Including Original "Paul H. Letters" Copyright © 1996-2017 Paul V. Heinrich - All rights reserved.



Friday, 21 February 2014

Musdtone Spheroids

Musdtone Spheroids

In "OT: musdtone spheroids" at
http://lists.drizzle.com/pipermail/rockhounds_lists.drizzle.com/2014-February/002510.html
Tim asked:

"Paul, is spheroid weathering of mudstones equivalent
to weathering of concretions from the host rock? Or is
it a completely different process?"

They are completely different processes. Spheroidal
weathering is a form of chemical weathering of
systematically jointed bedrock that results in the
formation of concentric or spherical layers of highly
decayed rock within a surficial zone of weathered
bedrock known as “saprolite.” When the physical erosion
exposes bedrock altered by spheroidal weathering,
these layers peel (spall) off in concentric shells much
like the layers of an onion as it is is peeled. This
process often leaves behind rounded boulders, known
as corestones. Spheroid weathering is also known as
either "onion skin weathering," "concentric weathering,"
and "spherical weathering." However, "onion skin
weathering" is also used describe a different type of
weathering that also can produce rounded boulders.

An example of spheroidal weathering can be seen in
"File:Concentric spheroidal weathering in
granite.JPG" at
https://commons.wikimedia.org/wiki/File:Concentric_spheroidal_weathering_in_granite.JPG

Some references:

Buss, H. L., P. B. Sak, S. M. Webb, and S. L. Brantley 2008
Weathering of the Rio Blanco quartz diorite, Luquillo
Mountains, Puerto Rico: Coupling oxidation,
dissolution, and fracturing. Geochimica et Cosmochimica
Acta. vol. 72, pp. 4488–4507
http://www2.dickinson.edu/departments/geol/sak%20publications/Buss%20et%20al%20%202008.pdf
http://www2.dickinson.edu/departments/geol/Faculty/sak_pub.html

Chapman, R. W., and M. A. Greenfield, 1949, Spheroidal
weathering of igneous rocks. American Journal of Science.
vol. 247, no. 6, pp. 407-429.

Heald, M. T., T. J. Hollingsworth, and R. M. Smith,
1979, Alteration of Sandstone as Revealed by Spheroidal
Weathering. Journal of Sedimentary Petrology. vol. 49,
no. 3, pp.s 901-909.

Ollier, C. D. (1971). Causes of spheroidal weathering.
Earth Science Reviews 7, 127-141.

Tim noted:

“I've seen both clearly defined concretions (sometimes
fossil bearing, thus my interest), and textures very
similar to the photos, both in mudstones (or I assume
so, I have often seen both occurring within a single
outcrop).”

Concretions are round. However, they form by the
cementation of sediments starting from a central point.
In case of concretions, mechanical weathering is simply
stripping poorly cemented matrix from the enclosed
well-cemented concretions to expose them.

Tim wrote,

“I should have known that throwing in a random
photo of rocks would be par for the course for an
article postulating about impacts of super-dense
celestial bodies and/or alien spacecraft. Otherwise
a great explanation!”

Offlist, I have been made aware of various YouTube
videos, which I was previously unaware. It is clear
from the Youtube videos that these features are
associated with the cones. I would suspect that the
pictures of spheroidal weathering are of the
weathered blocks of rock described as being part
the cone’s rubble. I would guess that the spheroidal
weathering is likely is part of a relict surficial
weathering zone (saprolite) that was broken up by
the formation of the cone.

Yours,

Paul H.