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
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.
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
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).
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).
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.
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