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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

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

enter image description here

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.

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?)

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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, which is treated by geologists differently from the intact, solid stuff. The "crumbly bits" have their own name: grus.

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 is the principal way granite becomes crumbly. Rain frequently picks up some acidity from the atmosphere, and infiltrates the granite. This acidic rain reacts with the feldspar and mica to produce clay minerals, primarily 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.

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Yes, this is a granite.

It's hard to know exactly which one. A geological map 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, 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.

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  • $\begingroup$ Great information. Thank you. Is there a part of this explanation that can account for the crumbliness? $\endgroup$ – BMS Sep 2 '18 at 2:00
  • $\begingroup$ @BMS it's called weathering. The other answer by Spencer explains it well. $\endgroup$ – Gimelist Sep 3 '18 at 1:41
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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.

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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..

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  • $\begingroup$ 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. $\endgroup$ – Gimelist 2 hours ago

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