Docstoc

Plate Tectonics and the El Nino Cycle

Document Sample
Plate Tectonics and the El Nino Cycle Powered By Docstoc
					                  Plate Tectonics and the El Nino Cycle
                                     Maria Gausman
                                        GLG 501
                                      April 26, 2003


There have been some publications in Eos, 1988, that address the possibility of tectonic
activity (earthquakes and magmatic heat) in the Pacific Ocean as a mechanism for the
synthesis of an El Nino event. Data was gathered on the frequency of earthquakes,
volcanic eruptions, tsunamis, and the Southern Oscillation Index (SOI) to find a
relationship between these events and El Nino occurrences. Graphing and correlating the
various tectonic activity data with the SOI, has shown no relationship between the events.

Introduction

         El Nino events, and the often-catastrophic climate changes that they bring with
them, have been a source for scientific and social exploration. Finding the mechanisms
that set El Nino in motion would make the phenomenon more predictable, which is a goal
for many scientific disciplines.
         In 1988, three scientists had proposed that activity of the crustal plates that are
found on the Pacific Ocean floor could be linked to El Nino.
         Daniel Walker, University of Hawaii, studied earthquake activity on the East
Pacific Rise (EPR), specifically at 20-40oS and 100-120oW. He gathered data on the
number, size, and distribution in time and space of earthquakes provided by the
International Seismological Centre of Berkshire, England. He chose to examine data
from 1964, when the World Wide Network of Standard Seismographs (WWNSS) was
implemented, to 1987. Walker converted earthquake magnitudes to energy values, and
divided those calculations by energy per month, number of earthquakes, and relative
strain release from 1964 to 1987. He then plotted relative strain release variation with the
SOI. Walker found that for five El Nino events in the study duration (1965, 1972-73,
1976-77, 1982-83, and 1986-87) five of the highest relative strain release variation values
occurred either just prior or at the beginning of the El Nino (Walker, 1988).
         Herbert Shaw and James Moore, U.S. Geological Survey, studied magmatic heat
input from the Pacific Ocean floor, and whether the input is substantial enough to create
temperature anomalies such as those observed during El Nino. They proposed a model
that states that general thermal circulation of the mixed layer in the equatorial Pacific is
periodically altered by thermal inputs from magmatic pulses. The pulses are expected to
correlate with El Nino occurrences. For this model, they used observations of fresh lava
flows recorded by GLORIA and SeaMarc side-looking sonar, determined size and
frequency of large flows, rate of emplacement of flood basalt flows, cooling rates of
magma, fluctuations in total magmatic power of oceanic spreading ridges (such as the
EPR), and the rise of heated water in the ocean. According to Shaw and Moore, thermal
input from the ocean floor can account for 10% of the heat needed to produce a
temperature anomaly (Shaw & Moore, 1988).
         Tectonic activity as a causal mechanism for El Nino events has not been a popular
hypothesis. Walker has been criticized harshly by the scientific community for linking
the two natural phenomenons (Shrimp News, 2003). Still, some efforts are being put
forth to support this hypothesis.

Data

        To see any relationships between tectonic activity and El Nino events, data on the
frequency over time of several types of activity produced from tectonic forces in the
Pacific Ocean was gathered. These activities include earthquakes, tsunamis, volcanic
eruptions, and submarine volcanic eruptions.
        All records are from and around the Pacific Ocean. Earthquake data spans from
1900 to 1994, and only includes quakes with a magnitude of 7 or greater on the Richter
scale (NEIC, 2003). Tsunami data spans from 1900 to 2002 (NGDC, 2003). Volcanic
eruptions (1967-2002) are activities such as volcanic gases venting to major pyroclastic
explosions (Smithsonian Institute, 2003). Submarine volcanic eruptions records are from
1900 to 1959 (NGDC, 1986). This data was then compared with the Southern Oscillation
Index (SOI) from 1900 to 2002, a good indicator of El Nino events (Commonwealth
Bureau of Meteorology, 2003).
        Figure 1 is a graph of all the above events and the SOI. The yellow bars indicate
El Nino events (Caviedes, 2001, Walker, 1988, Environmental News Network, 2003).


                             45
                             40
                                                                 1913-1915 El Nino

                                                                                     1918-1920 El Nino

                                                                                                         1925-1926 El Nino


                                                                                                                                1932-1933 El Nino




                                                                                                                                                                                                1957-1958 El Nino
                                      1901-1902 El Nino




                                                                                                                                                     1940-1941 El Nino




                                                                                                                                                                            1953-1954 El Nino




                                                                                                                                                                                                                     1965-1966 El Nino

                                                                                                                                                                                                                                         1972-1973 El Nino

                                                                                                                                                                                                                                                             1976-1977 El Nino

                                                                                                                                                                                                                                                                                 1982-1983 El Nino

                                                                                                                                                                                                                                                                                                     1986-1987 El Nino

                                                                                                                                                                                                                                                                                                                         1991-1995 El Nino

                                                                                                                                                                                                                                                                                                                                               1996-1997 El Nino
                             35
                             30
  Number of Events and SOI




                             25
                             20
                             15
                             10
                              5
                              0
                              -5
                             -10
                             -15
                             -20
                             -25
                               1900                       1910                       1920                                    1930                   1940                 1950                   1960                                1970                                 1980                                1990                                   2000
                                                                                                                                                                         Years

