Australia: The Land Where Time Began

A biography of the Australian continent 

Flood Basalts -Bigger and Badder

Flood volcanism is a process which transfers vast amounts of mass and energy to the surface of the earth from its interior over a relatively short expanse of time, geologically speaking, in an episodic manner, these events having occurred about 12 times over the last several million years. Geochronological evidence has been accumulating that suggests that during each of these flood basalt eruptions the generation and eruption of magma occurred over a period of about 1-3 My. This implies that the production rate of magma is much higher than in the main process of magma production at the surface of the Earth at the spreading centres on the boundaries between tectonic plates, the flood basalt events producing magma at a rate on the order of 10 million cu km/ year. There has been an increasing amount of research targeting flood basalt eruptions in an attempt to find the causes and consequences of such dramatic events. A central role in developing an understanding these events has been played by the Siberian Traps.

It has been proposed that the massive eruption of magma comprising the Siberian Traps might have been the first manifestation of a hotspot that is a still active source of magma as represented by Jan Mayen, a volcanic island in the North Atlantic (Morgan, 1981). This proposal was followed by a generalised theory linking flood basalts to hotspots that had been formed by mantle plumes that were superheated and buoyant, the inference being that mantle plumes have a dynamic role in the rifting that occurs between continents. Alternative mechanisms were proposed, a leading one of these "plume impact" models which proposes that rifting occurring in the lithosphere results in decompression of the mantle leading to melting which then rises buoyantly without the requirement for anomalous heating.

According to the author1 rifting that precedes decompression melting, according to the latter model, is prevented from occurring rapidly for mechanical reasons, the lower lithosphere being ductile and so under extension it does not break rapidly. The author1 suggests the decompression melting appears less consistent with the rapidity of the eruptions that has been observed than does the plume impact model. There is evidence, however, that volcanism is predated by extension in some cases, suggesting that some aspects, at least, of the decompression model are valid. The question is asked, what initiates extension if it is not the consequences of plume impact. One possibility is edge-driven convection (King & Anderson, 1998), that has been hypothesised to originate from discontinuities in the thickness and properties of the lithosphere.

Geochronology has been a key factor in defining the vastness of the provinces where the flood volcanics occur, as well as establishing the shortness of the periods in which the flood volcanic events occurred. The Central Atlantic Magmatic Province (CAMP) is a recent example, with its remnants being scattered across North America, northeastern South America, western Africa and western Europe. It has been proposed that prior to the opening of the central Atlantic the remnants of the CAMP had comprised a single contiguous province (Courtillot, 1994), the dispersed fragments being identified as an extensive flood basalt province, which was confirmed by precise dating (Marzoli et al., 1999).

The subsurface identity of the Siberian Traps almost 1,000 km to the west of the previously known limits of the province was documented by new dating reported by Reichow et al., (Reichow et al., 2002), as a result of analysis of drill-core samples from the West Siberian Basin (WSB). As a result of the new dating they have been linked for the first time as occurring in the same magmatic event.

By the use of the 40Ar/39Ar dating method it was shown (Reichow et al., 2002) that it is not possible to distinguish the age of the lavas of the WSB from those to the east that had previously been dated to 250 Ma by a similar dating method (Renne & Basu, 1991 originally dated as 248 Ma; revised after recalibration Renne et al., 1995). The new results suggest a total area for the Siberian Traps of 3.9 million km2. The new volume of magma for this expanded area is said to probably be 2-3 million km3, which clearly makes the Siberian Traps the largest volume of known flood basalts for a continental flood basalt province in the world.

About 300-200 Ma the WSB was subjected to rifting in the Late Palaeozoic or Early Mesozoic, bearing out the general relationship between extension and flood basalt magmatism. On the crucial point of whether rifting began before or after the beginning of volcanism existing data appear equivocal, as occurs in many other cases, the author1 suggesting the establishment of dating of these events should become a priority, and he also suggests that as research continues upward revision of  the areas of flood volcanic provinces will continue.

It has been shown by recent work (Lyons et al., in press 2002) that as far south as central Kazakhstan there is magmatism that is essentially the same age as in the Siberian Traps, and it is possible a swathe of contemporary magmatic activity may have extended, possibly in a semi-continuous manner, from there to the south of Lake Baikal. The author1 suggests it appears these complexes may represent the roots of silicic volcanic centres, the explosive eruptions of which would have provided a mechanism by which volcanogenic gases could be transported into the upper atmosphere.

There are important implications for the environment of flood volcanic events as a result of the increasing size estimates. The larger the volume of magma produced by a magma system the greater the likelihood is of it generating large quantities of climate-modifying gases, such as carbon dioxide and sulphur dioxide. According to the author1 it remain difficult to quantify the amounts of such gases that are actually released to the atmosphere by flood volcanism, though there is little doubt that the effects could be significant. It has been long noted in the geologic record that there is synchrony between flood volcanic events and mass extinction events. For the Siberian Traps, CAMP, and the Deccan Traps, the 3 biggest flood basalt events known of in the history of the Earth, there is a temporal correlation between the most severe mass extinctions at the close of the Permian, Triassic and Cretaceous, respectively, that has been established firmly. 

The author1 suggests the empirical connection between major flood volcanism and severe mass extinction events is even more intriguing as there is evidence hinting at impacts of large meteors that are also coincident with these events. The strongest evidence of a bolide impact is found in connection at the end-Cretaceous. The latest evidence to be uncovered suggests that an impact at the close of the Triassic may be connected with the CAMP and the mass extinction event at that time (Olsen et al., 2002), though in this case the evidence of impact is permissive rather than indicative.

The need for a unifying theory that is geophysically plausible is already clear to some Earth scientists, though other scientists consider the evidence for impacts coincident with major impact events is still too weak, with the exception with that at the close of the Cretaceous. The author1 suggests not many would dispute that proving the existence of an impact is much more of a challenge than documenting a flood basalt event, as it is difficult to miss millions of cu km of lavas, even when they are buried beneath 2 km or more of Siberian sediments. 

 

Sources & Further reading

  1. Renne, Paul R. "Flood Basalts--Bigger and Badder." Science 296, no. 5574 (June 7, 2002 2002): 1812-13.

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 25/06/2013
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading