Understanding through Discussion


Welcome! You are not logged in. [ Login ]
EvC Forum active members: 63 (9072 total)
151 online now:
Percy (Admin), Tanypteryx (2 members, 149 visitors)
Newest Member: FossilDiscovery
Post Volume: Total: 893,162 Year: 4,274/6,534 Month: 488/900 Week: 12/182 Day: 12/28 Hour: 1/0


Thread  Details

Email This Thread
Newer Topic | Older Topic
  
Author Topic:   Young earth explanations for Angular Unconformities
edge
Member (Idle past 940 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 182 of 202 (796803)
01-04-2017 6:34 PM
Reply to: Message 181 by Percy
01-04-2017 4:58 PM


Re: a review
I was wondering if that was the case - so what's the difference?

Weathering is a chemical attack on the rock. Erosion is the transport of that rock material. Solution of a limestone would also be erosion. In some cases weathering might be considered a part of erosion. In this case, I am trying to emphasize the difference because of Faith's statement about weathering at Siccar.

I don't think this could be the right image because I'm pretty sure Siccar Point is not artificial:

So again, I'm not sure which image you mean. Are we getting the Siccar Point and the "spheroidal weathering" cases confused?


That was in reference to the granite image.

I guess I don't know how to interpret those "boulders" if they weren't really boulders. I assumed they were granite boulders that were ground into spheroid shapes by the action of weather and by being scraped along fast-moving river bottoms. How did those shapes actually form? I did read this explanation from you:

By boulder, I'm referring to a size of rock fragment, as shown by this article:

https://en.wikipedia.org/wiki/Grain_size

As the cores of those spheroids fall out of their places they would become boulders.

But why would there be fractures along spheroid paths? And I guess the difference between "mechanically eroded" and "chemically weathered" is becoming important.

The fractures predate weathering. They allow access by water which slowly reacts with the rock.

This message is a reply to:
 Message 181 by Percy, posted 01-04-2017 4:58 PM Percy has seen this message

  
edge
Member (Idle past 940 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 184 of 202 (796817)
01-04-2017 10:05 PM
Reply to: Message 183 by Faith
01-04-2017 9:48 PM


Re: The decrepitude of both upper and lower blocks of strata
I don't get much out of your discussions with Percy about this or anything else. I still have no idea what you are saying here. It sounds like an irrelevant pedantic point. How does it change my observation that there is no difference in the appearance of the two blocks of strata -- doesn't matter if you want to call it erosion or weathering or whatnot, they look like they've been treated to exactly the same processes of deterioration from their original state to their current decrepit splintery state. I see no difference and so far nothing you have said has made sense of this.

As I said. The sandstone was deposited on eroded rocks that were being removed as they were being weathered. Hence, there was very little weathering of the older rocks before the sandstone was deposited.

This means that there should be little difference in the total amount of weathering, which was your observation.


This message is a reply to:
 Message 183 by Faith, posted 01-04-2017 9:48 PM Faith has replied

Replies to this message:
 Message 185 by Faith, posted 01-04-2017 10:18 PM edge has taken no action

  
edge
Member (Idle past 940 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 191 of 202 (796844)
01-05-2017 10:57 AM
Reply to: Message 187 by Faith
01-05-2017 12:15 AM


Re: The decrepitude of both upper and lower blocks of strata
Both segments of the unconformity look like they've been subjected to a very great degree of weathering or whatever has left them so desiccated and splintery looking.

If I stood out there for years in the weather along the North Sea, I'd look pretty desicated and splintery too.

Erosion removed the weathered material....How do you know that?

Well, for one, we see this happening all over the world including in the images of Siccar Point.

Beyond that, we can see rounded boulders of the lower sequence (below the unconformity) as a basal conglomerate just above the unconformity. So, we know that the lower rocks were eroded.

In fact, I think (IMO ... I've not actually been there) there are small channels of conglomerate in the sandstone that contain fragments of the lower rocks.

Why does it end up looking exactly like the rock above the unconformity, ...

You've lost me here. I thought you agreed that the rocks above and below were very different. That's how it looks to me. One is a red sandstone (above) and the other is a greenish-gray graywacke (below). I can describe other differences if you want.

... where presumably erosion did not remove the westhered material?

Well, it is being removed now, so I'm not sure what you mean.

If mean the previous period of erosion, sure. The upper rocks (the sand) were deposited after erosion of the lower layer.