                                   Number of                                                        Volcanic                                              SOI Average                                               Submarine Volcanic                                                                                                       Number of
                                   Tsunamis                                                         Eruptions                                                                                                       Eruptions                                                                                                                Earthquakes


Figure 1: Tectonic events and SOI from 1900-2002
Although the above graph shows that for several El Ninos, an increase of tectonic activity
is observed just before it, but usually the increase is pronounced in only one type of
activity (example, tsunami events and El Ninos 1940-41 and 1972-73). The same can be
said for just after El Nino (earthquakes and El Nino 1965-66, and tsunamis and El Nino
1991-95).
        Data analysis was done by correlating high frequency years for the individual
tectonic activities and the years just preceding an El Nino year. If tectonic events is the
mechanism that sets El Nino in motion, than increases in some or all of these events has
to happen just before the El Nino. For correlating the data, the years before El Nino
years are assigned a 1 and all other years are assigned a 0. The events were also
correlated to the SOI. Figure 2 summarizes the correlation coefficients found by these
methods.

                        Earthquakes           Tsunamis       Submarine          Volcanic
                                                             Eruptions          Eruptions
     SOI                   -0.005              0.089           0.108             -0.380
 Years prior to
                            0.112              0.112           -0.086            -0.058
 El Nino years
Figure 2: Table of correlation coefficients


Discussions and Conclusions

         The data gathered for this study and the analytical methods used to interpret the
data did not show any support for the hypothesis that tectonic activity triggers El Nino
events. The graph of the data shows that there is little relationship between increased
activities, such as earthquakes, tsunamis, volcanic explosions, and submarine volcanic
explosions, and negative SOI values. Correlating the activities to the SOI and to the
years prior to El Nino years also showed no relationship between the data. According to
this evidence, the conclusion is that tectonic activity plays no role in causing El Nino.

        Despite the conclusion, there needs to be some discussion around the data and that
perhaps this hypothesis should continue to draw interest from the scientific community.
        Walker only used data of earthquake occurrences from 1964 to 1987 because
before 1964, although the data was available, the reliability of it is questionable.
Earthquake data in this paper spans back to 1900 (Walker, 1988). Walker’s reasoning for
not including data older than 1964 is valid, yet only including data from 1964 to the
present generates a small list of observations. Finding earthquake records that included
quakes less than a magnitude of 7 on the Richter scale for an area as large as the Pacific
Ocean was not done for this study. Quantity verses quality is an issue with data on El
Nino, and the robustness of the data analysis done for this study may indicate that.
        Also, volcanic data for this study included a wide range of volcanic activities.
Besides pyroclastic explosions, activities such as gaseous venting, seismicity related to
the volcano, and ash plumes were included. Some activity from a volcano that lasts over
a year was counted as a separate event for the following year.
        When and how long El Nino events happen vary with varying sources. For this
study, three different sources were used to determine when in the past century El Ninos
happened and what their durations were. All three sources had differing information.
        In order to support the hypothesis of tectonic activity causing El Nino, frequency
data for tectonic activity may not provide enough information. For submarine volcanic
activity, monitoring data and research has only recently began to produce information
that can be tapped for researching effects on the ocean environment. Only since 1998,
the Pacific Marine Environmental Laboratory (a division of NOAA) has been monitoring
volcanic activity on the Juan De Fuca ridge, located approximately 250 miles from the
shores of Oregon and Washington State (NOAA Vents Program, 2003). Programs like
this will not only log frequency data, but perhaps more importantly, the size and
distribution of magmatic eruptions on the ocean floor. Shaw and Moore, in 1988,
calculated the amount of heat energy needed to generate a sea surface temperature
anomaly indicative of El Nino. Knowing how much fresh lava is introduced through
ridges and fissures on the ocean floor, and do the greatest quantity of fresh lava erupt
prior to El Nino may lend credence to both Shaw and Moore’s model and to this
hypothesis. Frequency data on earthquakes, or the eruption data gathered for this study
does not reflect the volume of fresh magma reaching the ocean floor.

References

Catalog of Submarine Volcanoes and Hydrological Phenomena Associated with Volcanic
Events: January 1, 1900 to December 31, 1959, National Geophysical Data Center,
Report SE-42, October 1986

Commonwealth Bureau of Meteorology,
http://www.bom.gov.au/climate/current/soi2.shtml, 2003

El Nino in History: Storming Through the Ages, Cesar N. Caviedes, University Press of
Florida, 2001

Environmental News Network, http://www.coaps.fsu.edu/lib/enso_sites.html, 2003

National Earthquake Information Center (NEIC),
http://neic/usgs/gov/neis/epic/epic_circ.html, 2003

National Geophysical Data Center (NGDC),
http://www.ngdc.noaa.gov/seg/hazard/tsevsrch_idb.shtml, 2003

NOAA Vents Program, http://pmel.noaa.gov/vents/nemo/realtime/index.html, 2003

Shaw, H. R., Moore, J.G., Magmatic Heat and the El Nino Cycle, Eos, vol 69, No. 45,
November 8, 1988

Shrimp News, http://members.aol.com/brosenberr/Home.html, 2003

Smithsonian Volcanism Program, http://www.volcano.si.edu/, 2003

Walker, D.A., Seismicity of the East Pacific Rise: Correlations with the Southern
Oscillation Index? Eos, vol. 69, No. 38, September 20, 1988

				
DOCUMENT INFO