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Author Topic: Geology Question of the Week  (Read 166896 times)

Offline Bass

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Re: Geology Question of the Week
« Reply #25 on: 30/11/2005 04:43:43 »
Neil- you’re on the right track.
This conundrum even stumps the folks that study plate tectonics (plate tectonicists?).

An initial short length, a bit of lengthening, and lots of shallow thrusting, is the best hypothesis put forward to date (quit panting Neil).

1.   Initial short length:  When the Rockies began forming, in
the Laramide orogeny (~75 to 55 million years ago), the coast was
several hundred kilometers closer.  The Farallon plate was being
subducted beneath North America close to the California-Nevada border.
2.   A bit of lengthening:  The Great Basin has been undergoing
extension of at least 2 cm/yr for the last 16 to 20 million years
(300-400 km)- so the subduction zone was even closer yet.  This is
probably due, in part, to the NA plate over-running the Farallon
spreading ridge.
3.   Lots of shallow thrusting:  The oceanic crust (Farallon
plate), being thinner and heavier than the continental plate, was
subducted beneath the NA plate- leading to compression and sliding
stacks of rocks along “thrust” faults long distances to the east.  
Apparently, instead of sinking into the mantle at the typical angle
of around 45 degrees, the Farallon plate stayed almost horizontal,
this stacked up thrust zones much farther to the east than the
norm.  Volcanism started around 50 million years ago, which is
additional evidence of a descending slab beneath the Rockies.  
Present uplift of the entire Rocky Mountain region indicates that
tectonic processes are still active- i.e. that the slab is still
scraping the bottom of the crust.

Here is a normal subduction zone


Compared to  horizontal slab subduction:

If you’ve ever been in a room with two or more geologists, you’d
soon discover that they can’t agree on much of anything.  In tune
with our disagreeable nature- there are several other hypotheses.
(BTW- Only a small piece of the Farallon plate exists today- called the Juan de Fuca plate off the coast of Oregon, Washington and British Columbia- the rest has been over-run by North America).

Subduction causes orogeny.
« Last Edit: 30/11/2005 04:52:05 by Bass »
 

Offline neilep

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Re: Geology Question of the Week
« Reply #26 on: 01/12/2005 03:42:39 »
Skip, you're very generous to say that I was on the right track, of course your explanation was going to be my next guess !!:D

....Thank you for the explanation. It's all very facinating stuff.

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Offline James Bowkett

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Re: Geology Question of the Week
« Reply #27 on: 01/12/2005 10:19:46 »
I'd agree with the theory of reduction spots. As for animal tracks, if you head to Almeria in Andalucia it is possible to examine large sedimentary basins and there is evidence of the Messianian salinity crisis, proof of which can be round the whole of the med. While there we saw deer tracks.

Can't find name of locality at present though.

James:)
 

Offline Bass

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Re: Geology Question of the Week
« Reply #28 on: 02/12/2005 00:20:18 »
The following pictures are from lava flows





QUESTION OF THE WEEK:
These flows have a very distinctive "ropey" surface texture.  Name the flow texture?

Bonus questions:
Name the type of lava?
What does this tell you about the explosiveness of the volcano?
Where is the source of the molten rock (lava)?


Subduction causes orogeny.
« Last Edit: 06/12/2005 01:52:40 by Bass »
 

Offline neilep

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Re: Geology Question of the Week
« Reply #29 on: 06/12/2005 03:07:32 »
Come on peeps..have a go...I'm stumped...This is one for Exodus.

Thanks for the quiz Bass.

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Offline ukmicky

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Re: Geology Question of the Week
« Reply #30 on: 06/12/2005 04:15:11 »
As i don't speak Hawaiian? i had to cheat. but because i cheated i learnt something, so in the end its all good.
but as a respectable member of society i feel it would be wrong of me to answer the questions.

Skip,
 I know what the bottom flow is called but just one question before i go to bed and place my head on my Pillow:) is the top picture taken underwater.

Michael                                      
« Last Edit: 06/12/2005 04:27:52 by ukmicky »
 

Offline Bass

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Re: Geology Question of the Week
« Reply #31 on: 06/12/2005 05:41:18 »
Oops!

I forgot to draw the little fishies in...

Actually, that is some sort of rabbit brush or small sagebursh growing on the flow- so not underwater.  This texture probably won't form underwater- the lava cools too fast.

Subduction causes orogeny.
« Last Edit: 06/12/2005 05:43:12 by Bass »
 

Offline Bass

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Re: Geology Question of the Week
« Reply #32 on: 07/12/2005 17:52:54 »
The “ropey” flows are called pahoehoe (pronounced like a pig squeal), the Hawaiian word for either “run like hell”, or “does it suddenly feel hot to you?”  

Actually, it means smooth lava, in contrast to aa, which is rough lava.

Pahoehoe forms when the surface of low-viscosity lava flows began to cool and develop a plastic skin,  As the lava continues to move underneath, it pushes the skin into lobes, giving it the characteristic “ropey” look.

