Geology Reference
In-Depth Information
2
Context
Basic concepts and primary observations. Defining the crust, mantle
and core. Distinguishing crust from lithosphere, continents from ocean
basins. The distribution of topography and heat flux over the sea floor.
Mantle convection occurs, remarkably enough, in the Earth's mantle. It is affected
by the crust, and part of the lithosphere plays a major role. There are peculiarities
near the boundary of the mantle with the core that may significantly affect mantle
convection, and that certainly tell us some important things about mantle convec-
tion. To discuss our subject sensibly, we had better be clear what all these terms
refer to: mantle, crust, core, lithosphere and so on. That is one thing this chapter
is about. There are also important constraints on mantle convection to be had from
the form of the Earth's topography, and from the geographic variation of heat flow
from the Earth's interior. These will also be summarised.
2.1 Crust, mantle, core
The major division of the Earth's interior is into crust, mantle and core. The
boundaries between these regions were detected seismologically, in other words
using the internal elastic waves generated by earthquakes, which are detected as
they emerge at the Earth's surface. The variation of seismic velocities, and density,
with depth in the Earth is shown in Figure 2.1. The boundary between the mantle
and the core is at a depth of about 2900 km, where the seismic velocities drop, the
shear velocity is zero and the density jumps.
The fact that the shear velocity is zero in the core indicates that it is liquid, except
for a smaller region at the centre, the inner core, which is solid. The high density
of the core is consistent with it being made mostly of iron, with some nickel and
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