<p>Yes, this is a granite.</p>

<p>It's hard to know exactly which one. A <a href="http://www.dnr.wa.gov/publications/ger_ofr73-1_geology_south_snohomish_co_pt2.pdf" rel="nofollow noreferrer">geological map</a> of the area shows the bedrock to be sedimentary rocks, so this piece of granite was transported in the creek from far away (but not too far, otherwise it will crumble completely).</p>

<p>If we go further east, for example with this <a href="https://pubs.er.usgs.gov/publication/i1963" rel="nofollow noreferrer">geological map</a>, we find that:</p>

<blockquote>
  <p>To the east, the higher-grade terrane is mostly the Chiwaukum Schist and related gneisses of the Nason terrane and invading mid-Cretaceous stitching plutons</p>
</blockquote>

<p>and</p>

<blockquote>
  <p>Rocks of the Cascade magmatic arc are mostly represented by Miocene and Oligocene plutons, including the Grotto, Snoqualmie, and Index batholiths. </p>
</blockquote>

<p>So it could be a very poor example of a high-grade granitic composition gneiss, or it could be a granite from Cretaceous, Miocene or Oligocene times. Any more information than that would require someone to study the rock in detail.</p>

CC BY-SA 4.0
2018-09-02T02:00:55.220Z
Great information. Thank you. Is there a part of this explanation that can account for the crumbliness?
BMS
CC BY-SA 4.0
2018-09-03T01:41:04.790Z
@BMS it's called weathering. The other answer by Spencer explains it well.
Gimelist
CC BY-SA 4.0
2018-09-01T05:56:20.947Z
2018-09-03T01:42:34.033Z
2018-09-03T01:42:34.033Z
Gimelist
<p>It has been burned.</p>

<p>No acidity etc. needed. If you throw one fist sized granit into a normal campfire (usually around 700 C) and leave it there to burn for some hours, when it cools it will crumble in your hands. As mentioned before "granite is made of crystals of different minerals" in intensive heat/cooling each mineral expands at a different rate and brakes the composition apart.</p>

<p>Tested myself many times in practise..</p>

CC BY-SA 4.0
2019-07-24T06:34:13.297Z
Most likely heating of a granite in a fire breaks it down not by expansion or contraction, but rather by decomposition of hydrous minerals like micas or amphiboles.
Gimelist
CC BY-SA 4.0
2019-07-24T04:43:45.570Z
2019-07-24T04:43:45.570Z
alise
<p>My understanding of why granite decomposes that way is because it's formed under ground and as the over-burnden is removed so is weight and pressure. The granite formations are rounded as the outside layers fall away. We have very few freeze/thaw cycles in Arizona (Phoenix area), and not a lot of rain, but, we have tons and tons of decomposed granite in Arizona. </p>

CC BY-SA 4.0
2018-09-05T22:13:09.683Z
2018-09-05T22:13:09.683Z
Tim Nevins
<p>Even rocks that have lasted for billions of years decompose when exposed to Earth's weather.</p>

<p>What you have is indeed granite, but a somewhat <a href="https://en.m.wikipedia.org/wiki/Decomposed_granite" rel="nofollow noreferrer">decomposed granite</a>, which is treated by geologists differently from the intact, solid stuff.  The "crumbly bits" have their own 
name: <strong><a href="https://en.wikipedia.org/wiki/Grus_(geology)" rel="nofollow noreferrer">grus</a></strong>.</p>

<p>Granite is made of crystals of different minerals  (mostly feldspar and quartz) that grew into each other as the parent magma cooled. The final rock is solid because of the irregular way the crystals fit into each other.</p>

<p>Granite weathers in several different ways. Cracks can form from hot and cold cycles. However, <a href="https://www.americangeosciences.org/education/k5geosource/content/rocks/what-is-chemical-weathering" rel="nofollow noreferrer">chemical weathering</a> is the principal way granite becomes crumbly.  <a href="https://en.wikipedia.org/wiki/Acid_rain" rel="nofollow noreferrer">Rain frequently picks up some acidity</a> from the atmosphere, and infiltrates the granite. This acidic rain reacts with the feldspar and mica to produce clay minerals, primarily <a href="https://en.wikipedia.org/wiki/Kaolinite" rel="nofollow noreferrer">kaolinite</a>. </p>

<p>This in turn makes it easier for water to get in the granite, and combined with heat cycles the process accelerates.
As a result, the joints between the crystals loosen up, producing rock in the same condition as your rock. </p>

CC BY-SA 4.0
2018-09-02T15:14:52.103Z
2018-09-04T23:05:53.583Z
2018-09-04T23:05:53.583Z
Spencer
<p>I found this crumbly rock embedded in the channel walls of a small creek in Everett, WA, USA. (North Creek that flows through McCollum Park.)  If possible, I'd like to know how the rock formed... what process leads to its formation? It's the crumbly part that I'm most curious about.</p>

<p>The shape is ellipsoid, abut 8 cm on a side:
<a href="https://i.sstatic.net/EWhqS.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/EWhqS.jpg" alt="enter image description here"></a></p>

<p>The entire thing is made of different types of 1-3 millimeter (or so) sized crystals.</p>

<p><a href="https://i.sstatic.net/14D3s.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/14D3s.jpg" alt="enter image description here"></a></p>

<p>I would have called it granite or some other relative; to my non-expert eye, I see feldspars, some quartz, and mica. Except that it's very crumbly. Here's a video that shows better the texture and how loosely-bound the crystals are to each other. You can flake them all off with your hand.</p>

<ul>
<li><a href="https://youtu.be/FAMZ0do98yU" rel="nofollow noreferrer">https://youtu.be/FAMZ0do98yU</a> &lt;-- Very useful video</li>
</ul>

<p>How is this formed? Are the minerals dissolved in the creek and they happen to have found a nice place to precipitate out? (If so, what would happen as time goes on? Would the rock grow larger with same crystal size, or larger crystal size but same overall rock size?)</p>

CC BY-SA 4.0
2019-08-29T19:44:22.547Z
NB to close-voters - this is *not* primarily a rock-identification question. It's asking how the rock was formed.
Semidiurnal Simon
<rocks><weathering>
How is this crumbly crystal-rich rock formed?
BMS
# How is this crumbly crystal-rich rock formed?