Pahoehoe only forms in basalt.  Basalt is a low-silica, high temperature lava- which means low viscosity.  Since basalt is low viscosity, it happily flows across the ground and doesn’t accumulate volatile components like its more explosive cousins, andesite and rhyolite.  That makes basalt eruptions, such as Hawaii and Iceland, safe enough to view and study up close.

The temperature and components of basalt argue for a deep source, most likely the uppermost mantle.  Basalt is by far the most common volcanic rock on earth.


Subduction causes orogeny.
« Last Edit: 07/12/2005 17:53:42 by Bass »
 

Offline Bass

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Re: Geology Question of the Week
« Reply #33 on: 08/12/2005 21:48:28 »
Pictured here is a primary depositional feature common in sedimentary rocks.  Note the change in attitude of the beds where the person is standing from those above and below.  



GQOTW:  Name this sedimentary feature.  

Bonus
What type of sedimentary rock is this?
Given the large scale of this feature, what was the most likely  environment during deposition of these sediments?


Subduction causes orogeny.
« Last Edit: 08/12/2005 21:50:38 by Bass »
 

Offline Bass

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Re: Geology Question of the Week
« Reply #34 on: 21/12/2005 20:53:21 »
The sedimentary feature here is called cross-bedding. Cross-beds are internal sedimentary structures formed by currents of wind or water. Cross beds are deposited on the lee side of ripples (subaqueous) or dunes (wind caused)- as granular sediment (usually sand) rolls down the steep advancing edge or "foreset slope" of the dune.  Commonly, the next sedimentary layer erodes the top of the cross-beds, leaving a sharply defined angular discontinuity.



The sedimentary rock pictured here is a sandstone.  The large scale of the cross-beds indicates it probably formed by wind-blown dunes- the depositional environment is most likely desert (possibly beach).  Water-laid cross-beds are much smaller.

Subduction causes orogeny.
« Last Edit: 21/12/2005 21:11:09 by Bass »
 

Offline Bass

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Re: Geology Question of the Week
« Reply #35 on: 31/12/2005 01:01:32 »

This photo is from Missoula, Montana.  Note the prominent horizontal banding, which is displayed quite well by snow on the mountain slope.  

GQOTW:  How were these horizontal bands formed?  (the underlying bedrock is oriented almost vertical)

Bonus:  What impact did this have on landforms in Washington state?

FYI:  The formation of these horizontal bands and their impact was one of the most controversial geologic debates in the 1930's to 50's.

Subduction causes orogeny.
 

Offline Soul Surfer

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Re: Geology Question of the Week
« Reply #36 on: 31/12/2005 14:53:24 »
That looks a bit like the stuff I saw on channel 5 the other night when they were talking about superfloods created by the failure of Ice dams on glacial lakes.

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Offline ukmicky

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Re: Geology Question of the Week
« Reply #37 on: 31/12/2005 16:57:40 »
Did the bands/layers start off horizontal but got pushed up into their vertical positions as the mountain went up

Michael  
HAPPY NEW YEAR                    
« Last Edit: 31/12/2005 16:58:08 by ukmicky »
 

Offline Bass

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Re: Geology Question of the Week
« Reply #38 on: 31/12/2005 19:25:18 »
Sorry if my question was confusing.  The underlying bedrock is much older than the horizontal banding- bears no relationship to the banding.
Soul Surfer is on the right track, but what formed the horizontal bands?

Subduction causes orogeny.
 

Offline neilep

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Re: Geology Question of the Week
« Reply #39 on: 31/12/2005 19:32:48 »
Watched an amazing program last night/this morning . following the expedition to discover what really happend at the bottom of the sea that caused the tsunami from last year. They went 4 months after the event an it was astonishing the pictures and technology used, and quite humbling too when you see the shear scale of things.

Seems there was an amzing uplift of about 40 meters by 750 miles width  !!!...the whole worked like a zipper and took just a few seconds. The scary thing is, only part of the fault rose up and another is expected !!...at any time.

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Offline kade04

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Re: Geology Question of the Week
« 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!
 

Offline DoctorBeaver

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Re: Geology Question of the Week
« 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)
« Last Edit: 03/01/2006 03:18:16 by DoctorBeaver »
 

Offline DoctorBeaver

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Re: Geology Question of the Week
« 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!
 

Offline Bass

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Re: Geology Question of the Week
« 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.
 

Offline Bass

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Re: Geology Question of the Week
« 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.
 

Offline DoctorBeaver

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Re: Geology Question of the Week
« 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
 

Offline Bass

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Re: Geology Question of the Week
« 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.
 

Offline kade04

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Re: Geology Question of the Week
« Reply #47 on: 06/01/2006 00:24:12 »
yeah! happy dance lol! can anyone ask questions?
 

Offline Bass

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Re: Geology Question of the Week
« Reply #48 on: 06/01/2006 05:21:08 »
Go for it Kade04

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Offline Bass

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Re: Geology Question of the Week
« 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.
 

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Re: Geology Question of the Week
« Reply #49 on: 09/01/2006 18:13:58 »

 

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