Page 186 - Global Tectonics

172 CHAPTER 7

formed in less than 3 Ma and the Kerguelen Plateau in
0 500 1000 Yemen 4.5 Ma. Most of the volcanic activity occurred in short,
Volcanics
km violent episodes separated by long periods of relative
Red Sea
Gulf of Aden tion, which include the periods of quiescence, are 12–
Afar quiescence. Estimates of the average rates of forma-
3 −1
3 −1
18 km a for the Ontong Java Plateau and 2–8 km a
10 N
for the Deccan Traps. Ontong Java’s rate of emplace-
Ethiopian Plateau ment may have exceeded the contemporaneous global
production rate of the entire mid-ocean ridge system
(Coffin & Eldholm, 1994).

The outpouring of large volumes of mafi c magma
Turkana
Depression in such short periods of time requires a mantle source.
Kenyan This characteristic has encouraged interpretations
Volcanics
0 involving deep mantle plumes (Sections 5.5, 12.10),
Lake
Victoria although the existence and importance of these fea-
tures are debated widely (Anderson & Natland, 2005).
East Mantle plumes may form large oceanic plateaux and
African some continental flood basalts also may be attributed

Plateau
to them. Beneath the Ethiopian Plateau and the
30 E 40 E 50 E
Kenya Dome (in the East African Plateau), extensive
volcanism and topographic uplift appear to be the
Figure 7.16 Map showing the location of Cenozoic
flood basalts of the Ethiopian Plateau and East African consequences of anomalously hot asthenosphere

Plateau (Kenya Dome) (after Macdonald et al., 2001, by (Venkataraman et al., 2004). The isotopic characteris-
permission of Oxford University Press). tics of the volcanic rock and the large volume of

mafic lava erupted over a short period of time
(Hofmann et al., 1997; Ebinger & Sleep, 1998) suggest
3
estimated at approximately 924,000 km (Latin et al., that a plume or plumes below the uplifts tap deep
1993). In Ethiopia (Fig. 7.16), layers of basaltic and felsic undegassed mantle sources (Marty et al., 1996; Furman
rock reach thickness of >2 km with a total volume esti- et al., 2004). As the deep plumes ascend they undergo
3
mated at 350,000 km (Mohr & Zanettin, 1988). The decompression melting with the amount of melt

eruption of such large volumes of mafic magma has depending on the ambient pressure (Section 7.4.2).
severe environmental consequences, such as the forma- Consequently, less melting is expected under thick
tion of greenhouse gases, the generation of acid rain, continental lithosphere than under thick oceanic litho-

and changes in sea level (Coffin & Eldholm, 1994; Ernst sphere. Nevertheless, the sources of magma that gen-
et al., 2005). The eruptions also make signifi cant contri- erated many LIPs are not well understood and it is
butions to crustal growth. likely that no single model explains them all. Ernst et
Some LIPs appear to form very quickly. For many al. (2005) review the many aspects of LIP research and

continental flood volcanics, 70–80% of the basaltic rock models of their formation, including links to ore
erupted in less than 3 million years (Menzies et al., deposits (Section 13.2.2).
2002). Geochronologic studies have shown that the
main flood event in Greenland (Tegner et al., 1998), the

Deccan Traps (Hofmann et al., 2000), and the bulk of 7.4.2 Petrogenesis of rift rocks
the Ethiopian Traps (Hofmann et al., 1997) all erupted

in less than one million years. Nevertheless, this latter The geochemistry of mafic volcanic rocks extruded at
example (Section 7.2) also shows that pulses of volca- continental rifts provides information on the sources
nism between 45 and 22 Ma contributed to the forma- and mechanisms of magma generation during rifting.
tion of the flood basalts in the Afar region. Submarine Rift basalts typically are enriched in the alkalis (Na 2 O,

plateaux probably formed at similar rates, although less K 2 O, CaO), large ion lithophile elements (LILE) such
2+
information is available from these types of LIPs. The as K, Ba, Rb, Sr, Pb and the light rare earths, and
North Atlantic Province and Ontong Java Plateau volatiles, in particular CO 2 and the halogens. Tholeiitic