Geology Reference
In-Depth Information
Figure 2.2. Sketch of the mantle, with crust and lithosphere roughly to scale. The
core is defined by the bottom boundary of the mantle. From Davies [4]. Copyright
by Elsevier Science. Reprinted with permission.
Earth's interior. There is now good evidence that it is due to a combination of a
change in composition and one or perhaps two pressure-induced phase changes.
Based on this information, and other sources that we will encounter through the
topic, Figure 2.2 is a sketched cross-section of the crust and mantle, roughly to
scale. The continental crust and some thicker parts of the oceanic crust show as
black at the surface, though their thickness is exaggerated. The lithosphere, defined
below, is outlined by the thin black lines. The lower boundary of the mantle outlines
the extent of the core. Within the mantle, the 660 km discontinuity is marked by
the long-dashed line. The D
region is outlined by dashed and dotted lines at the
bottom of the mantle. Other features in this sketch will be explained later in the
topic.
2.2 Lithosphere versus crust
The oceanic lithosphere plays a key role in the conception to be developed in this
topic. The role of continental lithosphere seems to be much less central, largely
because continental crust is different from oceanic crust. The distinctions between
crust and lithosphere, between continental and oceanic lithosphere, and between
continental and oceanic crust thus need to be clear, otherwise the discussion of
mantle convection will be confused. The distinctions are illustrated in cartoon form
in Figure 2.3, which is not to scale.
The continental crust is commonly about 35-40 km thick. This was first deter-
mined in 1909 when Mohorovicic identified the seismic discontinuity named after
him [5], also known as the 'moho'. The thickness is larger under mountain ranges,
