The National Science Foundation's (NSF) Tokyo Office periodically receives and disseminates reports on research developments in Japan that are related to the Foundation's mission. NSF-sponsored researchers currently working in Japan prepare many of these reports. These reports present information for use by NSF program managers and policy makers; they are not statements of NSF policy.
Special Scientific Report #00-04 (September 01, 2000)
Mr.
Lowell G. Kessel, a graduate student in Geological Sciences, University of California,
Santa Barbara, prepared the following report. Mr. Kessel is a
participant in the 2000 Summer
Institute sponsored in the United States by NSF/NIH/USDA and the Science
and Technology Agency and Japan Science and Technology Corporation in Japan.
Dr. Kazuhiko Kano of Geology Department at Geological
Survey of Japan in Tsukuba, hosted Mr. Kessel. Mr.
Kessel can be reached via email at: kessel@magic.ucsb.edu
1.0 Major Research Activities
The
goal of subaqueous volcanism, a subdiscipline of volcanology that has gained
recognition in recent years, is to gain an understanding of the roles of
volcanoes in the development of island arcs, massive sulfide ore deposits (i.e.,
Au, Ag, Cu, Pb), and volcanic hazards at sea. More than seventy percent of the
Earth’s surface is below sea level, and seafloor exploration has revealed that
there are more volcanic eruptions (e.g., silicic to maffic) occurring at the
bottom of the sea than on land. The effects of ambient water and hydrostatic
pressure on silicic volcanic eruptions in subaqueous settings are not clearly
understood. This is because deep marine silicic eruptions are not directly
observed, and observations of recent deep-water eruption deposits are incomplete
and rare. For this reason, most of our knowledge of deep-water silicic volcanic
processes comes from inferences based on ancient successions.
Subaqueous
volcanism has occurred in Japan in the geological past and continues to occur
south of Honshu, the largest of Japan's four principal islands.
Fortunately, four subaqueous volcanic deposits that have been documented
are located throughout Japan and represent significant deposits to study.
I
have visited these four field locations in Japan with volcanic rocks interpreted
to have been deposited subaqueously. Many of these deposits are of Miocene age
(~15Ma). The goal of this field study was to examine the relationships between
subaqueous volcanic deposits and the rocks underlying and overlying the volcanic
deposits and to find unique characteristics of subaqueous volcanic deposits that
can be used in future studies. Volcanic deposits, whether of subaerial or
submarine origin, are characterized by their distinguishable characteristics
such as composition, mode of eruption and transport, textural characteristics,
and relationships to neighboring rock units and their characteristics.
2.0 Principal Research Findings
The
areas I visited were, from north to south, (1) Akita, (2) Tokiwa, (3) Dogashima,
and (4) Matsue. All areas are known to have volcanic deposits emplaced in the
subaqueous environment. I looked at the stratigraphic relationships and textural
characteristics such as grading, sorting, and grain morphology. The following
are observations made during my field study in Japan.
2.1 Some characteristics of subaqueous silicic
pyroclastic flows
All
subaqueous volcanic deposits are associated with subaqueous sedimentary
deposits. These deposits range from near shore, offshore, and abyssal mudstone
deposits. Unfortunately, paleo-depth constraints for sedimentary strata are poor
and sometimes subject to contrasting interpretation. Therefore depth of
emplacement can be inferred with minor control on depth of water. Paleo-depth of
environment can be strongly supported, however, by fossil evidence such as
brachiopods, mollusks, foraminifera, chorals, or fish, contained within the
sediments.
Grading
characteristics of pyroclastic flows vary a lot in subaerial and subaqueous
deposits. It is commonly expected that water fluidized volcaniclastic flows
become normally graded in terms of all components except for large pumice blocks
which are buoyant and settle down to form large pumice layers. Unfortunately,
such phenomenon is commonly recognized in subaerial ignimbrite (pumice rich
pyroclastic flows) deposits. Therefore, this characteristic does not provide
clear evidence for the interpretation of the fluidizing agent (i.e., hot gas or
water) and but can be used in conjunction with other criteria.
Sorting
characteristics can be used to infer subaqueous eruption or emplacement of
silicic pyroclastic deposits. Large pumice blocks rise for a longer period
(i.e., minutes to hours) compared to smaller pumice fragments because of gases
trapped within vesicles and the very fine ash size fragments may become
entrained into the rising plume of heated water and gas because of the low
density and small weight. Thus, subaqueous silicic pyroclastic eruptions may be
depleted in the course size fraction as well as the very fine ash size fraction
based on buoyancy considerations of the material in a water medium. These
sorting characteristics may be significant in interpretations of the style of
eruption (if subaqueous) and emplacement mechanism.
Textural
characteristics such as grain morphology and grain size abundances can provide
some insight into the processes controlling the eruption style or transport/flow
properties (whether turbulent or laminar). There are many textures and
terminology to discuss and will not be discussed here.
My
conclusions obtained from my field work in Japan are that clear unambiguous
evidence for eruption and/or emplacement of pyroclastic flows remains to be
determined from examination of these deposits although inferential evidence such
as overlying or underlying sediments with fossils and textural characteristics
such as grading, sorting, and grain morphology combined can be used to identify
and document ancient subaqueous volcanic deposits as has previously been done.
Further
research will be conducted at the University of California, Santa Barbara (UCSB).
Samples collected in the field while in Japan have been sent to UCSB geological
sciences department where they can be analyzed by thin section for grain
morphology and grain size distribution. These textural features may provide
further information about the style of eruption and/or flow characteristics of
the deposits studied in Japan.
3.0 Research training
Visiting
the field areas described above has provided me the opportunity to examine some
well known subaqueous volcanic deposits in the greater island of Honshu, Japan.
Examination of the textural characteristics and stratigraphic relationships of
the rocks in the field was very beneficial to me in many ways. It (1) provided
me field experience in locating exposures in a highly vegetated region, (2)
allowed for sample collection, and (3) permitted comparison of personal
interpretation with previous interpretations and verification of previously
proposed models.
I
also learned to cooperate with foreign agencies and travel in a foreign country
as a researcher. This is very important and appreciated.
4.0 Contributions. This research experience in Japan has benefited me in ways
mentioned the previous sections. The knowledge and experience gained from my
research in Japan will be incorporated into ongoing and future studies upon my
return to the United States. My current research is directly related to the
research undertaken in Japan and will benefit as a result of my time working
with Dr. Kazuhiko Kano.