Geology Question of the Week

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Offline Exodus

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Geology Question of the Week
« on: 07/07/2004 01:40:51 »
The first question of the week:

YOU ARE OUT WALKING AND DISCOVER A RED SEDIMENTRY ROCK (E.G. A SANDSTONE) WHAT CAN A RED COLOURED ROCK SUCH AS THIS TELL YOU ABOUT THE EARTH'S CLIMATE WHEN IT WAS LAID DOWN?

AND FOR BONUS MARKS....

IF THIS ROCK CONTAINS SMALL GREEN AREAS, WHAT MIGHT HAVE LED TO THEIR FORMATION?

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Offline chris

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Re: Geology Question of the Week
« Reply #1 on: 09/07/2004 02:28:35 »
Well, it's red, so it must have some iron III oxide (Fe2O3) in it. And for it to have formed the oxide, there must have been some oxygen in the air / soil / sediment at the time...

I've no idea about the green inclusions - are they copper ?

Chris

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Offline Exodus

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Re: Geology Question of the Week
« Reply #2 on: 23/07/2004 00:30:02 »
Thats not bad... the redness is indeed down to the oxidation of iron and actually suggests that the environment was Warm and fairly dry...

The green inclusions are in fact the passage of old roots which have rotted underground and have subsequently caused reducing conditions. This in turn lead to iron reduction which is a charachteristic green colour...

So Overall, a warm dry environment with trees!

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Offline chris

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Re: Geology Question of the Week
« Reply #3 on: 28/09/2004 17:49:41 »
Come on Exodus, where's the next geology question of the week ?

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Offline Kiss

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Re: Geology Question of the Week
« Reply #4 on: 13/10/2004 11:32:04 »
I have visited a "early Ordovician red rock" locality a few month ago which is given to be a fresh water sedimentary system under hot climatic conditions. Two days collecting but I found no fossils there (which is not a surprise) but I've seen many sedimentary figures suggesting a shallow environment (ripples-marks) and probable dry periods (polygonal mud-cracks). All the sequence is almost reddish color, but a few layers are bright green. That seems to fit with the reducing condition period hypothesis exposed by Exodus. I'll buy that.
 

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Offline Exodus

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Re: Geology Question of the Week
« Reply #5 on: 13/10/2004 11:48:06 »
Kiss, the interesting thing was that there were what appeared to be round nodules of green rather than layers which had been formed by roots burrowing through the sands and producing reducing conditions upon breakdown.

I saw some great ripple marks whilst in the South Western Alps on some finer grained sandstones. One particular bedding plane had a trackway of bird footprints!

I guess the lack off fossils was down to the environmental setting of the time, it could well have been a braided river deposit? or it could have even been an ephemeral lake?

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Offline Kiss

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Re: Geology Question of the Week
« Reply #6 on: 13/10/2004 11:56:23 »
Honestly, when observing the basis of the sequence, it could be something like a melting glacier (brech+conglomerate) and I agree for the inner lake idea but really, the thickness suggest a long term pool.
 

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Offline Exodus

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Re: Geology Question of the Week
« Reply #7 on: 13/10/2004 12:52:16 »
Sounds interesting, where abouts was it?

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Offline Kiss

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Re: Geology Question of the Week
« Reply #8 on: 13/10/2004 13:10:35 »
I should be able to put a picture in this board. I've done some. See that this afternoon.
The location is Western France (Britanny).
 

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Offline Exodus

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Re: Geology Question of the Week
« Reply #9 on: 13/10/2004 15:35:10 »
Pictures will be great. I'm not too knowledgeable about the geology of North Western France i'm afraid, maybe i should look into it, never really thought about it!

So Kiss, are you a geologist?

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Offline Kiss

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Re: Geology Question of the Week
« Reply #10 on: 13/10/2004 15:42:23 »
Oh no Exodus [:D], I'm just a simple student but I love fossils and geology. I'm not a specialist huh [:I] sorry I'm so dumb superficial in this field. Very difficult disciplines.

Ok I will post some pics to have a look on. Must find how to host them first.
 

