Geology Reference
In-Depth Information
Uplift
A
B
Plate
Plume
D
Hotspot Swell
Plate
A
B
Motion
1000 km
C
D
1km
Plate
Plume
C
Figure 7.5. Sketch of a hotspot swell like that of Hawaii (Figure 7.1) in map
view (left) and two cross-sections (AB, CD, right), showing the relationship of
the swell to the plume that is inferred to be below the lithosphere. The swell is
inferred to be raised by the buoyancy of the plume material, which arrives as a
column (section AB) and then spreads laterally (section CD). This allows the rate
of flow of buoyancy and heat in the plume to be estimated.
of the plume material under the plate exerts an upward force. These forces should
balance, because the sea floor is not accelerating upwards or downwards and, as
discussed above, there is no other force to hold up the swell, such as the flexural
strength of the lithosphere.
Since the plate is moving over the plume, the parts of the plate that are already
elevated are being carried away from the plume. In order for the swell to persist,
new parts of the plate have to be continuously raised as they arrive near the plume.
This requires the arrival of new buoyant plume material under the plate (cross-
section AB). Thus the rate at which new swell topography is generated will be a
measure of the rate at which buoyant plume material arrives under the lithosphere.
The addition to swell topography each year is equivalent to elevating by a height
h
=
1 km a strip of sea floor with a 'width'
w
=
1000 km (the width of the swell)
and a 'length'
v
δ
t
100 mm (the distance travelled by the Pacific plate over the
plume in one year at velocity
v
=
=
100 mm/yr). The weight,
W
, of one year's worth
of new swell is then
=
−
W
g
(
ρ
m
ρ
w
)
wvh.
(7.3)
The relevant difference in density is that between the mantle (
ρ
m
) and sea water
(
ρ
w
) because both the sea floor and the moho are raised, and sea water is displaced.
(You can work this through by considering the vertical displacement of the moho,
and you will find the density of the crust cancels out.)
The requirement that the weight of the topography should balance the buoyancy
of the plume material then requires that
b
=
W
, so the rate of buoyancy flow up the
plume is
b
=
g
(
ρ
m
−
ρ
w
)
wvh.
(7.4)
