Geology Question of the Week

kade04 · « **Reply #40 on:** 03/01/2006 00:33:22 »

Im not sure but I think that is a feature created by the lake that builds up when a glacier blocks a river’s path. As the lake grows it moves up the side of the valley, the banding is created by the water as it erodes the valley wall and represents the shoreline at the time? Be kind im just a student!

DoctorBeaver · « **Reply #41 on:** 03/01/2006 03:16:48 »

Kade - I know nothing about geology, but wouldn't logic dictate that you would not, in that case, get steps? Surely the banks would be smoothed out. Steps would indicate that the water level rose, stayed level for a while, rose again, stayed level for a while and so on? Furthermore, many of the steps appear to be the same height which implies that many rises in the water level were the same. I'm not sure how likely that is to occur naturally, but I would have thought it highly improbable.

(I'm referring to the black & white photo, by the way)

DoctorBeaver · « **Reply #42 on:** 03/01/2006 03:46:14 »

quote:
The scary thing is, only part of the fault rose up and another is expected !!...at any time

Neil - apparently the same is true of the Pakistan earthquake. Some expert on TV the other day was saying that as little 1/10th of the stress has been relieved so far. That's scary too!

Bass · « **Reply #43 on:** 04/01/2006 20:49:10 »

quote:
Originally posted by kade04

Im not sure but I think that is a feature created by the lake that builds up when a glacier blocks a river’s path. As the lake grows it moves up the side of the valley, the banding is created by the water as it erodes the valley wall and represents the shoreline at the time? Be kind im just a student!

Awesome- your answer is right on!
As the level of glacial dam rose, the water level in the glacial lake rises and erodes a shoreline (or strand-line). Apparently the glacier would rise in discreet steps and stay stable for a short period of time, allowing the shorelines to develope. The fact that these lines are almost perfectly horizontal indicates they were formed by water.
There is some debate as to how many times the glacial dam built up and was then breeched, each time causing vast floods that poured through Washington state and formed the "Channel Scablands".
Beginning in the 1920's, geologist J Harlan Bretz argued that many of the landforms in Washington could only be formed by a stupendous flood- but his ideas were spurned by earth science establishment, who didn't believe in catasrtophic geologic events and couldn't envision a source for such enourmous volumes of water. It was years later before Glacial Lake Missoula was proposed as the source of the water- based on the shorelines so obvious in the photo.

Subduction causes orogeny.

Bass · « **Reply #44 on:** 05/01/2006 00:14:16 »

A quick summary and tour of the area:
http://www.glaciallakemissoula.org/story.html
http://www.uwsp.edu/geo/projects/geoweb/participants/dutch/VTrips/Scablands0.HTM

A very readable book

Glacial Lake Missoula and Its Humongous Floods by David D. Alt

Subduction causes orogeny.

DoctorBeaver · « **Reply #45 on:** 05/01/2006 03:08:00 »

Well I think I was sort-of right with what I said. If the level of the water rose in steps, then fair enough

Bass · « **Reply #46 on:** 06/01/2006 00:15:07 »

Sorry DoctorBeaver, didn't mean to ignore your answer- which as you point out is also right on!

Subduction causes orogeny.

kade04 · « **Reply #47 on:** 06/01/2006 00:24:12 »

yeah! happy dance lol! can anyone ask questions?

Bass · « **Reply #48 on:** 06/01/2006 05:21:08 »

Go for it Kade04

Subduction causes orogeny.

Bass · « **Reply #49 on:** 09/01/2006 18:13:58 »

Maybe this belongs in the "Science Photo of the Week" thread, but the reference to cross-beds in this thread made it relevant.

This photo from the Mars Rover Opportunity near the edge of Erebus Crater shows good evidence for the historical presence of water on Mars- note the "festoons" or cross-laminations (curved upward layers) that indicate water-formed ripples.
http://www.spaceref.com/news/viewsr.html?pid=19212

Subduction causes orogeny.

Bass · « **Reply #50 on:** 13/01/2006 05:08:05 »

Rocks can develop planar features for several reasons- the most common are bedding planes. The next most common is pictured here- in the top photo, bedding is obvious and aligned with the blue pencil, the second planar feature cuts across the bedding and is aligned with the black pencil. In the bottom photo, bedding is the crinkled colored layers while the second planar feature is almost vertical (aligned with knife):

GQOTW: Name this planar feature?

