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| Titel |
Columnar jointing pattern: characterization and interpretation |
| VerfasserIn |
György Hetényi, Fanny Garel, Etienne Médard, Benoit Taisne, Hannes B. Mattsson, Sonja Bosshard |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034560
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| Zusammenfassung |
| Columnar Jointing (CJ) is commonly observed in slowly cooling lava bodies. The
characteristic diameter of the columns is thought to depend on the cooling rate,
faster (respectively slower) cooling resulting in larger (smaller) thermo-elastic strain
yielding more slender (stout) columns. One of the goals of our CJ project is to
analyse whether the characteristic diameter of the columns is also dependent on other
factors, such as the chemical composition of the lava and the setting of the cooling
lava body. While the laboratory measurements are finished and the bibliography
compiled, we present our field measurements including photos, methods and first
results.
In order to compare different CJ sites, we develop a procedure to quantitatively
characterize the columns. On the field, we collect photos with a well visible scale, either
perpendicular or parallel to the columns. Then we use a self-developed, interactive software
that processes the photos and performs statistical analyses on the average length of a side (L),
on the average cross-sectional area (A), and on the number of sides of columns (N). Using
these values, the parabolic relation between A and L2 is determined as a function of N, and
compared to ideal polygons. Data from 25 sites across Europe show that the observed areas
are smaller than those of the ideal polygons by about 7%. To be able to use photos taken from
the side, we calculate the ratios of the distance to the 2nd and 3rd vertices to L, again as a
function of N. The averages of these ratios are as those of ideal polygons, but their
standard deviations suggest that the geometry of individual column cross-sections is
anisotropic in a random direction. This explanation also underlines the previous
result of smaller areas compared to ideal, and suggests that for a given area, the
total length of joints is not fully minimized. This is in line with earlier and also
present observations that the average number of sides at any CJ site is less than
6.
Preliminary results show that CJ sizes are not directly dependent on chemical
composition. The range of L at most sites varies between 8±3 cm to 50±15 cm,
although we observed a few sites with column sizes in the order of 1-3 m. The major
element composition measurements on rock samples from site where this information
was previously not available will help to conclude on this question. Composition,
however, may play an important role in an indirect way on the size of CJ. Our field
observations show that CJ occurs in different geological settings (lava flow, lava
lakes, necks and dykes), and that these might strongly influence the size of CJs
by defining the boundary conditions of the cooling lava body. Compilation of our
observations and further reading on visited CJ site geology will allow concluding on this
question. |
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