(Asked by: username_0 on 2018-09-01)

I found this crumbly rock embedded in the channel walls of a small creek in Everett, WA, USA. (North Creek that flows through McCollum Park.) If possible, I'd like to know how the rock formed... what process leads to its formation? It's the crumbly part that I'm most curious about.

The shape is ellipsoid, abut 8 cm on a side:
[![enter image description here](https://i.sstatic.net/EWhqS.jpg)](https://i.sstatic.net/EWhqS.jpg)

The entire thing is made of different types of 1-3 millimeter (or so) sized crystals.

[![enter image description here](https://i.sstatic.net/14D3s.jpg)](https://i.sstatic.net/14D3s.jpg)

I would have called it granite or some other relative; to my non-expert eye, I see feldspars, some quartz, and mica. Except that it's very crumbly. Here's a video that shows better the texture and how loosely-bound the crystals are to each other. You can flake them all off with your hand.

* <https://youtu.be/FAMZ0do98yU> <-- Very useful video

How is this formed? Are the minerals dissolved in the creek and they happen to have found a nice place to precipitate out? (If so, what would happen as time goes on? Would the rock grow larger with same crystal size, or larger crystal size but same overall rock size?)

**Tags:** rocks, weathering

> **Comments:**
> **username_1**: NB to close-voters - this is *not* primarily a rock-identification question. It's asking how the rock was formed.  

---

## 4 answers

### Answer by username_5 on 2018-09-02. Score: 8 | Accepted answer

Even rocks that have lasted for billions of years decompose when exposed to Earth's weather.

What you have is indeed granite, but a somewhat [decomposed granite](https://en.m.wikipedia.org/wiki/Decomposed_granite), which is treated by geologists differently from the intact, solid stuff. The "crumbly bits" have their own
name: **[grus](https://en.wikipedia.org/wiki/Grus_(geology))**.

Granite is made of crystals of different minerals (mostly feldspar and quartz) that grew into each other as the parent magma cooled. The final rock is solid because of the irregular way the crystals fit into each other.

Granite weathers in several different ways. Cracks can form from hot and cold cycles. However, [chemical weathering](https://www.americangeosciences.org/education/k5geosource/content/rocks/what-is-chemical-weathering) is the principal way granite becomes crumbly. [Rain frequently picks up some acidity](https://en.wikipedia.org/wiki/Acid_rain) from the atmosphere, and infiltrates the granite. This acidic rain reacts with the feldspar and mica to produce clay minerals, primarily [kaolinite](https://en.wikipedia.org/wiki/Kaolinite).

This in turn makes it easier for water to get in the granite, and combined with heat cycles the process accelerates.
As a result, the joints between the crystals loosen up, producing rock in the same condition as your rock.


---

### Answer by username_2 on 2018-09-01. Score: 7

Yes, this is a granite.

It's hard to know exactly which one. A [geological map](http://www.dnr.wa.gov/publications/ger_ofr73-1_geology_south_snohomish_co_pt2.pdf) of the area shows the bedrock to be sedimentary rocks, so this piece of granite was transported in the creek from far away (but not too far, otherwise it will crumble completely).

If we go further east, for example with this [geological map](https://pubs.er.usgs.gov/publication/i1963), we find that:

> To the east, the higher-grade terrane is mostly the Chiwaukum Schist and related gneisses of the Nason terrane and invading mid-Cretaceous stitching plutons

and

> Rocks of the Cascade magmatic arc are mostly represented by Miocene and Oligocene plutons, including the Grotto, Snoqualmie, and Index batholiths.

So it could be a very poor example of a high-grade granitic composition gneiss, or it could be a granite from Cretaceous, Miocene or Oligocene times. Any more information than that would require someone to study the rock in detail.

> **Comments:**
> **username_0**: Great information. Thank you. Is there a part of this explanation that can account for the crumbliness?  
> **username_2**: @username_0 it's called weathering. The other answer by username_5 explains it well.  

---

### Answer by username_3 on 2019-07-24. Score: 1

It has been burned.

No acidity etc. needed. If you throw one fist sized granit into a normal campfire (usually around 700 C) and leave it there to burn for some hours, when it cools it will crumble in your hands. As mentioned before "granite is made of crystals of different minerals" in intensive heat/cooling each mineral expands at a different rate and brakes the composition apart.

Tested myself many times in practise..

> **Comments:**
> **username_2**: Most likely heating of a granite in a fire breaks it down not by expansion or contraction, but rather by decomposition of hydrous minerals like micas or amphiboles.  

---

### Answer by username_4 on 2018-09-05. Score: 1

My understanding of why granite decomposes that way is because it's formed under ground and as the over-burnden is removed so is weight and pressure. The granite formations are rounded as the outside layers fall away. We have very few freeze/thaw cycles in Arizona (Phoenix area), and not a lot of rain, but, we have tons and tons of decomposed granite in Arizona.


---