Bonus: How does this planar feature form? What does it tell us about the history of the rock?

Subduction causes orogeny.

Bass · « **Reply #51 on:** 22/01/2006 01:46:06 »

Hint:

Wonderbra -
what most men first notice about a woman wearing a wonderbra.

Subduction causes orogeny.

Bass · « **Reply #52 on:** 17/02/2006 22:33:21 »

Cleavage!

More commonly called "rock cleavage" or more properly called either "axial planar cleavage" or "foliation".

Occurs in metamorphic rocks that have been subjected to pressure. The pressure and heat of metamorphism change the clay minerals in sedimentary rocks into mica (several varieties). Mica has a distict planar orientation, and it grows in the direction of least pressure- which will be perpendicular to the stress field.

Commonly, bedding planes will fold during metamorphism, also perpendicular to the greatest stress- so the foliation ends up being in the axial plane of the fold, hence "axial planar cleavage". The amount and orientation of foliation allows the observer to figure out the relative intensity of metamorphism and gives clues as to the true orientation of the original beds.

Subduction causes orogeny.

Ray hinton · « **Reply #53 on:** 24/02/2006 12:15:18 »

quote:
Wonderbra -
what most men first notice about a woman wearing a wonderbra.

their not very well blessed ?

RE-HAB IS FOR QUITTERS.

JimBob · « **Reply #54 on:** 05/03/2006 01:53:30 »

quote:
Originally posted by Bass

Maybe this belongs in the "Science Photo of the Week" thread, but the reference to cross-beds in this thread made it relevant.

This photo from the Mars Rover Opportunity near the edge of Erebus Crater shows good evidence for the historical presence of water on Mars- note the "festoons" or cross-laminations (curved upward layers) that indicate water-formed ripples.
http://www.spaceref.com/news/viewsr.html?pid=19212

Subduction causes orogeny.

Bass, may I politely disagree. I concur that this is evidence for water on Mars, but the structures to me look to be a series of seasonal dessication cracks that are stacked one upon another. To bad we can't get to the outcrop to determine the accuracy of the interpretation.

And orogeny causes uplift

Ophiolite · « **Reply #55 on:** 05/03/2006 02:29:56 »

Interesting. I am seeing a cross section through a finely laminated sequence; no cross bedding; no dessication cracks. It is good to know the art of geological equivocation is still alive and well.

JimBob · « **Reply #56 on:** 06/03/2006 01:22:46 »

Alright Ophialite (rock or snake?).

The structure just off of the end of the arrow is not a cleavage related feature. It lacks one side of being a closed square. This suggest dessication to me. There are traces of the same type of feature and one whith a chip "wedged" between the to sides of a upturned bed. But I am not going to make any more of this as we will never know for sure.

What bothers me more is the origin of the pellitoidal thingies that are all over the place. Are they desert, wind-generated concreations? Can you think of another explinaiton? Weatherd out of another rock? Or other?

I am open to all as I know the laminated are faily well explained.

Jim

Ophiolite · « **Reply #57 on:** 06/03/2006 02:57:03 »

[Ophiolite - ocean rocks welded to continental masses by subduction]
If you will imagine swinging the arrow around in a clockwise direction, till it is vertical, you will find it ends in a narrow column of rock, replete with laminations, slightly displaced from the adjacent blocks. I see post depositional collapse structures. Nothing more. However, you are correct: from this single photograph we are unlikely to reach a firm conclusion.

The small spherules I took to be blueberries, which were observed early on by both (?) rovers. These may be formed as concretion within the rock as a result of groundwater activity, then released by normal erosional processes. For example: http://www.spaceflightnow.com/news/n0406/16blueberries/

JimBob · « **Reply #58 on:** 06/03/2006 17:12:43 »

I can go along with collapse structurs. Also, the hematite concreation hypothisis was first on my list. I dared not hope for oolites eroded from a less dense carbonate. And I was taught that ophiolte was a mostly mafic igneous rock with a little metamophic thrown in that is metamorphosed in a subduction that probably failed - example: the highlands of Cuba. I am a rock pounder as well, but of a much ealier vintage.

Good to know I am not alone here.

Jim

Bass · « **Reply #59 on:** 08/03/2006 20:52:35 »

Since JimBob brought it up-

GQOTW: What are oolites and how do they form?

Subduction causes orogeny.

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