Basalt formations
co...@vlc.servicom.es
Hello to everybody in sci.geo.geology!
I just got back from my holidays in Scotland, and while I was
there I went to visit the isle of Staffa (where the famous
Fingles Cave is).
The entire island is formed of basalt columns, and is really
amazing. It is like an ordered spaghetti of rocks, all perfectly
fitted together.
How does such a thing form? The best the guide could do was
"volcanic activity" which doesn't help much when faced
with the amazing formations on the island.
Can anybody explain this? I'm not a geologist but don't
be afraid to get technical...
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Neil Dickey
co...@vlc.servicom.es wrote:
> Hello to everybody in sci.geo.geology!
>
> I just got back from my holidays in Scotland, and while I was
> there I went to visit the isle of Staffa (where the famous
> Fingles Cave is).
>
> The entire island is formed of basalt columns, and is really
> amazing. It is like an ordered spaghetti of rocks, all perfectly
> fitted together.
>
> How does such a thing form? The best the guide could do was
> "volcanic activity" which doesn't help much when faced
> with the amazing formations on the island.
>
> Can anybody explain this? I'm not a geologist but don't
> be afraid to get technical...
What you saw is called "columnar jointing." A joint is a crack in a
rock, accross which there has been essentially no movement. (As
differentiated from a fault, accross which there *has* been movement.)
Columnar joint sets arise during cooling of an igneous intrusion or
flow, commonly basalt, and are generally oriented normal to the cooling
surface or contact. Columnar joint sets may be straight or curved.
This is from Cas & Wright, "Volcanic Successionsi," 1987, p. 71:
"Flood basalts form extensive sheets of lava with very low aspect
ratios. . . . . Many of the larger flows of this type must have
ponded as vast lava lakes, taking years to tens of years to solidify,
as indicated by the well developed massive comumnar jointing."
"Cooling is accompanied by contraction, and takes place from the
cooling surfaces (principally the top and bottom of the flow) inwards.
The tensional stresses set up during contraction may produce regular
joint sets perpendicular to the cooling surfaces, and usually vertical
to sub-vertical in orientation. Well defined intersecting joint sets
may produce regular polygonal columns. The joint faces (and columns)
propagate inwards from the cooling surfaces as the 'cooling front'
advances inwards. This progressive propagation may be reflected by
complementary sub-horizontal joints within columns, or by a
segmentation pattern on the vertical joints, reflecting successive
propagation stages."
I note that columnar jointing has also been observed in dikes and
sills.
--
Best regards,
Neil Dickey
http://jove.geol.niu.edu/faculty/dickey/dnd.html
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