The Origin of the Kerguelen Plateau
by Shana Star Light
Honors Earth Science - Fall, 1999

Abstract

The Kerguelen Plateau is a large igneous province (LIP), almost completely below sea-level, located in the southern Indian Ocean. It has been, in recent years, examined closely to determine its nature and origin. It is of volcanic origin, having been formed by hotspot activity below the Indian Ocean in short, sporadic bursts of eruptive activity. This has been deduced by the study of extensive core samples collected by the Ocean Drilling Program (ODP). The samples collected by the ODP contained organic matter and fossils indicating that the plateau once stood above sea level. The plateau then subsided to its present position, with only three small islands extending above sea level. Knowledge of the Kerguelen Plateau and other LIPs gives us a glimpse into the interior of our planet and a chance to examine the processes that shape the surface of the earth.

KEY TERMS: Kerguelen Plateau, large igneous provinces, volcanism, Ocean Drilling Program, hotspots


Introduction

The Kerguelen Plateau is an elevated region of sea floor in the southern Indian Ocean approximately equidistant from Africa and Australia. It is the second largest oceanic plateau in the world with dimensions of about 500 by 2,100 km, with the long axis extending northwest-southeast between 46 and 64S. It is bordered to the northeast by the Australian-Antarctic Basin (Figure 1), to the northwest by Crozet basin, to the southwest by the African-Antarctic Basin, and to the south by the 3,500-m-deep Princess Elizabeth Trough (Shlich et al., 1988). The Kerguelen Plateau is a huge magmatic feature generally classified as a large igneous province (LIP). The hotspot responsible for the genesis of the Kerguelen Plateau became active at least 115 million years ago when India separated from Australia-Antarctica (Hassler and Shimizu, 1998). LIPs cover areas of up to millions of square kilometers (Charvis and Operto, 1995). No current active volcanic regions are as large as the extinct LIPs such as the Kerguelen Plateau (Bartlett and Wexler, 1999). The plateau has long invited speculation and debate concerning its origins.
 


Early Hypotheses

The immense size of the plateau led many geoscientists to conclude that it might be a fragment of Antarctica (Anonymous, 1988), separated and repositioned by the movements of the tectonic plates in the Indian Ocean. The idea that the lithospheric basement was composed of continental fragments from the breakup of Gondwanaland (Hassler and Shimizu, 1998) was also explored. Basalts found at several locations on the southern Kerguelen Plateau contain trace elements that would normally be present in a structure of continental origin. Also, the plateau has features similar to that of a continental passive margin (Hassler and Shimizu, 1998). Later research showed how these components could be present because of eruptive movement and volcanic episodes, and were not proof of a continental origin of the plateau.
 
 


The Ocean Drilling Program

It is currently accepted that the plateau is of volcanic origin of a spectacular manner. More than a kilometer of water covers the Kerguelen Plateau in all but three locations (Anonymous, 1988) so it has been very difficult for researchers to come to that conclusion. The Ocean Drilling Program (ODP) has gathered many samples and answered many questions about the Kerguelen Plateau and other LIPs. To study just the uppermost section (Figures 1 and 2), the five- to 25-kilometer thick crust, requires expensive drilling vessels (Baldauf et al., 1998). Even the deepest boreholes can extend downward only about 10 kilometers. Analysis of core materials has helped scientists to reconstruct the volcanic history of the region, including the timing of eruptions and the chemical composition of the lava (Anonymous, 1998).
 
 

Volcanism of the Kerguelen Plateau

Formation of the Plateau

To understand LIPs it is important to determine the nature of their formation. This has been difficult because the passage of time disguises the extent of ancient volcanic structures. Older igneous provinces are heavily eroded and are therefore disfigured and diminished in scale (Coffin and Mahoney, 1997). Scientists once believed that LIPs form slowly over tens of millions of years, as new oceanic crust forms at mid-ocean ridges. Based on radiometric dating, it is much more likely that they emerge from a series of volcanic firestorms that swiftly pump gases and rock fragments into the air and water, abruptly transforming the geology of large areas of the earth (Baldauf, 1998). Analysis of rock dating indicates that the Kerguelen Plateau mostly formed in about 4.5 million years (Rotstein and Schaming, 1990). Much of the volcanic activity associated with the formation of the plateau occurred in short, violent episodes separated by long periods of relative quiescence (Cottin et al, 1994). The global network of mid-ocean ridges has yielded between 16 and 26 cubic kilometers of ocean crust per year over the past 150 million years. Individual igneous provinces have generated new crust at rates comparable to or greater than sea floor spreading (Coffin et al., 1990).