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Offline Exodus

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Re: Geology Question of the Week
« Reply #11 on: 13/10/2004 15:44:53 »
quote:
Originally posted by Kiss

Oh no Exodus [:D], I'm just a simple student but I love fossils and geology. I'm not a specialist huh [:I] sorry I'm so dumb superficial in this field. Very difficult disciplines.

Ok I will post some pics to have a look on. Must find how to host them first.



don't be so harsh on yourself!!! sounds like you are keen! nice to have another rock lover on here... not many of us... [:(]

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Offline Exodus

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Re: Geology Question of the Week
« Reply #12 on: 13/10/2004 15:51:32 »
if you want a site to host them then go to

http://www.photobucket.com

its easy and free, then highlight the picture's details and input them in between the insert image code which may be found on the message toolbar!

« Last Edit: 13/10/2004 18:11:22 by Exodus »

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Offline Kiss

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Re: Geology Question of the Week
« Reply #13 on: 14/10/2004 07:06:18 »
Greatings,

It's my first picture posting. A friend of mine was kind enought
to open a guestfolder in her photopocket account.

The best upper red sandstone+shale exposure at Brehec (Western France).
Left is a volcanic dyke (bubble lava), center a 1/4 meter rejection fault.
The light green reducing episode is very well differencied.
Above, there are compact grey shales sequence.


Ripple-marks on the shale (my pic gives scale).
IMHO, the wavelenght of this sedimentation figure should indicate a very shallow water deposit.
A shame, there are no vertebrate foot-print and no vegetation evidence,
because of the early age of these rocks ! The whole succession is azoïc.
« Last Edit: 14/10/2004 07:08:37 by Kiss »
 

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Offline Exodus

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Re: Geology Question of the Week
« Reply #14 on: 14/10/2004 22:59:55 »
Really nice pics, post any other you get!!!

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Offline Notrenchgeology

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Re: Geology Question of the Week
« Reply #15 on: 18/01/2005 20:47:06 »


I would suggest to you that truly, the red color is indeed from iron (III) oxide, but that the green color would be from one of three possible sources, depending upon the size and orientation of the "mottles" that are present.

The first possibility that leaps to mind is the presence of glauconite, which as a semi- or pseudo- clay mineral, depending upon whether the mineral is ordered or disordered in molecular organization, lends the green color to so-called "greensand" deposits.  Glauconite also can be a part of ancient dolomite deposits, such as the Bonne Terre Formation dolomites (Upper Cambrian) we have here in Missouri, USA.

The second possibility may be illite, or an illitic shale deposits, which would be consistent with shallow-water or tidalite deposition.

The final possibility would be coloring due to the presence of bivalent nickel.  Nickel (II) sulfide, or millerite, is known to occur as acicular needles in limestones.  But what happens when these weather out of formation?  The bivalent nickel can become part of any shaly layers or laminae that are present that have any remaining anionic charge.  This has also been known to happen in the Cedar Valley Formation (Devonian) of the central United States.

Good question!

Notrenchgeology
 

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Offline Bass

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Re: Geology Question of the Week
« Reply #16 on: 15/09/2005 22:49:26 »
Great picture!
Reminds me of several sequences of siltstones and argillites (St. Regis Fm. and Missoula Group) in the Proterozoic Belt supergroup rocks in Montana and Idaho.
I agree with Notrenchgeology- the red is due to oxidized iron.  The green, however, is probably due to reduced iron.  My guess, by your comments, is that these rocks may be PreCambrian (lack of fossils).  Evidence for alternating oxidizing/reducing conditions is common in Precambrian rocks.

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Offline Bass

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Re: Geology Question of the Week
« Reply #17 on: 16/11/2005 01:26:56 »


Here is a photo taken during a recent trip to a famous park- I call it "the scream".  The feature is about 15 meters tall and is outdoors.

Identify the type of rock in the photo and how this feature was formed?

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« Last Edit: 16/11/2005 01:31:01 by Bass »
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Offline neilep

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Re: Geology Question of the Week
« Reply #18 on: 16/11/2005 03:16:34 »
I haven't a clue but what a fantatstic picture...and 'the scream' is very apt.....it looks salty !

