Columnar

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Columnar Joints
Giants Causeway, Ireland
http//www.ivanweb.net/images/Mondo/Irlanda/Giants
20Causeway,06.jpg
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Giants Causeway, Ireland
http//www.gaschurman.com/world/europe/northern20
ireland/giant20causeway/slides/giant20causeway2
013.jpg
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Mud cracks illustrate, in 2-d, the process of
volume contraction. As the mud dries, its clay
minerals contract, eventually pulling away from
one another when the contraction stress exceeds
the mud strength.
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In a lava flow, extend the mud crack process into
the third dimension, and you have columnar joints
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In a side view, you dont see the polygonal
pattern as well, but you do see the columnar
nature of the columns
1m
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Here, on the S. flank of E. Maui volcano, the
tops of the columns are exposed in a gully. The
surface is not horizontal because the flow
itself was emplaced on a slope the columns are
almost always perpendicular to the cooling
surface.
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Kepuni Gulch, East Maui
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Onomea, Hawaii
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Here are the tops of some 2 million year-old
columns above Makapuu, Oahu. Notice that
weathering has preferentially attacked the
fractures - eventually these will become
individual boulders.
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The next geologic event was cutting of the gully.
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Dikes also show columnar jointing, but because
dikes are essentially vertical (and therefore
have vertical cooling surfaces), their columnar
joints are horizontal. It is these pre-broken
fragments of dense dike rock that were often
made into adzes by Polynesians.
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A dike cutting diagonally across lava flows on
Kauai
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Devils tower (Wyoming), a remnant of a thick
lava flow or dome, is probably the most famous
example of columnar jointed rock in the USA.
Photos by Steve Mattox, from
http//volcano.und.edu/vwdocs/volc_images/north_a
merica/devils_tower.html