Environmental Engineering Reference
In-Depth Information
The
'
great volume
'
and
'
great areal extent of outcrop
'
criteria are in some cases
dif
cult to apply because of limited surface outcrop and the effects of extensive
erosion. LIPs, because of their size, have been recognized as a distinct class of
igneous rocks for many decades (e.g. Eldholm & Coffin,
2000
). Specialist study
groups and comprehensive reviews (e.g. Ernst
et al
.,
2005
) have included a strong
emphasis on LIPs of Precambrian times. Giant dike swarms are particularly abun-
dant in areas of Precambrian outcrop because rocks erupted at shallower depths,
such as
flows and sills, have commonly been eroded away. Studies have in some
cases attempted to identify LIPs as being characterized by distinctive minor-element
and isotopic composition; however, Herzberg
s(
2011
) initiative in characterizing
LIP basalts as having been erupted at relatively high temperatures seems more
promising as an identi
'
cation criterion. It has been suggested that noble-gas signa-
tures re
ecting diffusion from the hot basal interiors of long-lived LLSVPs and
LSVPs are also distinctive of LIPs (Mukhopaday,
2012
;Burke
et al
.,
2008
).
LIPs have been suggested to be of deep-seated hot source origin because
their formation requires partial melting of a large volume of mantle rock in a short
time. Because such melting makes basalt it must take place dominantly at shallow
depths (typically
<
c
. 100 km). The implication is that LIP origin is likely to
require the rise of bodies of hot ultrama
c rock from depth. Bodies of that kind
have, for the past 50 years, been postulated to exist and have been called
'
mantle
plumes
(see, e.g., Wilson,
1963
; Morgan,
1972
).
We have found that LIPs have been erupted above deep mantle plumes gener-
ated on the CMB. We are currently testing the hypothesis that LIPs, hotspot
volcanoes and rocks derived from sources within the continental mantle litho-
sphere (e.g. kimberlites) erupt near the Earth
'
or
'
deep mantle plumes
'
'
s surface close to vertically above
plume generation zones (PGZs) on the edges of LLSVPs (Burke
et al
.,
2008
;
Torsvik
et al
.,
2006
,
2010
). We find it useful to distinguish oceanic plateaus (OPs),
which are LIPs erupted within the world ocean, from LIPs called continental large
igneous provinces (CLIPs), which are erupted within the continents (Eldholm &
Cof
n,
2000
).
3.2.1 Oceanic plateaus (OPs)
We recognize nearly 20 OPs (73
155 Ma) within the oceans of the world (18 listed
in
Table 3.1
and shown in
Figure 3.1
that we can con
-
dently reconstruct to their
locations at the time of their eruption). That is about an average of one preserved
for every 10 Myr since the formation age of the world
oor; the
youngest recognized OP, the Sierra Leone Rise is about 73 Myr old. We discern
no periodicity of eruption within OPs but emphasize that the population is small
and the length of time represented by the life of the world
'
s oldest ocean
'
s current ocean
oor