Researchers believe that the basement basalts of the Kerguelen Plateau erupted when the area was either above sea level or very close to the surface (Anonymous, 1988). Drill samples and seismic-reflection surveys demonstrate that when the Kerguelen plateau first erupted, it stood above sea level. Soil and vegetation probably covered parts of the plateau because there are claystones, siltstones, and small pieces of wood in the sediment blanketing the basalts (Anonymous, 1988). The plateau moved below sea level slowly. Limestones containing fossils of urchin-like creatures and other animals testify that the plateau was once at a shallow depth (Anonymous, 1988). It only gradually subsided to its present position deep in the water (Anonymous, 1988). As the tectonic plates of the Indian Ocean moved, they transported the landmass of Kerguelen away from its volcanic source. The crust cooled, contracted, and gradually subsided to below sea level (Bartlett and Wexler, 1999).
 
 

The Kerguelen Plume

Seismic studies of the Kerguelen Plateau present convincing evidence that oceanic plateaus consist primarily of volcanic rocks (Balauf et al., 1998). Beneath the islands of the Kerguelen Plateau, pieces of mantle of diverse provenance are present (Hassler and Shimizu, 1998).

Scientists have proposed that the earth's mantle circulates in two different modes: 1) the dominant one which involves large scale convection that nudges plates across the surface causing sea floor spreading and continental drift and 2) hotspot activity (Baski, 1999). The basement of the Kerguelen Plateau is formed from molten mantle rock of both types. About 1/10 of the heat now escaping from the mantle does so in the form of deep-rooted, narrow plumes of warmer than average material that rises through the mantle. When it reaches the base of the lithosphere, a plume decompresses and partially melts producing an upwelling of magma and a long-lasting locus of volcanic activity known as a hotspot (Lawver et al., 1994).

The most basic information needed to determine how these massive volcanic constructions arise is the total volume of lava and intrusive rock in an igneous province and the rate at which it formed. The volume of basaltic rock content in LIPs can indicate the dimensions of the hot plumes in the mantle that led to their genesis (Rotstein et al., 1990). Only a small portion of the plume material melts and reaches the crust. There is presumably a smaller amount of melting occurring at greater depths because of the intense pressure, which tends to keep mantle rocks in a solid state (Coffin and Eldholm, 1993). Less melting should take place under thick continental lithosphere than below thin oceanic lithosphere. The molten, basaltic portion of the plume, which is responsible for the surface volcanism and subsurface igneous intrusions, accounts for from 5 to 30 percent of the plume's total volume (Gibson et al., 1989).

Large plumes may contain at least some material from the lower mantle, located more than 670 kilometers below the surface. There is some discrepancy among geophysicists as to whether the whole mantle mixes or whether the upper and lower parts of the mantle behave as independent systems that circulate separately (Baski, 1994). Coffin and Eldholm (1993) favor models that allow for at least some interaction between upper and lower parts of the mantle. They believe that the largest plumes originate in the lower mantle in the D" layer, a region with unusual seismic properties which lies just outside the Earth's core.

Wherever mantle plumes originate, their attributes and effects at the surface depend strongly on the temperature, composition, and physical state of the material they encounter just beneath the lithosphere (Charvis and Operto, 1995).
 
 

Hotspot Activity

To evaluate the area and volume of LIPs, geologists examined modern-day analogues (models) of the gigantic igneous provinces (Anonymous, 1999). The Kerguelen mantle plume is considered responsible for extensive hotspot volcanism in the Indian Ocean basin (Figure 2), including the Ninetyeast Ridge, Broken Ridge, and the Kerguelen Plateau (Hassler and Shimizu, 1998). Lab experiments suggest that a drawn-out tail of hot material should lag behind the spherical head of the plume, yielding a long-lived, focused source of magma (Baski, 1999). Over millions of years, the ocean floor migrates over the hotspot sites due to plate motions - where lava erupts at the surface, gradually it constructs a linear submarine ridge or a sequence of islands and seamounts (Baski, 1999).
 


Conclusions

LIPs provide the only known record of ancient deep earth dynamics according to Mike Coffin (ENN, 1999), who led the most recent ODP expedition at the Kerguelen Plateau, and that is part of the significance of these studies. The effects that the formation of LIPs have on the world is only speculated, but they are thought to be immense. Understanding the formation of the Kerguelen Plateau will have a great impact on the understanding of the break-up that occurred between India, Antarctica, and Australia about 130 million years ago (Bartlett and Wexler, 1999). The strange nature of the formation of LIPs such as the Kerguelen Plateau may lead to insight regarding the nature of the plate-tectonics, volcanism, and other mechanisms that shape the surface of the earth.
 

[WORD COUNT: 1497]