Irregular angular rock surfaces at Spirit site

"will e" wrote in message
...
Michael, A joint is a fracture or crack in a rock without observable
relative movement between the sides of the crack. Shrinkage joints are
caused by tensional forces set up in a rock body as a result of cooling ( in
igneous rocks this may be columnar, like those at Devil's Tower) or by
desiccation in sedimentary rocks. Sheet joints are more or less parallel to
intrusive bodies such as granite and other plutons. Tectonic joints are a
direct result of folding or thrusting of rocks and can form complex sets of
cracks depending on the rock type and resolution of forces. Joints caused by
meteorite impact radiate in a cone shape out from the point of impact. To
answer your original question: Sets of joints intersect, some at 90 degrees
and others oblique (less than 45 deg.) The result of jointing gives rocks,
clasts, of all sizes their overall shape. Once they reach the surface, then
the effects of wind, ice, water, thermal changes,biological activity, become
important to either reshape the rock or to break it down further. Hope this
helps, Will Estavillo


Good answer.

George wrote:

"Pedro Rosa" wrote in message
om...

" George" wrote in message

. ..

"will e" wrote in message
...

Hi Michael, Look up "dreikanter". These are ventifacts, rocks faceted by
wind (and sand). Common in many of earth's dry locations. (deserts and
glacial). Will Estavillo

That may be true to an extent. However, as this material appears from

analysis

to be basalt, it is not unusual for it to fracture in angular patterns.

I've

also been looking to see if any of the rocks show evidence of columnar

jointing.

So far I haven't seen any.

IMHO that would happen mostly in cones or dickes. Now Mars is known
for its shield volcanoes. Mars seems to have had a very fluid magma.
So columnar jointing may be hard to find...


Unless it was quentched rapidly, say, perhaps in a body of liquid water?



Hey George,
I've just written an article on columnar jointing for outdoor
people. Everywhere I looked, multiple sources mentioned slow uniform
cooling. Not one said anything about columnar jointing through
quenching. Having personally quenched red hot quartz in water, it tends
to fracture irregularly/explode. fracture internally--not form anything
which is even reasonably regular. And there are the blobby pillow
basalts on earth...
Got some cites for columnar jointing through quenching?
Jo

"Jo Schaper" wrote in message
...
George wrote:

"Pedro Rosa" wrote in message
om...

" George" wrote in message

. ..

"will e" wrote in message
...

Hi Michael, Look up "dreikanter". These are ventifacts, rocks faceted by
wind (and sand). Common in many of earth's dry locations. (deserts and
glacial). Will Estavillo

That may be true to an extent. However, as this material appears from

analysis

to be basalt, it is not unusual for it to fracture in angular patterns.

I've

also been looking to see if any of the rocks show evidence of columnar

jointing.

So far I haven't seen any.

IMHO that would happen mostly in cones or dickes. Now Mars is known
for its shield volcanoes. Mars seems to have had a very fluid magma.
So columnar jointing may be hard to find...


Unless it was quentched rapidly, say, perhaps in a body of liquid water?



Hey George,
I've just written an article on columnar jointing for outdoor
people. Everywhere I looked, multiple sources mentioned slow uniform
cooling. Not one said anything about columnar jointing through
quenching. Having personally quenched red hot quartz in water, it tends
to fracture irregularly/explode. fracture internally--not form anything
which is even reasonably regular. And there are the blobby pillow
basalts on earth...
Got some cites for columnar jointing through quenching?
Jo

You may be right. It was only a suggestion. But I would think that it doesn't
cool too slowly or it would have a more coarse matrix, would it not? We don't
have much basalt in Kentucky. In fact, we don't have any. But I understand
that pillow basalts, having been quenched in water, does cool rather rapidly.