Fossil shrinkage crack patterns
deutsche Version

Shrinkage of bulk materials appears in two different ways: In the simplest case, a shrinking chunk of matter becomes smaller, which is the very meaning of "shrink". Harder to imagine is shrinkage without size reduction, with the surface kept fixed, which gives rise to shrinkage stress. Most often reality is between these limits.
If the stress exceeds the strength of the material, cracks may arise. Usually the material is anisotropic, as wood, and the stress and strength, too, are anisotropic. Considering that stress and strain are quantities made up of 6 components each, and elasticity, which interrelates 
stress and strain, is a material property with up to 21 components, and there are multi-parameter non-elastic material properties, too, and the set of numbers describing the mechanical situation may vary throughout the material, and the cracks feel each other's presence via local stress fields, one need not wonder why the cracks arrange themselves in configurations which most often are not easily understood. What seems to be certain is that wide cracks had formed in an early stage when the matter was still soft but very narrow ones had formed in an advanced state of mineralisation. (Narrow cracks may become wider near the surface afterwards, as explained in Fossil Wood News 16 in connection with Fig.4 there.)turtle stone, fragment of septarian boulder
Widely known because often pictured are the crack networks with polygonal meshes. The 3D-structure can be columnar, as with the conspicuous more or less regular basalt columns, or not columnar but irregularly polyhedral, as in Fig.1, or something in between.

Fig.1 (right): Fragment of a septarian boulder, also known as turtle stone, with smooth surface below and a 3D-network of wide cracks dividing the shrunken matter into irregular polyhedra. Carboniferous limestone (?), Gullane beach, East Lothian, Scotland. Width 25cm.
shrinkage cracks in peat, silicified
Fig.2 (left): Shrinkage cracks in silicified peat with variable coupling to the peat texture: along and perpendicular to the layers on the left but unrelated to the layers on the right. Cut face of Lower Permian chert, Döhlen basin, Germany.

The crack pattern may show preferential directions, depending on the anisotropy of strength. The cracks on the left part of Fig.2 obviously preferred the easy paths
of weak strength along the peat layers. Secondary cracks across the layers came later. (The same sequence of crack formation is seen in wood misinterpreted as charcoal, there Fig.5.) On the right, the peat was virtually isotropic, as a result of either advanced decay or advanced silicification.
Shrinkage cracks in wood which did not take the easy way with the grain
(see example) indicate that there must have been shrinkage stress whose anisotropy was higher than the anisotropy of strength.
Shrinkage cracks standing by their own, without the shrunken matter in between, are seen in Fig.3. Apparently they had formed as a local phenomenon during an early stage of the silicification of wood which had been degraded such that it had lost its anisotropy. The cracks became filled with silica and were left over while the shrunken wood vanished for reasons unknown.siliceous casts of shrinkage cracks in petrified wood

Fig.3: Silica casts of shrinkage cracks formed in degraded wood, seen on a fracture face of Lower Permian petrified wood, Döhlen basin, Germany. Width of the picture 4cm.

In view of a variety of shrinkage crack phenomena occurring before, during, and after mineralisation, one need not try to explain some of them as fossil charcoal as done at the Naturkunde Museum Chemnitz [1]. The crack pattern on the stem surface in Fig.4 resembles that one in Fig.1, and the inside is divided into separate fragments by shrinkage cracks similar to those in Figs.1,2,3. Also it could not be explained how a whole tree trunk would turn into a neat arrangement of thousands of separate pieces of charcoal (Fig.5) without collapsing into a loose heap or going up in smoke and ashes.
(See Fossil Wood News 9.) Similar structures are interpreted as charcoal in [2].
alleged fossil charred wood surfacesection of fossil tree trunk allegedly consisting of charcoal
Detail of a fossil tree trunk surface (far left) and section allegedly consisting of silicified charcoal, after [1], Bild 449, 450.

wide cracksAddendum 2013:
Fig.6 (right): Shrinkage cracks in degraded wood in chert from Döhlen basin. Width of the picture 5cm.

The compressed wood seen in Fig.6 as a constituent of silicified peat has formed wide shrinkage cracks during an early stage of silicification, similar to those in Figs.4,5, which may serve as another argument against the interpretation of fossil wood with wide cracks as fossil charcoal.

Addendum 2015
: After stubbornly ignoring the plain facts presented to them repeatedly for about ten years, the authors [1,2] have changed their mind and do no more propagate the silly charcoal interpretation favoured at the Naturkunde-Museum Chemnitz, where a charred recent tree trunk has been shown for comparison, meant to support the notion of silicified charcoal. Now they offer the old interpretation as shrinkage cracks in decaying wood as if it were a new idea. (For references see Fossil Wood News 9.)

H.-J. Weiss     2012,  completed 2013, 2015

[1]  R. Rössler: Der versteinerte Wald von Chemnitz. Museum für Naturkunde Chemnitz, 2001, 179.
[2]  R. Noll, V. Wilde :  Conifers from the „Uplands“ – Petrified wood from Central Germany, 
       in: U. Dernbach, W.D. Tidwell : Secrets of Petrified Plants, D'ORO Publ., 2002, 88-103

quartz crystal with wood inside
Fossil Wood News  17

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