Geoscience Reference
In-Depth Information
1Ga(10
9
years ago). An extensive accumulation of
basalt or olivine near the Earth's surface during
accretion forms a buoyant layer that resists sub-
duction. An extensively molten, slowly cooling,
upper mantle, and a very slowly cooling deeper
mantle are predicted.
A magma ocean freezes from the bottom but
a thin chill layer may form at the surface. As
various crystals freeze out of the ocean they
will float or sink, depending on their density.
On the Moon, plagioclase crystals float when
they freeze and this is one explanation of the
anorthositic highlands. On the much larger
Earth, the aluminum enters dense garnet crys-
tals and a deep eclogite-rich layer is the result.
Although a magma ocean may be convecting vio-
lently when it is hot, or being stirred by impacts,
at some point it cools through the crystallization
temperatures of its components and the subse-
quent gravitational layering depends on the rela-
tive cooling rate and sinking rates of the crys-
tals. Meanwhile, new material is being added
from space and is processed in the magma ocean.
A chemically stratified Earth is the end result.
Accretional and convective stirring is unlikely to
dominate over gravitational settling.
Magma is one of the most buoyant products
of mantle differentiation and will tend to stay
near the surface. A hot, differentiated planet
cools by the
heat-pipe cooling mechanism
of mantle convection
; pipes or sheets of
magma remove material from the base of the
proto-crust and place it on top of the basaltic
pile, which gets pushed back into the mantle,
cooling the interior. As a thick basalt crust cools,
the lower portions eventually convert to dense
eclogite -- instead of melting -- and delaminate.
This also cools off the interior. As the surface
layer cools further, the olivine-rich and eclogitic
parts of the outer layer become denser than the
interior and subduction initiates. At this point
portions of the upper mantle are rapidly cooled
and the thermal evolution of the Earth switches
over to the plate-tectonic era. Plate tectonics is a
late-stage method for cooling off the interior, but
it is restricted to those parts of the interior that
are less dense than slabs. A dense primitive atmo-
sphere and buoyant outer layers are effective
insulators and serve to keep the crust and upper
6
5
4
3
2
Melting
Zone
1
0
0
1
2
3
4
5
6
7
Radius (10
3
km)
Fig. 1.3
Schematic temperatures as a function of radius at
three stages in the accretion of a planet (heavy lines).
Temperatures in the interior are initially low because of the
low energy of accretion. The solidi and liquidi and the melting
zone in the upper mantle are also shown. Upper-mantle
melting and melt-solid separation is likely during most of the
accretion process. Silicate melts, enriched in incompatible
elements, will be concentrated toward the surface
throughout accretion. The Earth, and perhaps the mantle, will
be stratified by intrinsic density, during and after accretion.
The Melting Zone in the upper mantle or a near-surface
magma ocean processes accreting material. Temperature
estimates provided by D. Stevenson.
years, around 4.55 billion years ago. The core
was forming during accretion and was already
in place by its end. There was likely not a
core-
forming event
.
Accretional calculations, taking into account
the energy partitioning during impact, have
upper-mantle temperatures in excess of the melt-
ing temperature during most of the accretion
time (Figure 1.3). If melting gets too extensive,
themeltmovestowardthesurface,andsome
fraction reaches the surface and radiates away
its heat. A hot atmosphere, a thermal bound-
ary layer and the presence of chemically buoy-
ant material at the Earth's surface, however, insu-
lates most of the interior, and cooling is slow.
Extensive cooling of the interior can only occur
if cold surface material is subducted into the
mantle. This requires a very cold, thick thermal
boundary layer that is denser than the under-
lying mantle. This
plate tectonic mode
of mantle
convection -- with subduction and recycling --
may only extend back into Earth history about
