Environmental Engineering Reference
In-Depth Information
A strong spherical harmonic degree two in geoid and seismic tomography
suggests that the Earth
'
is mantle is divided into two boxes by subducted plates.
This can be traced back at least 400 Myr (Storey,
1995
; Greff-Lefftz and Besse,
2012
). The two boxes extend from the transition zone to the core
-
mantle boundary
(CMB). Present-day convection in the lower mantle is dominated by a quadrupolar
mode, in which superplumes rise from roughly antipodal equatorial regions
under Africa and the central Paci
c. Surface hotspots may rise from distinct depths
(Courtillot
et al
.,
2003
): seven from the deepest part of the lower mantle, probably
anchored on chemical heterogeneities deposited in the D
00
layer. Six out of seven of
these primary hotspots are found at the margins of the superplumes (
Figure 15.3
).
The original locations of
flood basalts at the time of eruption are also along
the edges of the present-day seismic low-velocity anomalies imaged above the
have proposed that the edge of the Indo-African superswell at the CMB has not
moved signi
cantly with respect to the spin axis of the Earth in the past 300 Myr.
Greff-Lefftz and Besse (
2012
) have proposed an extension of this analysis
back to 410 Ma.
flood basalt volcanism
A nagging problem remains the limits of time resolution imposed by geochrono-
logical methods. Seen from a general perspective, most continental
15.4 The tempo of
flood basalts
appear similar: the volumes of extruded lava are on the order of 1 Mkm
3
(spread
less than a factor 10) and their durations are on the order of 1 Myr. Yet,
their environmental consequences are diverse. The factors that could explain this
diversity include geographical location of eruptions, height of volcanic plumes,
composition, amounts and
fluxes of volatiles or the presence of sediments intruded
by the rising magma. We have focused on (and attempted to test) the hypothesis
that the tempo of volcanism is the culprit: eruption would take place as a short
series of intense, brief volcanic pulses, and the separation in time between these
pulses, compared to the time required by the ocean to return to equilibrium (a few
thousand years), could be the main factor.
We returned to the Deccan (Chenet
et al
.,
2007
,
2008
,
2009
), and then to the
Karoo in Lesotho and South Africa (Moulin
et al
.,
2011
, unpublished work). Other
groups have documented the Columbia (Mankinen
et al
.,
1985
,
1987
; Jarboe
et al
.,
2008
), North Atlantic Igneous Province (NAIP) (Greenland) (Riisager
et al
.,
2002
,
2003
), Central Atlantic Magmatic Province (CAMP; Knight
et al
.,
2004
) and the
Siberian Traps (Pavlov
et al
.,
2011
; also see
Chapter 5
). This has recently been
studies for the other Phanerozoic continental
flood basalts. A key piece of