I presume it's Yellowstone Park eh ?

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« Last Edit: 16/11/2005 03:17:38 by neilep »
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Offline Bass

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Re: Geology Question of the Week
« Reply #19 on: 17/11/2005 00:44:58 »
Neil
You hit the hot-spot right on the head, Yellowstone it is.
More later

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Offline Bass

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Re: Geology Question of the Week
« Reply #20 on: 22/11/2005 22:29:09 »
The photo was snapped at Mammoth Hot Springs near the nothern edge of Yellowstone Park.  The rocks are made of travertine, which is deposited from the hot springs.  Mammoth Hot Springs has the most abundant deposits of travertine in the park.  
In most of Yellowstone, the geysers and hot springs precipitate sinter (or geyserite)- composed of silica which the thermal waters dissolve as they pass through the volcanic material.  Silica sinter accumulates slowly- in the order of a few centimeters per century.
Because travertine is calcium carbonate, the thermal waters at Mammoth must pass through older, probably Mississipian-aged, limestones.  Travertine accumulates much more rapidly than does silica sinter, in the order of 40 to 70 cm/year- thus the massive formations of travertine at Mammoth.  The travertine also dissolves easily, so as the pathways of the thermal waters change, voids often form- which is what I believe created part of the formation in the picture.
As an aside- the thermal springs at Mammoth are not as hot as those found more central to the park- suggesting that they have travelled some distance before bubbling up to the surface.  The travertine preceipitates as the water cools at the surface, forming small pools.  As these pools grow, the water flows over the edge of the pool where it is shallowest, further cooling the water and leading to more precipitation- in this way the edges of the pools build up.

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Offline Bass

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Re: Geology Question of the Week
« Reply #21 on: 25/11/2005 23:22:06 »
Plate tectonics predicts that mountains will form at the edge of plates.  This seems to hold true for almost all mountain ranges on earth- even those ranges found in continental interiors (Urals, Himalayans, Alps) formed along the edge of a tectonic plate.

Question of the week:  THE ROCKY MOUNTAINS (USA) ARE APPROXIMATELY 2000 KILOMETERS FROM THE EDGE OF THE NORTH AMERICAN PLATE- HOW DID THE ROCKY MOUNTAINS FORM (AND WHY ARE THEY STILL GROWING TALLER) SO FAR FROM THE EDGE OF THE PLATE?



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Offline ukmicky

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Re: Geology Question of the Week
« Reply #22 on: 25/11/2005 23:58:00 »
Hiya skip
I'm not quite sure how it all works. But are they the type of mountain that is created by the folding of the earths crust, an area of land that sits in between two plates that are pushing together. Something like that, I think

Michael                                      
« Last Edit: 26/11/2005 00:12:33 by ukmicky »

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Offline Bass

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Re: Geology Question of the Week
« Reply #23 on: 26/11/2005 03:48:10 »
Your hypothesis is spot on for the Himalayas, Alps and a few other mountain ranges- but there is no evidence of a junction of plates along the Rockies.  As far as I know- the North American plate extends all the way across the US (with the exception of a small piece in southern California).

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Offline neilep

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Re: Geology Question of the Week
« Reply #24 on: 26/11/2005 03:53:09 »
Thanks for a great question Skip.

I'm stumped !..the mountains are not on the edge of the plates and Michaels
 answer above doesn't cut it either eh ?...and yet these mountains are
 growing !...is there some sort of lava making activity happening below,
 beneath the Rocky Mountains that is pushing them up ?...there has to be
 something that is making more mountain.


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« Last Edit: 27/11/2005 04:30:15 by neilep »
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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 »
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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[:)]
 

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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 »
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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 »

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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 »
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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 »
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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 »
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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 »
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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.

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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 »

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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?

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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.

Men are the same as women.... just inside out !!
Men are the same as women, just inside out !

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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!
 

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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 »
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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!
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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.

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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


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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
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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!

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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?
 

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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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