This message is a reply to:
 Message 187 by Faith, posted 01-05-2017 12:15 AM Faith has replied

Replies to this message:
 Message 192 by Faith, posted 01-05-2017 1:44 PM edge has replied

  
edge
Member (Idle past 940 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 194 of 202 (796857)
01-05-2017 2:28 PM
Reply to: Message 192 by Faith
01-05-2017 1:44 PM


Re: The decrepitude of both upper and lower blocks of strata
I've been saying from the point I entered this thread that the upper and lower segments of the Siccar unconformity look about equally weathered. The appearance of weathering (or erosion or whatever is the cause of the obvious breakdown of the rocks into their splintery form) is the only comparison I've commented on. The point has been from the beginning that I see no evidence of the usual interpretation of angular unconformities, that there is a difference of millions of years between the tilted lower portion and the upper horizontal portion of such formations.

I'm not sure how to explain it any more simply.

As the rocks are weathered, the weathered parts are carried away by mechanical erosion. This exposes more fresh rock to weathering. That way, the rock always looks fresher than one might expect. The exposed surface is being constantly renewed.

If that were the case, I argued, the lower section should be utterly reduced to a small pile of splintery rock at a location like Siccar Point with the constant battering of the elements. MILLIONS of years. I don't think such a huge time span can be encompassed by anybody's mental set, it's beyond comprehension. But sedimentary rock should disintegrate under the conditions at Siccar Point in far far less time.

And they do. The question is, how long has that particular part of the rock been exposed to weathering.

Why do you think that geologists carry hammers? It's to break through the part of a rock that is weathered to see the original rock characteristics.

The upper segment is identified as Devonian, the lower as Silurian, the usual difference in age between these time periods being in the millions. This is all I've been referring to from the beginning. I thought you answered earlier in terms that reduced the difference in age. Now you are answering in terms of the appearance of age. On the appearance of age I argue that the differences couldn't possibly reflect millions of years.

They don't.

You understand that the Silurian rocks were being weathered in the Devonian, right up until they were being covered by the sand.

We can't even tell when the weathering started. That's one of the tricky things about unconformities. They seem like a fairly simple concept, but when you really get into it, there's a lot of complications and implications.

How long does it take? How long has the upper section been there?

The upper section started forming in the Devonian. The lower section was eroded sometime after it was deformed. All we really know is that erosion ended in the Devonian.

Even that section should have been reduced to rubble by now according to the usual time spans proposed by standard geology. That any of the formation is still standing at all is testimony to a much shorter span of time than that.

Not really. Erosion is a process, it starts when the rock is exposed to the elements and it ends when deposition begins anew.

The topic is still pretty confused it seems to me.

Well, frankly, it is confusing to most people. But most people will try to understand the processes acting over time.

This message is a reply to:
 Message 192 by Faith, posted 01-05-2017 1:44 PM Faith has replied

Replies to this message:
 Message 196 by Faith, posted 01-05-2017 8:31 PM edge has taken no action

  
edge
Member (Idle past 940 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 195 of 202 (796858)
01-05-2017 2:32 PM
Reply to: Message 193 by PaulK
01-05-2017 2:17 PM


Re: The decrepitude of both upper and lower blocks of strata
The age of the fold would seem far more relevant than the time the greenwacke was deposited. Exposure time is what matters, not age.

This is very true (except that it is a graywacke). Just because the lower rocks are Silurian in age does not mean that they were eroded in the Silurian.

In geology terms, the erosional surface cuts the folded rocks, therefor it is younger than the folding.


This message is a reply to:
 Message 193 by PaulK, posted 01-05-2017 2:17 PM PaulK has taken no action

  
edge
Member (Idle past 940 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


(1)
Message 202 of 202 (796883)
01-06-2017 4:17 PM


Since there seems to have been some confusion here regarding the processes of weathering and erosion, here is a simplified explanation that may help clear up further discussion about unconformities.

http://www.onegeology.org/.../earthprocesses/weathering.html

Alternatively, here is a (more technical) wiki article on shear zones:

https://en.wikipedia.org/wiki/Shear_zone

A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening (crustal) blocks stay relatively unaffected by the deformation. ...

I have bolded the part about shear zoned being planar in nature and not rough/irregular such as what we see at Siccar Point, etc.


  
Newer Topic | Older Topic
Jump to:


Copyright 2001-2018 by EvC Forum, All Rights Reserved

™ Version 4.1
Innovative software from Qwixotic © 2022