<p>I am not sure those are pegmatites. They may simply be granitic dikes. But in either case, you would need to assess the regional geology to look for potential sources of this material, which would be a granitic type magma of younger age than the host rock. You could then map out the size, orientation, and frequency of these dikes to tie them back into their parent unit. Geochemical fingerprinting and age dating could help, too. But there is no way to look at that picture, or a given outcrop, and tell the direction. Remember that the geometry of the third dimension, the one in and out of the picture or outcrop, always needs to be kept in mind and is typically unknown.</p>

<p>I am pretty sure it was not top down, though!</p>

CC BY-SA 4.0
2019-09-11T04:11:30.087Z
Yes, you are absolutely correct. No doubt.
Daniel
CC BY-SA 4.0
2019-09-10T18:38:35.527Z
*Geologic Principles—Cross-cutting Relationships* at https://www.nps.gov/articles/geologic-principles-cross-cutting-relationships.htm
 says *Pegmatite dikes exposed in Painted Wall cliff face. The light colored dikes are younger than the dark rock.* I understand that the dike is the light colored veins, I am correct?
Alessandro Jacopson
CC BY-SA 4.0
2019-09-09T15:53:24.683Z
2019-09-09T15:53:24.683Z
Daniel
<p>The following picture (<em>Painted wall</em> cliff in the Black Canyon of the Gunnison National Park, Colorado, USA) shows a pegmatite dike: is it possible to tell what was the flow direction? From thin (see below the printed 1 in the picture) to thick (2) or from thick to thin?</p>

<p><a href="https://i.sstatic.net/aMkPS.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/aMkPS.jpg" alt="enter image description here"></a></p>

CC BY-SA 4.0
2019-09-12T01:18:00.383Z
Who said that they had to go up or down? They could also go laterally, either "forwards" or "backwards". This is most likely what you're seeing here, and the "branches" are not the dyke combining or splitting, it's just parallel lateral movements of the magma.
Gimelist
CC BY-SA 4.0
2019-09-12T06:20:55.430Z
@Gimelist I am not a geologist, I just have questions that can be silly. When you say *most likely* you mean that the flow was probably "entering" or "exiting" from the plane of the picture, am I correct?
Alessandro Jacopson
CC BY-SA 4.0
2019-09-12T06:22:58.523Z
Yes, exactly. Magma rarely flows directly "up". It flows in oblique sheets, with the flow direction overall being up, but it can have a large "sideways" component. A nearly-vertical outcrop cuts along this plane in an arbitrary orientation, and it is very challenging to tell which way the magma was going just from this image.
Gimelist
CC BY-SA 4.0
2019-09-12T08:03:35.340Z
@Gimelist I would love to upvote your answer
Alessandro Jacopson
CC BY-SA 4.0
2019-09-12T08:51:51.320Z
that’s fine, I already have more than enough rep 
Gimelist
<igneous>
Is it possible to know what was the flow direction in a pegmatite dike?
Alessandro Jacopson
# Is it possible to know what was the flow direction in a pegmatite dike?

(Asked by: username_0 on 2019-08-17)

The following picture (*Painted wall* cliff in the Black Canyon of the Gunnison National Park, Colorado, USA) shows a pegmatite dike: is it possible to tell what was the flow direction? From thin (see below the printed 1 in the picture) to thick (2) or from thick to thin?

[![enter image description here](https://i.sstatic.net/aMkPS.jpg)](https://i.sstatic.net/aMkPS.jpg)

**Tags:** igneous

> **Comments:**
> **username_1**: Who said that they had to go up or down? They could also go laterally, either "forwards" or "backwards". This is most likely what you're seeing here, and the "branches" are not the dyke combining or splitting, it's just parallel lateral movements of the magma.  
> **username_0**: @username_1 I am not a geologist, I just have questions that can be silly. When you say *most likely* you mean that the flow was probably "entering" or "exiting" from the plane of the picture, am I correct?  
> **username_1**: Yes, exactly. Magma rarely flows directly "up". It flows in oblique sheets, with the flow direction overall being up, but it can have a large "sideways" component. A nearly-vertical outcrop cuts along this plane in an arbitrary orientation, and it is very challenging to tell which way the magma was going just from this image.  
> **username_0**: @username_1 I would love to upvote your answer  
> **username_1**: that’s fine, I already have more than enough rep   

---

## 1 answers

### Answer by username_2 on 2019-09-09. Score: 2 | Accepted answer

I am not sure those are pegmatites. They may simply be granitic dikes. But in either case, you would need to assess the regional geology to look for potential sources of this material, which would be a granitic type magma of younger age than the host rock. You could then map out the size, orientation, and frequency of these dikes to tie them back into their parent unit. Geochemical fingerprinting and age dating could help, too. But there is no way to look at that picture, or a given outcrop, and tell the direction. Remember that the geometry of the third dimension, the one in and out of the picture or outcrop, always needs to be kept in mind and is typically unknown.

I am pretty sure it was not top down, though!

> **Comments:**
> **username_0**: *Geologic Principles—Cross-cutting Relationships* at https://www.nps.gov/articles/geologic-principles-cross-cutting-relationships.htm  says *Pegmatite dikes exposed in Painted Wall cliff face. The light colored dikes are younger than the dark rock.* I understand that the dike is the light colored veins, I am correct?  
> **username_2**: Yes, you are absolutely correct. No doubt.  

---

