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Narumi Takahashi talks with
ScienceWatch.com and answers a few questions about
this month's Emerging Research Front Paper in the field of
Geosciences.
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Article: Crustal structure and evolution of the Mariana
intra-oceanic island arc
Authors: Takahashi, N;Kodaira, S;Klemperer,
SL;Tatsumi, Y;Kaneda, Y;Suyehiro, K
Journal: GEOLOGY, 35 (3): 203-206 MAR 2007
Addresses: Japan Agcy Marine Earth Sci & Technol, Kanazawa
Ku, 3173-25 Showa Machi, Yokohama, Kanagawa 2360001,
Japan.
Japan Agcy Marine Earth Sci & Technol, Kanazawa Ku,
Yokohama, Kanagawa 2360001, Japan.
Stanford Univ, Dept Geophys, Stanford, CA 94305 USA.
Japan Agcy Marine Earth Sci & Technol, Yokosuka, Kanagawa
2370061, Japan.
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Why do you think your paper is highly
cited?
The main reason is probably that the scenario of the crustal growth shown
in this paper is quantitative and simple. Therefore, it is also easy to
cite.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
In this paper, we discussed how crusts grow and how continental materials
are generated as a target of crustal structures of the Izu and Mariana arcs
in combination with seismology and petrology.
As the result of this study, we found that the Mariana arc has continental
materials inside, despite the fact that it has evolved through a process of
subduction between two oceanic crusts without having had any continental
materials, the same as the Izu arc. In addition, the mantle velocities are
commonly slow when compared with a typical mantle.
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(Top) Fig. 1 : Crustal structure cross the Mariana arc in
direction of E-W (after Takahashi et al., 2007). No shaded
area is resolved area in this study. Numerals are P
wavespeed (km/s).
(Bottom) Fig. 2: Simple schematic figure of the crustal
growth model.
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It is well known that an oceanic arc such as the Mariana arc contains
various types of rocks from heavy dense materials like basalts to lighter
materials like ryolites. Using the diversities of rocks and crust/mantle
structures obtained by this study, we constructed a crustal growth scenario
to make continental materials and concluded that heavy crustal materials,
which are differentiated by the generation of light continental materials,
are transformed to the mantle, changing the total crustal density from
heavy to light.
The structural commonalities between the Mariana and Izu arcs are a new
discovery and the construction of the crustal growth model is the
synthesis.
Would you summarize the significance of your paper
in layman's term?
Generally speaking, there may be some individuals who believe that the
ground is fixed and unchangeable, whereas the ground is constantly evolving
through geologic periods as part of the evolution of the planet. This paper
is one of the papers which outline how continents have evolved. Without
geologic change, there could have been no possibility of human life on
Earth.
How did you become involved in this research and
were any particular problems encountered along the way?
The theme of my doctoral thesis was the study of a typical oceanic crust.
The area of concentration was the Izu-Bonin region. However, the cost of
conducting such seismic surveys is quite high, requiring large sources and
many observatories. Therefore, the study had been stopped for a long time.
The initiative of the 2003 Mariana seismic surveys, a collaboration between
the US and Japan, was begun with the US-Japan Nankai Trough 3-D seismic
reflection survey in 1999. It is important to note that such studies on
crustal growth are one of the scientific targets of the
IODP (Integrate Ocean Drilling Program).
Where do you see your research leading in the
future?
The crustal growth process as examined by this study includes many
ambiguous elements. For example, it has remained uncertain how much water
content is included in subducting oceanic crust, although it is an
important element in the discussion of magma generation.
As it is unknown what kind of role is played by the dense crustal materials
transported in the mantle, I would like to help advance such crustal growth
studies.
Do you foresee any social or political
implications for your research?
The study of crustal growth is still in quite a fundamental stage. I hope
that the crustal growth scenario could be part of a universal one outlining
the evolving process which resulted in the making of continents after the
birth of our planet.
Narumi Takahashi, Ph.D.
Senior Scientist
Institute for Research on Earth Evolution (IFREE)
Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
Yokohama, Kanagawa, Japan
Web
KEYWORDS: seismic structure; intra-oceanic island arc; crustal growth;
Izu-Bonin-Mariana arc; IZU-BONIN-MARIANA; CONTINENTAL-CRUST; CENTRAL JAPAN;
ORIGIN; BASIN; MANTLE.
