Cooling columns in the Bishop Tuff
Shall we return now to the volcanic tableland north of Bishop, California?
Yes, let's shall.
Today's topic: cooling columns in the Bishop Tuff. Like all volcanic rocks, the Bishop Tuff erupted hot and cooled off as it "set." This change in temperature led to a change in volume, and the upper welded layer, known affectionally to its friends as "Ig2," lost enough volume and was stiff enough that it developed a set of polygonal fractures, which propagated downward, mostly vertically. This divided up the Ig2 into blocky columns, which now topple over where exposed along normal fault scarps:
View is to the south/southeast. White Mountains in the distance. Simon Kattenhorn for scale.
Here's a Google Map of this locality. It's just north of where that syn-deformational drainage channel flows down a relay ramp into a graben.
Up atop the footwall block, you can see some of these columns separating from one another, opening up zig-zag-shaped fractures as the columns nearest the scarp rotate outward into the graben. The resulting gape gets filled in with sediment, like a rift valley in miniature:
A close look at the columns themselves (next three images along the fault scarp) reveals some of the lovely smaller structures that serve as decoration and fodder for the structural geologist. Consider, for instance, this delectable "crack panel" showing the arrest lines as the fractures which define the column propagated downward a few inches at a time:
A closer look: the arrest lines are ~horizontal on a ~vertical column:
Also, this caught my eye:
I think what we're seeing here is two intersection "hackle fringes" on the corner of a block of Bishop Tuff Ig2. I don't think they're arrest lines, so they don't appear to have anything to do with columnar jointing. The prominent "ruffley" set of hackles on the left appear to have all formed as part of a single episode of jointing. That joint apparently cross-cut an earlier joint which left the less-prominent hackle fringe on the right (hackles at a ~10 degree angle to the first set of hackles). At least I think that's what's going on here... Would anyone else care to offer another "read" on this outcrop?
Yes, let's shall.
Today's topic: cooling columns in the Bishop Tuff. Like all volcanic rocks, the Bishop Tuff erupted hot and cooled off as it "set." This change in temperature led to a change in volume, and the upper welded layer, known affectionally to its friends as "Ig2," lost enough volume and was stiff enough that it developed a set of polygonal fractures, which propagated downward, mostly vertically. This divided up the Ig2 into blocky columns, which now topple over where exposed along normal fault scarps:
View is to the south/southeast. White Mountains in the distance. Simon Kattenhorn for scale.
Here's a Google Map of this locality. It's just north of where that syn-deformational drainage channel flows down a relay ramp into a graben.
Up atop the footwall block, you can see some of these columns separating from one another, opening up zig-zag-shaped fractures as the columns nearest the scarp rotate outward into the graben. The resulting gape gets filled in with sediment, like a rift valley in miniature:
A close look at the columns themselves (next three images along the fault scarp) reveals some of the lovely smaller structures that serve as decoration and fodder for the structural geologist. Consider, for instance, this delectable "crack panel" showing the arrest lines as the fractures which define the column propagated downward a few inches at a time:
A closer look: the arrest lines are ~horizontal on a ~vertical column:
Also, this caught my eye:
I think what we're seeing here is two intersection "hackle fringes" on the corner of a block of Bishop Tuff Ig2. I don't think they're arrest lines, so they don't appear to have anything to do with columnar jointing. The prominent "ruffley" set of hackles on the left appear to have all formed as part of a single episode of jointing. That joint apparently cross-cut an earlier joint which left the less-prominent hackle fringe on the right (hackles at a ~10 degree angle to the first set of hackles). At least I think that's what's going on here... Would anyone else care to offer another "read" on this outcrop?
Labels: california, faults, joints, meetings, structure, travel, volcano, weathering
3 Comments:
This is clearly alien cuneiform script from an extraterrestrail visitation - rough translation: "arrived here thursday, nobody around - Thanksgiving holiday, will try again monday"
(apologies to Stan Freberg)
Beautiful pictures, Callan. I'm not sure the last one is twist hackle as it would imply the entire left joint face grew in one growth increment (as opposed to the pulsed increments that leave the horizontal panels in your other photos) in order to leave hackle all along its margin. In fact, I'm not sure if I've ever seen twist hackle on a cooling joint - possibly due to the nature of the stress field (isotropic). An interesting issue... Also, the left joint can only intersect the right one, not crosscut it. It's always difficult to tell from a photo but another interpretation is that these may be growth increments that were advancing into material still near the glass transition temperature, resulting in blunted crack tips for each increment before growth again advanced (this may explain the somewhat rounded or bulbous look to these features). Whatever they are, they're pretty cool!
Great thoughts, Simon! You're absolutely right that "the left joint can only intersect the right one, not crosscut it." That's me being sloppy.
For the record, I'm not under the impression that the last photo shows a cooling joint. Maybe I should have posted it in a separate post, under a different title. It could be a fracture related to some other stress -- perhaps later fault-inducing stresses, or the tumbling of large blocks down the slope of the scarp. But you're right -- definitely not the cooling-induced tensional stresses that generate columns.
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