of the Earth’s most active areas
developed. By contrast, the layering of the of volcanic and plutonic activity. Oceanic
majority of oceanic crust is well defi ned into islands are a third distinct, but less prolifi c
three distinct layers. However, the nature of oceanic setting for igneous activity.
these layers, in particular layers 2 and 3, may
change quite markedly with depth.
2 Thickness. The thickness of continental crust
averages 40 km but is quite variable, thinning 2.8 THE MANTLE
to only a few kilometers beneath rifts and
thickening to up to 80 km beneath young
mountain belts. Most oceanic crust has a 2.8.1 Introduction
remarkably constant thickness of about 7 km,
although layer 1, the sedimentary layer, The mantle constitutes the largest internal subdivision
increases in thickness towards ocean margins of the Earth by both mass and volume, and extends
that are not characterized by ocean trenches. from the Moho, at a mean depth of about 21 km, to the
Differences in the thickness and the creep core–mantle boundary at a depth of 2891 km. On a
strength (Section 2.10.4) of continental crust gross scale it is believed to be chemically homogeneous,
make the lower crust of continental regions apart from the abundances of minor and trace elements,
much more likely to deform pervasively than and formed of silicate minerals. The mineralogy and
in the lower layers of oceanic crust (Section structure of the silicates change with depth and give rise
2.10.5). to a transition zone between 410 and 660 km depth,
3 Age. Continental crust is as at least as old as which separates the upper and lower mantle.
Mantle materials are only rarely brought to the
4.0 Ga, the age of the oldest rocks yet
discovered (Section 11.1). On a very broad scale surface, in ophiolite complexes (Section 2.5), in kimber-
lite pipes (Section 13.2.2), and as xenoliths in alkali
the oldest crust consists of Precambrian cratons
or shield areas that are surrounded by younger basalts. Consequently, most of our information about
the mantle is indirect and based on the variation of
orogenic belts, both active and inactive. Oceanic
crust, however, is nowhere older than 180 Ma, seismic velocities with depth combined with studies of
mineral behavior at high temperatures and pressure,
and progressively increases in age outwards
from oceanic ridges (Section 4.1). Oceans are and in shock-wave experiments. Geochemical studies of
meteorites and ultramafic rocks are also utilized in

consequently viewed as essentially transient
features of the Earth’s surface. About 50% of making predictions about the mantle.
the surface area of the present day ocean fl oor
has been created during the last 65 Ma, implying
that 30% of the solid Earth’s surface has been 2.8.2 Seismic structure
created during the most recent 1.5% of of the mantle
geologic time.
4 Tectonic activity. Continental crust may be The uppermost part of the mantle constitutes a high
extensively folded and faulted and preserves velocity lid typically 80–160 km thick in which seismic
evidence of being subjected to multiple tectonic velocities remain constant at a figure in excess of

events. Oceanic crust, however, appears to be 7.9 km s or increase slightly with depth. This part of
−1
much more stable and has suffered relatively the mantle makes up the lower portion of the litho-
little deformation except at plate margins. sphere (Section 2.12). Beneath the lithosphere lies a low
5 Igneous activity. There are very few active velocity zone extending to a depth of approximately
volcanoes on the great majority of the 300 km. This appears to be present beneath most regions
continental crust. The only major locations of the Earth with the exception of the mantle beneath
of activity are mountain belts of Andean type cratonic areas. From the base of this zone seismic veloc-
(Section 9.8). The activity within the oceans is ities increase slowly until a major discontinuity is
very much greater. Ocean ridges and island arcs reached at a depth of 410 km, marking the upper region