Comments on: Basalt of Sibley Volcano

By: Darryl Matthew Andrews

Darryl Matthew Andrews — Thu, 22 Mar 2012 23:14:43 +0000

A majority of the ppl in the bay area don’t know about this. I learned this in my Geology course at CC that this volcano is 10 million years old and it has been sealed for years. Amazing, but scary

By: SteveN

SteveN — Fri, 15 May 2009 23:35:44 +0000

And the jointing facilitated significant groundwater intrusion, as evidenced by the numerous quartz-filled amygules one finds in the “Sibley basalt” vesicles toward the northern extent of the park trails

By: Holly

Holly — Mon, 06 Apr 2009 16:26:24 +0000

Oh, that makes sense! Great explanation. Thanks.

By: Andrew

Andrew — Fri, 03 Apr 2009 00:55:28 +0000

Holly, it’s the same reason that a square bar of soap turns round the more you use it: the edges of the block are attacked from two sides, the corners from three sides, the faces from just one side.

By: Holly

Holly — Thu, 02 Apr 2009 23:18:23 +0000

Hi Andrew,
I don’t have a lot of experience with geology, so forgive me if my question is naive. Does the weathering cause the spheroidal shape all on its own, or is that due to some internal structure of the basalt? Maybe it was a pillow lava, and it formed in rounded layers? Or are the joints really just naturally round, even if the rock has no round characteristics? That is so weird. I wonder why that happens.
-Holly